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Introduction to Session 1
Co pyright
© IFA t: Ap pli ~d
~!~aSllrel1l~1l1S
SESSION I
in Mineral and ;Vletallurgical Pro(e~sil1g.
Trallsvaal. S. Africa 1988
INTRODUCTION TO SESSION 1 G. Sommer ,\/mSllrl'lII('n/ (JIld Co ntrol Division, Counril Jin iVIin Pro/ Tl'rhn%,I.,r;,·, Prim/I' Bag X3 J05 , Rand/n/lg 2125, SUII/h AJi-iw
One of the highest concentrations of hardrock milling circuits in the world is found in the gold-mining area of the Transvaal and orange Free State. Approxi~ately 100 Mt of gold-bearing ore is milled annually, for which the consumption of electricity is more than 3000 million kilowatt hours. If the milling circuits operating in the other South African mineral-processing operations were to be included, these figures would be almost doubled. The installed cost of a conventional milling circuit of lOOt per hour is about 4 million rands, and therefore a high availability at maximum throughput is desired so that the best use can be made of the capital employed. Because of the sheer magnitude of these figurs, it is important to examine the basic reasons for milling. For any relatively impervious ore, milling is necessary to reduce the size of the particles, and thus to expose a sufficiently large surface area of the valuable mineral to the lixiviant. Hence, the more finely the valuable mineral is disseminated throughout the gangue, the smaller is the required particle size range. In the south African gold-mining industry, the target particle-size distribution of the milling operation for optimum gold recovery is gene rally about 80 per cent smaller than 75um. It is assumed that such a single point measurement is related to the full size distribution of the particles in the pulp. Finer milling is considered to be uneconomical because, although it exposes more of the gold for dissolution, it produces an increased proportion of fine sI imes, which have a deleterious effect on filtration and thickening, and also leads to increased losses of the gold in downstream processing. In addi tion, the power requirements for finer grinding are higher, and so is the consumption of the grinding medium. A typical relationship between profitability (as distinct from gold recovery) and fineness of grind in a typical milling - cyanidation circuit is represented in Figure 1, but the specific optimum grind will depend on the ore, price of gold, etc. In the milling of South African platinum ore, the particle size of the product is even more critical since the recovery of platinum by flotation is more sensitive to particle-size distribution than is the recovery of gold by cyanidation - material that is too fine or too coarse will not float properly. The curve in Figure 1 shows that, for a gold recovery plant to operate at optimum profitability, the particle size of the milled product must be controlled within
;r-----
Prof itabilit y
1
I
Optimum product·size distribution
t4"-
I
80 %
< 75~m
I 70
80 Fi neness of grind , %
90
< 75IJ.m
FIGURE 1. A typical relationship between profitability and finene ss of gr ind in milling
fairly narrow limits, and the availability of the milling circuit at maximum throughput must be high. The configuration of milling circuits varies considerably from mine to mine. In consequence, each circuit can be regarded as a unique entity. However, except in run-ofmine milling operations, the last stage in the circuit generally consists of one or more large pebble or ball mills, which are fed with water and crushed or partially milled ore. The mill discharge is pumped to a hydrocyclone, the underflow from which is recycled to the mill and the overflow sent to thickening and leaching. As the grinding medium in a mill is worn away, it is replenished with small quantities of new grinding medium (balls or pebbles) , which are fed to the mill at intervals. The following four papers deal with the measurements and some of the subsequent control strategies in milling operations. Or Moys will deal with four measurement techniques which can be used to provide a better insight into the milling operation. Or Vermeulen will present his interpretation of the signal obtained from a conductivity bolt. Or Moys will then present Or J .A. Herbst I s paper on the smart sensor developed for the control of media charging into the mills. Finally in this first session of the Workshop the presentation of Mr Lange which deals with an important measurement in the run-of-mine milling operation, namely to provide advanced information on the feed size distribution to such a circuit. Run-of-Mine (ROM) milling differs from conventional milling in that the ore fed to the circuit comes directly from the mining operations, i.e. without pre-treatment by a crusher plant. The larger pebbles and rocks in the feed to the ROM mill form the main medium by which grinding is achieved. The measurement of the rock-size distribution could not only be used in the control of a milling circuit but also in the development of the most appropriate mining and blasting strategies.
© IFA t: Ap pli ~d
~!~aSllrel1l~1l1S
SESSION I
in Mineral and ;Vletallurgical Pro(e~sil1g.
Trallsvaal. S. Africa 1988
INTRODUCTION TO SESSION 1 G. Sommer ,\/mSllrl'lII('n/ (JIld Co ntrol Division, Counril Jin iVIin Pro/ Tl'rhn%,I.,r;,·, Prim/I' Bag X3 J05 , Rand/n/lg 2125, SUII/h AJi-iw
One of the highest concentrations of hardrock milling circuits in the world is found in the gold-mining area of the Transvaal and orange Free State. Approxi~ately 100 Mt of gold-bearing ore is milled annually, for which the consumption of electricity is more than 3000 million kilowatt hours. If the milling circuits operating in the other South African mineral-processing operations were to be included, these figures would be almost doubled. The installed cost of a conventional milling circuit of lOOt per hour is about 4 million rands, and therefore a high availability at maximum throughput is desired so that the best use can be made of the capital employed. Because of the sheer magnitude of these figurs, it is important to examine the basic reasons for milling. For any relatively impervious ore, milling is necessary to reduce the size of the particles, and thus to expose a sufficiently large surface area of the valuable mineral to the lixiviant. Hence, the more finely the valuable mineral is disseminated throughout the gangue, the smaller is the required particle size range. In the south African gold-mining industry, the target particle-size distribution of the milling operation for optimum gold recovery is gene rally about 80 per cent smaller than 75um. It is assumed that such a single point measurement is related to the full size distribution of the particles in the pulp. Finer milling is considered to be uneconomical because, although it exposes more of the gold for dissolution, it produces an increased proportion of fine sI imes, which have a deleterious effect on filtration and thickening, and also leads to increased losses of the gold in downstream processing. In addi tion, the power requirements for finer grinding are higher, and so is the consumption of the grinding medium. A typical relationship between profitability (as distinct from gold recovery) and fineness of grind in a typical milling - cyanidation circuit is represented in Figure 1, but the specific optimum grind will depend on the ore, price of gold, etc. In the milling of South African platinum ore, the particle size of the product is even more critical since the recovery of platinum by flotation is more sensitive to particle-size distribution than is the recovery of gold by cyanidation - material that is too fine or too coarse will not float properly. The curve in Figure 1 shows that, for a gold recovery plant to operate at optimum profitability, the particle size of the milled product must be controlled within
;r-----
Prof itabilit y
1
I
Optimum product·size distribution
t4"-
I
80 %
< 75~m
I 70
80 Fi neness of grind , %
90
< 75IJ.m
FIGURE 1. A typical relationship between profitability and finene ss of gr ind in milling
fairly narrow limits, and the availability of the milling circuit at maximum throughput must be high. The configuration of milling circuits varies considerably from mine to mine. In consequence, each circuit can be regarded as a unique entity. However, except in run-ofmine milling operations, the last stage in the circuit generally consists of one or more large pebble or ball mills, which are fed with water and crushed or partially milled ore. The mill discharge is pumped to a hydrocyclone, the underflow from which is recycled to the mill and the overflow sent to thickening and leaching. As the grinding medium in a mill is worn away, it is replenished with small quantities of new grinding medium (balls or pebbles) , which are fed to the mill at intervals. The following four papers deal with the measurements and some of the subsequent control strategies in milling operations. Or Moys will deal with four measurement techniques which can be used to provide a better insight into the milling operation. Or Vermeulen will present his interpretation of the signal obtained from a conductivity bolt. Or Moys will then present Or J .A. Herbst I s paper on the smart sensor developed for the control of media charging into the mills. Finally in this first session of the Workshop the presentation of Mr Lange which deals with an important measurement in the run-of-mine milling operation, namely to provide advanced information on the feed size distribution to such a circuit. Run-of-Mine (ROM) milling differs from conventional milling in that the ore fed to the circuit comes directly from the mining operations, i.e. without pre-treatment by a crusher plant. The larger pebbles and rocks in the feed to the ROM mill form the main medium by which grinding is achieved. The measurement of the rock-size distribution could not only be used in the control of a milling circuit but also in the development of the most appropriate mining and blasting strategies.