PERFORMANCE EVALUATION OF BALL MILL AND SHAKING TABLES AT RAHMAN
HYDRAULIC TIN SDN BHD
SITI HAMIDAH BINTI NOORHAKIMI
UNIVERSITI SAINS MALAYSIA
2019
PERFORMANCE EVALUATION OF BALL MILL AND SHAKING TABLES AT RAHMAN
HYDRAULIC TIN SDN BHD
by
SITI HAMIDAH BINTI NOORHAKIMI
Thesis submitted in fulfilment of the requirements for the degree of
Bachelor of Engineering with Honours (Mineral Resources Engineering)
July 2019
DECLARATION
I hereby declare that I have conducted, completed the research work and written the dissertation entitled “Performance Evaluation of Ball Mill and Shaking Tables at Rahman Hydraulic Tin Sdn Bhd”. I also declare that it has not been previously submitted for the award of any degree or diploma or other similar title of this for any other examining body or university.
Name of Student : Siti Hamidah Binti Noorhakimi Date : 2nd July 2019
Signature:
Witness by,
Supervisor : Assoc. Prof. Ir. Dr. Syed Fuad Saiyid Hashim Date : 3rd July 2019
Signature:
ACKNOWLEDGEMENT
The success and final outcome of this project required a lot of guidance and assistance from many people and I am extremely privileged to have got this all along the completion of my project. All that I have done is only due to such supervision and assistance and I would not forget to thank them.
I would like to thank my supervisor, Assoc. Prof. Ir. Dr. Syed Fuad Saiyid Hashim for the patient guidance, encouragement and advice he has provided throughout my time as his student. I have been extremely lucky to have a supervisor who cared so much about my work, and who responded to my questions and queries so promptly. At many stages in the course of this research project I benefited from his advice, particularly so when exploring new ideas. His positive outlook and confidence in my research inspired me and gave me confidence. I would also like to thank all the staff at School of Materials and Mineral Resources Engineering, especially Mr. Junaidi Ramli, Mrs. Mahani, Mr. Mokhtar, Mr. Hasnor who helped me a lot by giving me full guidance in handling the equipments in the workshop and laboratory.
I take this oppurtunity to thank School of Materials and Mineral Resources Engineering for giving me an oppurtunities to use the facilities and equipments until I finished my entire project.
I owe my deep gratitude to my parents, Noorhakimi Bin Ismail and Rosnah Binti Hussin for their support and being pillar of my strength in my life.
Siti Hamidah Binti Noorhakimi.
TABLE OF CONTENTS
DECLARATION ... ii
ACKNOWLEDGEMENT ... iii
TABLE OF CONTENTS ... iv
LIST OF TABLES ... vii
LIST OF FIGURES ... viii
LIST OF SYMBOLS ... xi
LIST OF ABBREVIATIONS ... xii
ABSTRAK ... xiii
ABSTRACT ... xiv
CHAPTER 1 INTRODUCTION ... 1
1.1 Introduction ... 1
1.2 Problem Statement ... 4
1.3 Objectives ... 5
1.4 Scope of the Studies ... 6
CHAPTER 2 LITERATURE REVIEW ... 7
2.1 Introduction ... 7
2.2 Ball Mill Operation ... 13
2.3 Particle Breakage Mechanism in a Ball Mill ... 15
2.3.1 Breakage by Impact or Compression ... 17
2.3.2 Breakage by Abrasion ... 18
2.3.3 Breakage by Attrition ... 19
2.4 Effect of Breakage Mechanism on Mineral Liberation ... 20
2.5 Factors Affecting Ball Mill Efficiencies ... 21
2.5.1 Size of Grinding Media ... 21
2.5.2 Speed of Mill ... 22
2.5.3 Pulp Density ... 24
2.5.4 Charge Volume ... 25
2.5.5 Circulating Load ... 25
2.6 Gravity Concentrations by Shaking Tables ... 26
2.7 Working Principles of Shaking Tables ... 28
2.8 Stratification and Hindered Settling ... 30
2.9 Operating Parameters of Shaking Tables ... 32
CHAPTER 3 METHODOLOGY ... 35
3.1 Introduction ... 35
3.2 Samples ... 35
3.3 Drying of Samples ... 37
3.4 Sampling ... 37
3.4.1 Coning and Quartering ... 38
3.4.2 Jones Riffle Splitter ... 39
3.5 Sieving ... 42
3.6 Magnetic Separation ... 43
3.7 Tinning Test ... 44
3.8 Optical Microscopy Analysis ... 45
CHAPTER 4 RESULTS AND DISCUSSION ... 46
4.1 Introduction ... 46
4.2 Particle Size Distribution ... 46
4.2.1 Particle Size Analysis of Ball Mill 1 ... 47
4.2.1(a) Feed Ball Mill 1 Particle Size Analysis Results ... 49
4.2.1(b) Product Ball Mill 1 Particle Size Analysis Results ... 49
4.2.2 Particle Size Analysis of Ball Mill 2 ... 50
4.2.2(a) Feed Ball Mill 2 Particle Size Analysis Results ... 52
4.2.2(b) Product Ball Mill 2 Particle Size Analysis Results ... 52
4.2.3 Particle Size Analysis of Ball Mill 3 ... 53
4.2.3(a) Feed Ball Mill 3 Particle Size Analysis Results ... 55
4.2.3(b) Product Ball Mill 3 Particle Size Analysis Results ... 55
4.2.4 Overall Ball Mill Performance ... 56
4.2.5 Particle Size Analysis of Shaking Tables ... 57
4.2.5(a) Concentrate Shaking Table 1-15 ... 59
4.2.5(b) Concentrate Shaking Table 16-23 ... 59
4.3 Tinning Test Result ... 61
4.4 Optical Microscopic ... 62
4.4.1 Ball Mill 1 ... 63
4.4.1(a) Feed ... 63
4.4.1(b) Product ... 66
4.4.2 Ball Mill 2 ... 68
4.4.2(a) Feed ... 68
4.4.2(b) Product ... 70
4.4.3 Ball Mill 3 ... 72
4.4.3(a) Feed ... 72
4.4.3(b) Product ... 75
4.4.4 Shaking Table ... 77
4.4.5 Tailing Shaking Table ... 79
CHAPTER 5 CONCLUSION AND FUTURE RECOMMENDATIONS .... 82
5.1 Conclusion ... 82
5.2 Recommendations for Future Research ... 83
REFERENCES ... 85
LIST OF TABLES
Page
Table 2.1 Effect of variables on shaking tables performance ... 34
Table 4.1 Feed Ball Mill 1 (FBM 1) ... 47
Table 4.2 Concentrate Ball Mill 1 (CBM 1) ... 48
Table 4.3 Feed Ball Mill 2 (FBM 2) ... 50
Table 4.4 Concentrate Ball Mill 2 (CBM 2) ... 51
Table 4.5 Feed Ball Mill 3 (FBM 3) ... 53
Table 4.6 Concentrate ball mill 3 (CBM 3) ... 54
Table 4.7 Ball mill comparison ... 56
Table 4.8 Concentrates Table 1-15 (CT 1-15) ... 57
Table 4.9 Table 4.8 Concentrate Table 16-23 (CT 16-23) ... 58
LIST OF FIGURES
Page
Figure 1.1 Shaking tables at KBM processing plant ... 3
Figure 1.2 Ball mill at KBM processing plant ... 4
Figure 2.1 Mineral locked with gangue and illustrating effect of breakage on liberation ... 8
Figure 2.2 Example cross sections of ore particles ... 9
Figure 2.3 Motion of charges in grinding mill ... 14
Figure 2.4 Types of motion in a grinding mill a) cascading b) cataracting c) rolling ... 15
Figure 2.5 Types of particle breakage a) impact b) compression c) abrasion... 19
Figure 2.6 Schematic drawing of shaking table view ... 29
Figure 2.7 Segregation action across the surface of a table ... 30
Figure 2.8 Segregation action profile for a riffle of a table ... 30
Figure 2.9 Segregation of particles due to horizontal shaking motion ... 32
Figure 3.1 Kota Bunyih Mill processing plant ( sampling point ) ... 36
Figure 3.2 Drying of samples for two days ... 37
Figure 3.3 Coning and quartering ... 39
Figure 3.4 Jones Riffle Splitter ... 40
Figure 3.5 Feed ball mill 1 (FBM 1) ... 40
Figure 3.6 Feed ball mill 2 (FBM 2) ... 40
Figure 3.7 Feed ball mill 3 ... 41
Figure 3.8 Concentrate ball mill 1... 41
Figure 3.9 Concentrate ball mill 2... 41
Figure 3.10 Concentrate ball mill 3... 41
Figure 3.11 Concentrate tables 1-15 ... 41
Figure 3.12 Concentrate tables 16-23 ... 41
Figure 3.13 Tailing tables 16-23 ... 42
Figure 3.14 Tailing tables 1-15 ... 42
Figure 3.15 Sieving by automatic sieve shaker ... 43
Figure 3.16 Weighing of sample retained on each sieve sizes ... 43
Figure 3.17 Zinc granules ... 44
Figure 3.18 Reaction between zinc and hydrochloric acid ... 44
Figure 3.19 Complete reaction between zinc and hydrocloric acid ... 44
Figure 3.20 Stereo zoom microscope Kunoh Robo ... 45
Figure 4.1 Particle size distribution curve of ball mill 1 feed and product ... 48
Figure 4.2 Particle size distribution curve of ball mill 2 feed and product ... 51
Figure 4.3 Particle size distribution curve of ball mill 3 feed and product ... 54
Figure 4.4 Particle size distribution of ball mill feeds and products ... 56
Figure 4.5 Particle size distribution curve of shaking table concentrate... 58
Figure 4.6 Particle size distribution curve of shaking table concentrate and tailing ... 60
Figure 4.7 Before tinning test ... 61
Figure 4.8 After tinning test, cassiterite present was coated with zinc ... 61
Figure 4.9 Picture of stereo zoom microscopy... 62
Figure 4.10 Grain size at +3.35 mm ... 63
Figure 4.11 Grain size at 1.7 mm to 3.35 mm... 63
Figure 4.12 Grain size at 850 µm to 1.7 mm ... 64
Figure 4.13 Grain size at 425 µm to 850 µm ... 64
Figure 4.14 Grain size at 212 µm to 425 µm ... 65
Figure 4.15 Grain size at 106 µm to 212 µm ... 65
Figure 4.16 Grain size at +850 µm ... 66
Figure 4.17 Grain size between 425 µm to 850 µm ... 66
Figure 4.18 Grain size between 212 µm 425 µm ... 67
Figure 4.19 Grain size between 106 µm to 212 µm ... 67
Figure 4.20 Grain size at +3.35 mm ... 68
Figure 4.21 Grain size between 850 µm to 3.35 mm ... 68
Figure 4.22 Grain size between 425 µm to 850 µm ... 69
Figure 4.23 Grain size between 106 µm to 425 µm ... 69
Figure 4.24 Grain size at +1.00 mm ... 70
Figure 4.25 Grain size between 425 µm to 1.00 mm ... 70
Figure 4.26 Grain size between 250 µm to 425 µm ... 71
Figure 4.27 Grain size between 106 µm to 250 µm ... 71
Figure 4.28 Grain size at +3.35 mm ... 72
Figure 4.29 Grain size between 1.7 mm to 3.35 mm ... 72
Figure 4.30 Grain size between 850 µm to 1.7 mm ... 73
Figure 4.31 Grain size between 425 µm to 850 µm ... 73
Figure 4.32 Grain size between 212 µm to 425 µm ... 74
Figure 4.33 Grain size between 106 µm to 212 µm ... 74
Figure 4.34 Grain size at +1.7 mm ... 75
Figure 4.35 Grain size at 850 µm to 1.7 mm ... 75
Figure 4.36 Grain size at 425 µm to 850 µm ... 76
Figure 4.37 Grain size between 212 µm to 425 µm ... 76
Figure 4.38 Grain size at +425 µm ... 77
Figure 4.39 Grain size between 212 µm to 425 µm ... 77
Figure 4.40 Grain size between 106 µm to 212 µm ... 78
Figure 4.41 Grain size at +425 µm ... 78
Figure 4.42 Grain size at 850 µm ... 79
Figure 4.43 Grain size at 425 µm ... 79
Figure 4.44 Grain size between 300 µm to 425 µm ... 80
Figure 4.45 Grain size between 212 µm to 300 µm ... 80
Figure 4.46 Grain size between 106 µm to 212 µm ... 81
LIST OF SYMBOLS
NC Critical speed of mill
D Diameter of mill
F80 Diameter 80% feed mass has smaller particle size P80 Diameter 80% product mass has smaller particle size D50 Diameter 50% feed mass smaller than and larger than D80 Diameter 80% feed mass has smaller particle size NC Critical speed of mill
D Diameter of mill
LIST OF ABBREVIATIONS
RHTB Rahman Hydraulic Tin Sdn Bhd MSC Malaysia Smelting Corporation
KBM Kota Bunyih Mill
FBM Feed Ball Mill CBM Product Ball Mill
CT Concentrate Shaking Table TT Tailing Shaking Table USM Universiti Sains Malaysia
PENILAIAN PRESTASI PENGISAR BEBOLA DAN MEJA AYUN DARIPADA RAHMAN HYDRAULIC TIN SDN BHD
ABSTRAK
Bertujuan untuk mempertingkatkan pemulihan produk kasiterit di Rahman Hydraulic Tin Sdn Bhd, pengisar bebola dan meja ayun telah digunakan dalam loji pemprosesan untuk menghasilkan zarah halus daripada suapan kasar. Prestasi pengisar bebola dan meja ayun semasa penghasilan mineral sangat penting. Oleh itu, prestasi peralatan dinilai untuk membantu mengoptimumkan peralatan pemprosesan.
Mikroskop optik, analisis pembebasan mineral, pengagihan saiz bijian dan penilaian mineral bersekutu telah dijalankan ke atas sampel yang didapati daripada bebola pengisar dan meja ayun. Ujian timah dilakukan untuk menentukan zarah kasiterit.
Untuk bahagian pertama, selepas pecahan interpartikel, kelakuan pemecahan bahan sampel akan dikaji dan diterangkan dari segi taburan saiz zarah. Yang terakhir dinilai dengan menggunakan mikroskop zum Kunoh Robo untuk mengkaji pembebasan mineral bernilai. Ciri-ciri pembebasan dan pematahan yang disebabkan oleh pengisaran bermampat dianalisis. Di samping itu, cadangan juga akan diperincikan berhubung dengan pemprosesan bijih dan kemungkinan masalah yang harus dielakkan semasa penggunaannya atau pemprosesannya sendiri. Keputusan menunjukkan bahawa pembebasan bijih berlaku pada saiz kasar (<850 μm). Selain itu, mineral galian juga didapati dalam amang dalam saiz yang agak kasar (<600 μm) yang menunjukkan bahawa pemisahan graviti kurang efisien dalam perawatan mineral bersaiz kasar.
PERFORMANCE EVALUATION OF BALL MILL AND SHAKING TABLES AT RAHMAN HYDRAULIC TIN SDN BHD
ABSTRACT
Aiming to enhance the production and recovery of cassiterite concentrate from Rahman Hydraulic Tin Sdn Bhd, ball mill and shaking table has been used in processing line to produce fine particles from the coarse feed. The performance of ball mill and shaking table during the recovery of a mineral is very important. Therefore, the performance of the equipment’s was evaluated to help optimize the processing equipment. The optical microscope, mineral liberation analysis, grain size distribution and mineral association assessment were carried out on sample obtained from ball mill and shaking table. The optical microscope, mineral liberation analysis, grain size distribution and mineral association assessment were carried out. Tinning test were conducted to determine the cassiterite particles respectively. For the first part, after the interparticle breakage, the breaking behavior of the sample material was studied and described in terms of particle size distribution. The latter was evaluated by using the stereo zoom microscope Kunoh Robo to observe the liberation of the valuable minerals. The characteristics of liberation and fracture caused by compressive grinding was analysed. In addition, recommendations will also be detailed in relation to the processing of the ore and possible problems that should be avoided during its use or its own processing. Results show that the liberation of ore occurs at coarser size (<850 µm). Moreover, the valuable minerals were also found in tailings in relatively coarser size (<600 µm) which indicated that the gravity concentration were less efficient in treating coarser size minerals.
CHAPTER 1 INTRODUCTION
1.1 Introduction
Generally, cassiterite is known as tin oxide mineral, tinstone, stannic oxide, and tin (IV) oxide having a chemical composition of SnO2. Cassiterite is the precious metal having commercial value that helps developing the growth of the economy. It is the most important tin source and it can be recovered by mining of cassiterite. The is a theory saying that cassiterite containing tin was 78.77 percent. In a rare case, when chemically pure, cassiterite is said to contain about 78.6 percent of tin but, when it became contaminated with impurities, the tin content will vary in between 73 and 75 percent. Cassiterite is primarily present in two types of deposits. It occurs initially as a primary intruding component of certain late stage granite intrusions and is found both in veins and cracks on granite rock as well as in the surrounding country rock. The second type of deposit comes from secondary source and take place as deposits of alluvial or placer and detrital. Around 70 percent of tin global productions is obtained from placer deposits.
Cassiterite is clearly related to highly acidic granitic rock and it is interestingly significant that cassiterite is not present on the ground in one single tin field in the world except near granite or granite rocks. Due to cassiterite’s chemical stability, each primary deposit contributes to secondary accumulations that is currently the most important source of tin. The bulk of the world's supply of cassiterite comes from stanniferous alluvial deposits derived from mineralized areas in their neighborhood. In the Northern Transvaal region of South Africa the main source of tin is the deposits of the region of Rooiberg, Leeuport and Union Tin, which form roof on arkosites and shales of the Transvaal Precambrian System. Since cassiterite is ductile, malleable, not being easily
oxidized in the air and thus resistance to corrosion, it can be utilized in a variety of applications such as solder, chemical compounds production, tin plates, batteries, and alloys. It is also used primarily as a coating to prevent corrosion. Tin became the primary metal (>60 percent) in solder after the use of lead in electronics was banned.
At present the majority of global cassiterite is mined in Malaysia, Indonesia, Bolivia, Nigeria, Myanmar, Thailand, and parts of China represent around 95 percent of world production of cassiterite, while the remaining countries produce only smaller amounts. Malaysia was previously, in the South Asian Tin Belt, one of the biggest and largest production companies in the world until the mid-1980s. Tin was an initial driver of early economic development in the Malaysia Peninsula throughout the 20th century, the non – renewable natural resources. A large proportion of export earnings was accounted for in that century. Tin mining in Malaysia is considered as a sunset industry where the access to the remaining reserves are accessible in line with the needs of the manufacturing and leisure sectors before the tin price collapsed, forcing the closing of many tin mines in 1985. The major tin production was from placer tin deposits or alluvial from states of Perak and Selangor. The source of many of the alluvial deposit can be traced to the primary tin veins and ore bodies found at contact zone of tin-bearing granites and other rocks. These days, the higher demand towards metal including tin seems to be seen in the development of many previously abandoned tin fields and new resources in Malaysia. Now, in Malaysia, Rahman Hydraulic Tin Sdn. Bhd (RHTB) is one of the mining company that carried out the largest mining activities in the states of Perak which contributes and develops the country’s economy.
Rahman Hydraulic Tin Sdn Bhd (RHTB) is located in Klian Intan, Hulu Perak, Perak. RHTB operates as a subsidiary of Malaysia Smelting Corporation (MSC). The
mine commences its operation in the year 1907 and treats cassiterite ore to engender a tin concentrate. RHTB consist of five processing plant which is known as Palong 1, Palong 2, Kota Bunyih Mill, Tin Mill, and Mini Mill. The main focus is on the Kota Bunyih Mill processing plant which consists of three units of a ball mill (4 feet × 8 feet) utilized for grinding or regrinding and several units of shaking tables which are utilized in gravity concentration. Both equipment’s required an evaluation performance to improve the production and recovery of tin concentrate.
Currently, the grinding and gravity concentration circuits in Kota Bunyih Mill are quite inefficient in achieving their aspired final product due to the interlocking minerals which are not fully liberated during the grinding process which affecting the final products that produce in the shaking tables. So, a study of the performance of both ball mill and shaking tables was conducted in order to improve the recovery of the products through the experimental lab works. The experimental works focus on the qualitative characterization of the feed, products, and tailings from ball mill and shaking tables in Kota Bunyih Mill processing plant.
Figure 1.1 Shaking tables at KBM processing plant
1.2 Problem Statement
Tin is one of the most important commodities that contribute to an increase in the income and economic development of Malaysia. Tin, like many other metals, saw better demand towards the end of the year as a more bullish global economic outlook developed. The mineral and metal industry is highly dependent on it. In the country, it is the primary sources which are widely used in the tin smelting company (MSC) to produce tin and tin based products. The purpose of this project is to evaluate the performance of ball mill and shaking table by characterizing the site's ore, by analyzing the liberation, and mineral distribution to increase the recovery of the final products produced by both ball mill and shaking tables.
Ball mill is often used in the final stage of comminution. However, in the case of ball mill, there are some shortcoming that exists in the ball mill that affects the grinding efficiency and the performance of milling. This shortcoming causes the ball mill not to operate under optimum conditions which can lower the production. As for shaking tables, the main problems have been its limitation in treating particles in the
Figure 1.2 Ball mill at KBM processing plant
relatively coarse size range. In coarse size ranges, the fluid and viscous forces become most commonly relative to the gravity and this, in turn, affects the separation efficiency.
Recently an effort has been put in to develop an efficient gravity separator for the treatment of fines materials, which has resulted in considerable success. From the results obtained, we will be able to increase the recovery of the current processing plant along with providing some suitable parameters and recommendations for the equipment’s to run at their optimum conditions.
1.3 Objectives
The objectives of this project are as follow:
1) To evaluate the efficiency of ball mill and shaking table at Kota Bunyih Mill processing plant.
2) To determine the liberation rate and performance of ball mill and shaking table at Kota Bunyih Mill.
3) To provide recommendations of possible parameters that can be change to improve the efficiency of the ball mill and shaking tables.
1.4 Scope of the Studies
Rahman Hydraulic Tin Sdn Bhd (RHTB) which operates the largest open-pit mine in Malaysia in the State of Perak continued to focus on the pit optimization, enhancement of ore processing plants and improving its environmental standards for further improvements in the operating efficiencies. The interest is now moving to the evaluation of the performance of the ball mill and shaking tables in RHTB to improve their efficiencies in order to produce fine-grained materials. This thesis reports that the work concerned with the evaluation of the performance through qualitative study of the equipments feed, products and tailings of cassiterite ore for improving the efficiencies.
This final year projects is divided into two main parts. The first part focused on the particle size distribution of the minerals while the second parts focused on the mineral liberation studies of the cassiterite to determine the success of ball mill to perform its grinding operations to produce a finer products.
CHAPTER 2 LITERATURE REVIEW
2.1 Introduction
Comminution has become one of the important processes in mining industries, especially in mineral processing. It is a collective term for processes which reduces the grain sizes of material. The goals of comminution is to achieve liberation at the coarsest possible particle size. If such a goals is achieved, then not only the energy will be save but also will reduce the amount of fine particles produced by any subsequent separation stage becomes much easier and cheaper to operates. The throughput and performance also can be increased. (Wills and Finch 2016). In mineral processing, comminution can also be referred to as crushing (used for coarse material) and grinding (used for fine material). For materials which are not completely liberated from their original grain size, comminution is highly recommended. The mineral must be initially liberated or unlocked before the separation process can be undertaken since most of the minerals are finely disseminated and intimately associated with gangue mineral. This process is accomplished by comminution in which the particle size of the ore is progressively reduced until the clean particles of the mineral can be separated by such methods as are available. (Wills 2005).
Full liberation is rarely achieved in practice, although the ore is ground down to less than of the desired mineral’s grain size. This is illustrated by Figure 2.1, which shows a lump of ore containing a grain of valuable mineral with a breakage pattern superimposed that divides the lump into cubic particles of identical volume (for simplicity) and of a size below that of the mineral grain. It can be judged that each particle produced containing mineral also contains a portion of gangue. Complete
liberation has not been attained, but the bulk of the major mineral (the gangue mineral), however, has been liberated from the valuable mineral.
The particles of “locked” (or “composite”) mineral and gangue are known as middlings, and further liberation from this fraction can only be achieved by further comminution. The “degree of liberation” refers to the percentage of the mineral occurring as free particles in the broken ore in relation to the total mineral content in locked and free particles. Liberation can be high if there are weak boundaries between mineral and gangue particles, which is often the case with ores composed mainly of rock-forming minerals, particularly sedimentary minerals. This is sometimes referred to as “liberation by detachment.” Usually, however, the adhesion between mineral and gangue is strong and during comminution the various constituents are cleft across the grain boundaries; that is, breakage is random. Random breakage produces a significant amount of middlings. Approaches to increasing the degree of liberation involve directing the breaking stresses at the mineral grain boundaries, so that the rock can be broken without breaking the mineral grains (Wills and Atkinson, 1993). For example,
Figure 2.1 Mineral locked with gangue and illustrating effect of breakage on liberation
microwaves can be used, which cause differential heating among the constituent minerals and thus create stress fractures at grain boundaries (Kingman et al., 2004)
Figure 2.2 shows the cross-section of the ore particle and indicates the dilemma of liberation often encountered by the ore processor. The A part constitutes the valuable mineral and AA part is rich in valuable mineral but strongly linked with the gangue mineral. Cominution can lead to several fragments of mineral, gangue particles, and illustrated particles being fully released. Part 1 particles is rich in minerals and is classified as a concentrate since it still has an acceptable low level of interlocking with the gangue in order to produce sellable concentrate levels. Type 4 parts may be classified as tailings, because the small quantity of mineral present is an acceptable mineral loss.
Particles of types 2 and 3 are likely to be ranked as middlings, although the re- grinding level required to promote the release of the mineral from particle 3 is greater than that of particle 2. In practice, ores are ground to an optimum grind size, determined by laboratory and pilot scale testwork, to produce an economic degree of liberation. The concentration process is then designed to produce a concentrate consisting predominantly of valuable mineral, with an accepted degree of locking with the gangue minerals, and a middlings fraction, which may require additional grinding to promote optimum mineral released. The tailings should be mainly composed of gangue minerals.
Figure 2.2 Example cross sections of ore particles
Since grinding is the last stage in communition, the minerals particles are reduced in size by the combination of impact and abrasion inside the mill either in dry or wet condition. In addition, there are many types of grinding mill such as rod mill, ball mill, tube mill, pebble mill, batch mill, etc. Usually, the grinding process is performed in a cylindrical steel vessel containing grinding media which rotating about its axis. The grinding media are free to move inside the mill which communiting the ore particles. As the main purpose of grinding is to produce the controlled product size, so correct grinding is required to be the key to a good mineral processing. Under-grinding of the ore, however, will produce a product which is too coarse with a low degree of liberation for economic separation, thus poor recovery and enrichment ratio will be obtained in the concentration stage. (Wills 2005). Overgrinding needlessly reduces the particle size of the subsequently liberated major constituent (gangue mineral) and may reduce the particle size of the minor constituent (valuable mineral) below the size required for most efficient separation. (Wills 2005).
Grinding can be done by several mechanisms, including impact or compression, due to forces applied almost normally to the particle surface, chipping due to oblique forces or abrasion due to forces acting parallel to the surfaces. These mechanisms will alter the particles and change their shape over certain limits which are determined by their degree of elasticity, which will cause them to break. (Wills, 2005). Grinding is usually performed in wet conditions, although in certain applications, dry grinding is used. When the mill is rotated, the mixture of medium, ore, and water, which are known as the mill charge, will intimately be mixed, the medium communiting the particles by any of the above mechanisms depending on the speed of rotation of the mill and the structures of the shell liner.
In a ball mill, communition process is performed by using steel balls as the grinding medium since the steel balls have a greater surface area per unit weight as compared to rod mill. The diameter ratio of the ball is usually ranging from 1.5 to 1 and less. Grinding process in a ball mill is affected by the point contact of balls with the ore particles and the given time so that any degree of fineness can be achieved. The process is completely random so the probability of a fine particle being struck by a ball is the same as that of a coarse particle. The product from an open-circuit ball mill, therefore, exhibits a wide range of particle size and over-grinding of at least some of the charge becomes a problem. Closed-circuit grinding in a ball mill usually providing a low residence time for the particles is almost always used in the last stages to overcome this.
There are several factors that influence the efficiency of ball mill. Gupta and Yan, (2006) states that the major factors which are directly influencing the behavior of the load are the constitution of the charge, the rotational speed of the mill and the type of motion of individual pieces of the medium in the mill. Since the liner profile and the speed of the mill are generally fixed operating parameters by considering the fact that the liners wear very slowly. So, as a result, the type of motion of particles does not change on average. This implies an industrial point of view that flexibility will be most of the time limited only to the constitution of the charge.
Earlier studies by Concha et al. (1992) were able to get a 12 percent increase in the capacity of an industrial mill circuit by systematically optimizing the ball charge mixture. Because of this, particular attention should be paid to making sure that the charge is sensibly controlled. For this reason, a clear description of the effect of mill load composition will allow one to better model the grinding process and in that way will provide a tool to find possible increases in mill performance.
The effect of ground conditions on the response of ore can be studied by changing one condition at a time and figured out information from the data. (Egbe, E.A.P 2012). Man, Y.T. (2002) indicates that mill speed affects throughput and energy consumption. The size of ball influence mill throughput, power consumption and product size (Fuerstenau, D.W., Lutch, J.J. and De, A. 1999), (Kotake, N., Daibo, K., Yamamoto, T., Kanda, Y. 2004). It is reported by (Trumic, M., Magdalinovic, N., Trumic, G. 2007) that each grain size has an optimum size of the ball.
Sahoo, A. and Roy, G. K. (2008) discovered that ball mills usually operate at speed ranging from 65 percent to 75 percent of the critical speed. Overloading of the ball mill leads the fines to accumulate at the toe of the mill, which results in a cushioning effect on the balls upon the impact. In the opposite way, when the loading material is low, excessive ball to ball contact retard the rate of breakage. (Gupta, A. and Yan, D.S.
2006). The optimum breakage response of an ore requires the maximum production of particles of the desired size while minimizing the production of the untreatable fines.
This way that effects of milling conditions on response must be known in addition to the grinding of an ore.
In addition, a number of researchers (Deniz, 2012., Bwalya et al., 2014; Petrakis et al., 2016) carried out the studies about the effect of feed particle size and grinding media size on the grinding kinetics of different ores. (Khumalo et. Al., 2006) postulated that generally larger sized grinding media would break larger particles quicker, but a finer product would be obtained by use of smaller balls. However, the use of smaller grinding media is believed to support abrasion and attrition (Katubilwa et al., 2011) which are energy inefficient breakage mechanisms (King, 2001).
2.2 Ball Mill Operation
Ball mills utilize steel balls as the only grinding medium. They work effectively over a wide range of length to diameter aspect ratios of 1.5 to 1.0 and even less (Schlanz 1987, Napier-Munn, Morrel et al. 1996, Wills and Napier-Munn 2006). Based on discharge mechanism, two main forms of ball mills exist. The overflow mill has an exit hole at the discharge trunnion which is larger than the inlet, generating a hydraulic gradient which drives the slurry through the mill. A scalping screen is normally required to collect smaller or worn-out grinding medium which may overflow with the pulp.
Power consumption may be up to 15 percent less than the other type although in terms of milling efficiency are nearly equal.
The grate discharge mill is fitted with discharge grates between the cylindrical mill shell and the discharge trunnion. This type requires a lower pulp density which reduces retention times, resulting to little overgrinding but also discharges a large fraction of coarse particles, necessitating closed circuit configuration (Gupta and Yan 2006, Wills and Napier-Munn 2006). There are several factors influencing ball mills efficiency which will be highlighted in following sections.
The distinctive feature of tumbling mills is the use of loose crushing bodies, which are large, hard, and heavy in relation to the ore particles, but small in relation to the volume of the mill, and which occupy slightly less than half the volume of the mill.
Due to the rotation and friction of the mill shell, the grinding medium is lifted along the rising side of the mill until a position of dynamic equilibrium is reached, when the bodies cascade and cataract down the free surface of the other bodies, about a dead zone where little movement occurs, down to the toe of the mill charge.
Regardless of the grinding media, all tumbling mills have the same basic principles for breaking. When the mill rotates, the grinding charge is increased by liners from a level surface position preventing the charges from slipping so that the media is shifting with the shell and tumbling over the weight of the charges. The design in which the load tumbles (Figure 2.4) is associated with the rotational speed of the mill.
Cascading describes the portion of the media that favor to rolls down towards the toe of the mill, resulting in an abrasive comminution, which is then resulting in finer grinding and greater wear of liner. This action leads to a comminution effect, resulting in a more coarser product. Most mills currently use the combination of the movement of cascading and cataracting.This was due to the concept that most of the grinding action at the end of the load occurs in a mill, where it does not only directly affect the charge, but also receives the shock transferred from the cataracting substance.(Wills and Napier-Munn 2006).
Figure 2.3 Motion of charges in grinding mill
2.3 Particle Breakage Mechanism in a Ball Mill
With knowledge of the fracturing mechanism of a particular ore, it can be possible to properly design and select a more efficient comminution machine. The application of an external force results in particle fractures. The stress develops inside the particles when an external force is applied. If this stress surpasses the final stress, it breaks down (Rao 2011). The way the particle fractures are dependent on the nature of the particle material and its inner structure and how the force is applied to the particle (Rumpf 1965, Schönert 1996, Gupta 2003). The size reduction devices used today break particles with different types of forces.
The particle strength is defined as the applied stress at the very first breaking point. Here in case, the breaking strength shall be defined as the force of the first fracture by unit area of a particle cross section, whereas the breaking energy is the work that must be applied to make it fracture (Fuerstenau and Han 2003). The breaking energy is the component of the applied force over the resulting deformation. It is important to realize that the actual strengths of materials are much lower than their theoretical strengths (Fuerstenau and Han 2003), which is due to material defects in the inner structure.
Figure 2.4 Types of motion in a grinding mill a) cascading b) cataracting c) rolling
The theoretical strength is based on the fact that the material is homogenous.
However, defects occur always in ordinary bulk materials as lattice failures, grain boundaries and micro-crack. Stress levels are much higher at these defect than in other body parts. Due to the higher stress, fractures at these points will start. Thus, because of these defects, the actual strength is less than theoretical strength. The energy consumed by material fractures or ruptures increases the extent of these ruptures. The production of a new surface areas (specific free surface energy) and the plastics deformation of the material at the crack tip are part of the energy that are used by the cracks. (Fuerstenau and Han 2003).
Materials can basically be classified as ductiles or brittles. A ductile material usually breaks into two pieces when stressed to failure. The stress of a fragile material causes shattering or breaking down into many parts of various dimensions that cannot control the fracture paths. Since ores act as a brittle material, the fracture pattern presents difficulties in grinding by trying to create fractures in certain limits without having any control over the fracture process.
Griffith (1921) showed that materials fail by crack propagation when this is energetically feasible, i.e. when the energy released by relaxing the strain energy is larger than the energy of the new surface produced. Brittle materials relieve the strain energy mainly by crack propagation, whereas "tough" materials can relax strain energy without crack propagation but instead by the mechanism of plastic flow. Here the atoms or molecules slide over each other and energy is consumed in distorting the shape of the material. Crack propagation can also be inhibited by encounters with other cracks or by meeting crystal boundaries (Wills and Napier-Munn 2006).
Generally, grinding can be done by several mechanisms, including impact or compression, due to forces applied almost normally to the particle surface, chipping due
to oblique forces or abrasion due to forces acting parallel to the surfaces. These mechanisms will alter the particles and change their shape over certain limits which are determined by their degree of elasticity, which will cause them to break (Wills,1992).
The relative motion between media is responsible to a great extent for the grinding action. Ball media are entrained in a tumbling motion which engenders some interactions. During these interactions’, the media in the ball mill collide or roll over each other. Depending on the type and the magnitude of the interaction, particles break following a certain pattern.
King (2001) argued that in a ball mill, particle break primarily by impact or crushing and attrition. It seems however that the impact from the breakage is predominant at a more coarser particle size whilst attrition is the main reduction mechanism at much finer sizes. In between these two extremes, the breakage mechanism is composed of some combination of impact and abrasion. In this study, breakage is considered to be a result of the impact, abrasion, and attrition only.
2.3.1 Breakage by Impact or Compression
Breakage by impact in a ball mill occurs when forces are normally applied to the particle surface. It is also referred to as breakage by compression. King (2001) expounded this mechanism of fracture and showed that it encompasses shatter and cleavage. Fracture by cleavage occurs when the energy applied is just sufficient for the load comparatively few regions of the particle to the fracture point, and only a few particles result. The progeny size is comparatively close to the original particle size.
This type of fracture occurs under conditions of slow compression where the fracture at once relieves the loading on the particle.
When an irregular particle is broken by compression such as in crushing, the products will fall into two distinct size ranges either coarse particles resulting from the induced tensile failure or fines particles resulting from compressive failure near the points of loadings or by shear at projections. The amount of fines particles produced can be reduced by minimising the area of loading which is often done in compressive crushing machines by using corrugated crushing surfaces (Wills and Napier Munn, 2006).
Fracture by shatter, on the other hand, occurs when the applied energy is well more than that required for fracture. Under these conditions many areas in the particle are over-loaded and the result is a comparatively large number of particles with a wide spectrum of sizes. This occurs under conditions of rapid loading such as in a high- velocity impact (Kelly and Spottiswood, 1982). In an impact breakage mechanisms, due to the rapid loading, a particles experiences a higher average of stress while undergoing strain than is necessary to achieve simple fracture and tends to break apart rapidly, mainly by tensile failure. Many areas in the particles are overloaded and resulting in a comparative greater number of particles having wide size distribution. An impact breakage causes an immediate fracture with no residual stresses.
2.3.2 Breakage by Abrasion
Abrasion is seen as a surface phenomenon which takes place when two particles move parallel to their plane of contact. Small pieces of each particle are broken or torn out of the surface, leaving the parent particles largely intact. Abrasion fracture occurs when insufficient energy is applied to cause a significant fracture of the particle.
Abrasion breakage also occurs due to the particle – particle interaction which may occur when a mill is fed too fast causing the contacting particles thus will increase the degree
of compressive stress and hence shear failure. (Wills and Napier Munn, 2006, Rao, 2011). Rather, localized stressing occurs, and a small area is fractured to give a distribution of very fine particles (effectively localized shatter fracture) (Kelly and Spottiswood, 1982).
2.3.3 Breakage by Attrition
When a ball mill is performing at its low speed, grinding is a result of rubbing action within the ball mass and between the ball mass and the mill liners. The size reduction depends mainly on the surface areas of the media in interaction (Hukki, 1954).
This breakage mechanism is known as attrition. It is caused by the relative movement between powder and individual grinding media components in the mill. In the relative motion of particles and media, very small particles happen to be nipped between large balls or between large balls and mill liners. The rubbing together of the two media or of media and liners will result in the production of a significant number of very fine particles compared to the parent size. For that reason, it would be fair to assume that attrition is largely responsible for the breaking of particles that have become smaller than the voids between the grinding media and that the stresses induced in the particle nipped between the two media or between the media and the liners are not large enough to cause break (King, 2001).
a) b) c)
Figure 2.5 Types of particle breakage a) impact b) compression c) abrasion
2.4 Effect of Breakage Mechanism on Mineral Liberation
Usually valuable mineral are not free but commonly coexist with several other minerals known as gangue minerals in mineral processing. Mineral processing aims mainly on the separation between valuable minerals and gangue minerals. The valuable minerals must be substantially free in order to separate valuable minerals from the gangue by breaking the host rock into sizes that are small enough to be recovered by the subsequent separation process. The simplication of particle texture is also part of the size reduction. This process of simplified particle texture is reffered to as liberation.
(Gaudin, 1939).
The degree of liberation varies with the size of the final products. In order to separate valuable minerals from the gangue, it do not have to be fully liberated. Even poorly liberated, particles with an optimal floatation size may float more rapidly than liberated ground particlesand at the same time degree of liberaton, intermediate sized particles float faster than coarse particles, say Sutherland (1989). The results of his study therefore show that the floatation rates are not only influenced by size but also by liberation and the interaction between libeartion and particle size is present.
Therefore it is necessary to know how communition changes the composition distribution of the particle in order to optimize the mineral processing circuit and unit operations. The understanding of how the compositional distribution particles changes because of communition is crucial for understanding the ways particles broken within grinding machine. The breakage mechanism within the machine defines the resulting distribution of particle size of the products.
2.5 Factors Affecting Ball Mill Efficiencies
The efficiency of a ball mill is influence by several factors such as the size of grinding media, speed of mill, pulp density, hardness of the ore, and many other factors.
A number of variables influence their performance will be highlighted in following paragraphs.
2.5.1 Size of Grinding Media
The efficiency of grinding depends on the surface area of the grinding medium.
Grinding in a ball mill is effected by contacts between ball and ore particles. The angle of nip is important and ball sizes must be carefully chosen in relation to the largest and hardest particles in the feed. balls should be as small as possible and the charge should be graded such that the largest balls are just heavy enough to grind the largest and hardest particles in the feed. A seasoned charge will consist of a wide range of ball sizes and new balls added to the mill are usually of the largest size required. Undersize balls leave the mill with the ore product and can be removed by passing the discharge over screens. Jankovic (2003) reported that coarser grinding media were found to be more efficient for the coarser feed sizes but less effective on the finer feed sizes.
Various formulae have been proposed for the required ratio of ball size to ore size, however, none of which is entirely satisfactory; the practice of charging balls to a tumbling mill is a matter of experience as well (Concha et al, 1992). The capacity of a mill increases with decreasing ball diameter, due to the increase in grinding surface, to the point where the angle of nip between contacting balls and particles is exceeded.
2.5.2 Speed of Mill
The speed of mill is one of the most important variables to be considered in the experiment. It is essential to control the mill speed in an operating mill, because the speed affects motion behavior, power draw, product size, and liner/ball wear. It must be carefully optimized to obtain maximum collision energy. The energy input exerted on the material by ball milling will depends on how fast the ball mill rotates. It should be highlighted that the maximum speed of rotation is a great importance on particle liberation and the morphology of the material being milled. In this connection, there will be some disadvantage if the speed of mill lower or higher than the optimum value.
For instance, a very low rotational speeds leads to very long periods of milling and a large inhomogeneity in the alloy because of insufficient kinetic energy input. (P.R Soni, 1999).
At relatively lower mill speeds, the medium would roll down to the toe of the mill and attrition grinding takes place, thereby producing finer grinding, but increasing liner wear (Wills & Napier-Munn, 2006). At higher speeds, impact grinding mainly occurs as a result of the cataracting motion of grinding balls; hence it provides a coarser product with reduced liner wear. Increasing speed in an operating mill would generate more impact breakage, which normally enhances coarse rock breakage, resulting in higher throughputs. The amount of cascading motion also decreases and as a result, coarser product would be produced rather than finer one. The breakage rate at coarser particle sizes increases at higher speeds, while the breakage rate in the lower size range decreases.
At the critical speed of the mill, the centrifugal force generated is big enough to have the small particles, next to the shell liners, stuck to the shell liners for the complete
revolution of the mill (Rowland, 2002). The following formula is used for determining the critical speed (D in meters):
Nc =
42.305√𝐷 (1)
Where Nc is the critical speed of the mill, D is the mill diameter in meters. In practice, the magnitude of calculated critical speed is usually increased by approximately 20 percent. Generally, the mills can be operated at a mill speed of 70-80 percent of the critical speed.
Liddell and Moys (1988) reported that mill speed and filling both affected the position of ball charge in a laboratory mill. An increase in mill speed results in an increase in the measured torque until reaching a maximum point and after that it decreases sharply. Variations in mill speed up to about 80 percent of critical speed have not changed the toe position. The position changes at higher mill speed and hence, the lifting charges fall down onto the liners. The shoulder position depends upon speed and the filling.
At a constant mill speed, the measured torque or mill power has a parabolic relationship with mill filling, which is zero torque at mill fillings of 0 and 100 percent.
It is noteworthy that in order to avoid unwanted liner wear, the lifting charges should land on the toe of mill and does not impact the liner. At the toe of the charge, the majority of the grinding in the mill occurs and it has not just direct impact of the cataracting medium onto the charge, but also the shock generated is transferred to the ore part behaving in cascading manner. The medium and ore layer next to liners stick tighter on them than the rest of the charge.
Since ball mill operated at higher speeds than rod mills, larger balls cataract and impacts on the particles. So, the work input to a mill increases in proportion to the speed and ball mills work at high speed possible without centrifuging. Normally it is in 70 to 80 percent of the critical speed. The higher speeds usually being used to increase the amount of cataracting taking place in order to break down hard or coarse feeds.
2.5.3 Pulp Density
The pulp density of the feed entering ball mill must be as high as possible, consistent with the ease of flow through the mill. It is extremely important that the balls are coated with a layer of ore. A pulp will increases metal-to-metal contact, thus, giving increased steel consumption and reduced efficiency if too dilute. Ball mills should operate between 65 and 80 percent solids by weight, depending on the ore. Fine – grinding circuits may required lower pulp densities as the fineness of the particles increases with increase in viscosity of the pulp. A number of researchers (Shi and Napier-Munn, 2002; Kawatra and Eisele, 1988; Klimpel, 1982, 1983, 1984; Moys, 1989; Shi, 1994;) have been discussed on the major factors that affecting the pulp rheology and its effect on the grinding circuit. It was found that not only the the rheological type (Newtonian or non-Newtonian) would also affect ball milling performance rather than pulp density.
Previous study by Lux, J. and Clermont, B (2004) found that Pulp density has a large influence on the grinding efficiency. In this case, the optimum is in the region of 73.4 percent or 1.989 kg/l. If one goes below that, the higher dilution will ‘flush’ the fines out of the mill and reduce the overall residence time in the mill. This will result in a coarser grind with a slightly steeper particle size distribution curve. Increasing the
density will make the pulp more sticky and the ball charge expands. The balls are being coated and the grinding efficiency decreases.
2.5.4 Charge Volume
It is important to ensure that the mill is not underfilled or overfilled with the charge. Overloading of the charges can cause the accumulations of fine particles at the toe of the mill which leads to cushioning effect which roots the breakage due to absorbtion of the impact. When the rock load is low, the rate of breakage of the rock were reduced as the impact of ball-to-ball contact. This required the operator to compute the optimum quantity of each parameter to achieve the desired product size and to sustain the fixed output rate and at the same time will maximizing the energy eficiency.
The charge volume proposed is about 40 to 50 percent of the internal volume of the mill. Approximately 40 percent of this mill are empty space. (Wills, 2005). The energy input to a mill increases with the ball charge and reaches a maximum at a charge volume of approximately 50 percent but for several reasons 40 to 50 percent is rarely exceeded. In overflow mills, the volume of charge is about 40 percent. The optimum mill speed increases with the volume of charge as the increased wight of the charge reduced the amount of cataracting that taking place.
2.5.5 Circulating Load
A lower circulating load allows lower residence times and a finer discharge of the ball mill. The optimum circulating load therefore constitutes a good compromise between over-grinding and minimization of coarse particles at the mill discharge. This
means that matching the classifier cut size to the product size of the mill with maximum water split to overflow. (Wills 2005).
2.6 Gravity Concentrations by Shaking Tables
Concentrations are one of the functions of mineral treatment. The concentration can be defined simply as a mineral upgrade. This means increasing the fraction of the valuable material against the waste material. With a number of different process, this can be achieved. Successive stages of grinding are the features of many gravity separators. It is known that the performance of the gravity separator is based on particle size, density, and specific gravity.
Each gravity separator has an optimal size range, under specific operating conditions, which provides the highest separation rate for a certain mineral. In the past, exact data on the recovery by size have been produced which required samples to be screened and classified into size classes (typically for less than 45 μm particles). When sample classification is done using machinery such as a cyclosizer, particle density has a significant effect on the separation sizes which makes it harder to evaluate the results.
There are many types of gravity separators such as shaking tables, centrifugal jigs, conventional jigs and spirals.
The shaking tables are relatively old devices, but still play a major role in mineral processing. In general, they treat finer materials than jigs, but with lower capacity. Shaking tables remain used for 0 - 6 mm coal purification as well as to concentrate heavy non-sulfide minerals such as cassiterite, scheelite and gold. (M.
Tshazi 2016). Gravity concentration by using shaking tables is a more recommended technique as it is environmentally friendly because the only reagent used is water and applicable for recovery of cassiterite as cassiterite minerals having a specific gravity
ranging from 6.9 to 7.1 which are quite a large variation as compared to gangue minerals with a specific gravity of 2.6. (Wills 2005).
A particle’s specific gravity is describe as the proportion of the particle density to some reference or standard substance. Water is usually referred as the standard substance having a specific gravity of 1. Particles of less than 1 are floating on the water while the particles with more than 1 are sinking. Shaking table will separate the particles into products, middlings and tailing. The shaking tables are highly selective when used properly with a high upgrade ratio, and we can see the separation and make modifications or adjustments.
Shaking table is usually used in commercial mining but was not used by small miners because of its relatively high cost. (Mitchell, C.J., 1997). Moreover, it can only handle low feed capacity, but it requires large floor area and frequent operator attention in order to check and make adjustment as feed should be sized. Cassiterite recoveries could be upgraded by either tabling, washing, and magnetic or electrostatic separation.
The final product is practically pure cassiterite. On the contrary, cassiterite deposit can be reduced to an appropriate size by typical crushing and grinding process.
Gravity concentration by shaking tables can only perform effectively for ore at certain size ranges approximately between 105 µm to 600 µm. (Ismail, I., Md Muzayin, A., and Salmah, B. 2011)( Md Muzayin, A., and Nazwin, A. 2001). At the processing plant, the comminution must be carried out on the rock containing cassiterite in order to liberate the minerals and to enable its concentration by physical means. (Ismail, I., Md Muzayin, A., and Salmah, B. 2011)( Md Muzayin, A., Ismail, I., and Salmah, B.
2012). However, as the size of liberation may be below 105 µm and given that the separation process of the shaking tables is only suitable for mineral separation in a relatively coarser size range by using the same methods for separation of fine cassiterite
is quite challenging. To obtain good results and higher recoveries of final products, shaking tables are recommended to be fed by particles smaller than 2 mm for this process (Schubert 1989).
Moreover, Siqing Liu (2011) stated that fine grinding need to be conducted to get a high quality tin concentrate and a great loss of tin cannot be avoided when treating the ore by the gravity concentration. Based on the facts above, a two-stage separation, with a low-intensity shaking table was selected to test the ore. Sandy, A. H. (2004) suggest that maximising the recoveries is done by targeting recovery in a certain fraction of size or fraction with the highest value mineral loading. In this study, the -600 µm size range of sample was used for further separation tests. In the ground sample, 33.2 percent of SnO2 was distributed in the size range below 105 µm and this fine material also has to be treated. The performance of each process was measured through the grade and the recovery of SnO2. It was found that only those liberated and coarser SnO2 were able to be separated from their gangue minerals.
2.7 Working Principles of Shaking Tables
Shaking tables are consisting of a plate with riffles that are inclined at certain degree with the lower edge in the opposite of the feed side. An engine moves the plate back and forth. Longitudinally, the movements of the particles along a deck are caused by the mechanism of vibration which superimposes a slow forward stroke and rapid return. Wash water is introduced longitudinally along the feed side by a shower. The water relatively closed to the surface is slowed off by the friction of water on the surface when a flowing film of water flows over a flat, inclined surface causing the increase in velocity towards the surface. If the mineral particles enter the film, small particles will not move as rapidly as large ones because they are immersed in the slow – moving
section of the film. Particles having high specific gravity move much slower than light particles and thus the frequency and amplitude of this motion can be adjusted for the lateral displacement of material.
The water from water hoses is supplied to the table by cross – flow water where its intensity can be adjusted. This water streams creates flow of energy across the table causing the slimes which are very small and low – density particles to be suspended from the table. Since heavy particles are less affected by the flow of water, it tends to follow the back and forth direction of the tables compared to light particles which flow slightly to the middling and light section of the table.
In general, apart from slimes, small particles move farther lengthwise than coarser particles of the same density. This can lead to the overlapping of coarse dense and small light particles. To avoid this effect, it is crucial to create a narrow grain size fractions, before feeding the shaking table. For using shaking tables efficiently a proper calibration is mandatory. Also, the whole plate should be moistened by water and stay moistened during the whole process. The working fundamental of a shaking table can be seen in the simplified drawing below (Figure 2.6).
Figure 2.6 Schematic drawing of shaking table view
2.8 Stratification and Hindered Settling
Stratification due to the nearly horizontal action of the table deck and the flow of water is not the only mechanism at work on the table. There is some suggestion that hindered settling may also assist in the separation in some minor way. The stratification due to the shaking motion of the deck and flow of water is referred to as table stratification. Under this process, the small particles will segregate towards the bottom of the bed, behind the riffles, while the large particles collect towards the top.
The size of the stratification area however depends on the complexity of separation and the capacity to be processed. It should exceed one-third of the deck surface. The more difficult the separation, the greater area that is required to obtain a
Figure 2.7 Segregation action across the surface of a table
Figure 2.8 Segregation action profile for a riffle of a table
proper stratification. For example, the stratification area is large when separating frosted beans from saleable beans because there is little difference in weight. However, the stratification area become smaller when the rocks are remove from beans because there is a large difference in its weight.
Higher capacity also requires a larger stratification area. As soon as the material is stratified, the vibrating effect of the deck pushes the heavier layers in contact with the deck toward its high side. At the same time, the ligther layers which are at the top of the bed and do not touch the vibrating deck will float downhill toward the low side of the deck. As the material flows downhill from the feed end to the discharge end of the deck the vibrating action gradually converts the layers of vertical stratification to a horizontal separation. As the material reaches the discharge end of the deck, the separation is complete. Heavier materials must concentrated at the high side of the deck, light materials should be concentrated at the low side of the deck and intermediate materials will be in between.
For a mixture of mineral densities in the feed, there will be a mid-layer of particles where the large heavy and small light particles will overlap as indicated in Figure 2.7. As the cross flowing water flows over the riffles it can cause eddy currents to penetrate the mobile bed before rising to flow over the next riffle. This rising current of water can lift the finer particles to higher positions in the bed by a hindered settling type action and this can assist in the segregation of heavy and light minerals. This effect of hindered settling along any individual riffle is likely to be small but the cumulative effect along the entire series of riffles on the deck might be sufficient to effect the separation of the fine light particles away from the large heavy particles in the bed.
Hindered settling on a table is more effective if the particles in the feed are closely sized.
Classification of the table feed improves the performance of the table and increase the capacity.
The discharge from the gravity separator is a continuously graded product ranging from the heaviest particles to the lightest particles. In practice, however, this continuous grade is broken down into three products : (1) a heavy or acceptable product;
(2) a light or reject product; and (3) a small middling product which is fully separated.
In processing where rocks of other heavy trash are present a fourth product is sometimes separated consisting of rocks and some good product for further processing.
2.9 Operating Parameters of Shaking Tables
Some operating parameters which affect the operation of the shaking tables are the size of particle and its density, the shape of the particle, the design of the riffle, the shape of the deck, the flow of water and feed, the speed and stroke of the table and the deck slope. The effect of these variables are summarised in Table 2.1 (R.O. Burt, 1984).
The correct operation of the table has the middling fraction discharged at the diagonally opposite corner of the table to the feed. For any feed variation, the operating parameters are adjusted to maintain this separation point. The particle shape is not a major factor
Figure 2.9 Segregation of particles due to horizontal shaking motion
