Table of Contents
The SAG milling of ores with markedly variable grinding rates presents definite process control challenges. Decisions must be made regarding the selection of the most appropriate combination of mill volumetric load level, percent solids, and mill speed for the combination of ore types being processed at the moment.
Circuit Description
Chino’s concentrator is described in a paper presented at the 1983 meeting of the Arizona AIME Section. Each of the 28 x 11.5-ft variable speed SAG mills is in closed circuit with a vibrating screen. SAG mill speed may be varied automatically between 65 and 73% of critical by the use of a liquid rheostat arrangement. Measured variables include new feed rate recirculating load, feed and recycle particle size distribution, process water flows, trunnion bearing pressure and motor power draw. All measurements are transmitted to a centralized control room.
One conclusion which may readily be drawn from the data is that control strategies which are based on controlling SAG mill operation at specific power draw or bearing pressure levels will be faced with some major difficulties. One problem involves the selection of the most appropriate power or bearing pressure value at which to operate. Since process conditions vary so widely, no single best operating condition exists. R
Sag Mill Process Control Challenges
To date, however, the major emphasis has definitely been oriented toward process stabilization activities utilizing SAG mill power or bearing pressure as opposed to true adaptive or optimization attempts. There are two major reasons for this. The first of these relates to the complexity of the SAG process itself. Multiple grinding mechanisms are at work in a SAG mill with size reduction occurring due to impact, by attrition, and by a nipping action between larger rocks and balls.
The second major factor which has impeded the development of more advanced and effective SAG mill control procedures is the lack of available instrumentation to measure important SAG mill process variables. For example, the volumetric load level within a SAG mill is perhaps the single most important factor which characterizes the mill’s operation.
Effects of Major Process Variables on SAG Performance
Identification of the most economical SAG mill operating conditions requires knowledge of the response of the SAG mill to variations in control parameters such as mill discharge percent solids, SAG speed, and volumetric load level, and to the effects of SAG feed size and ore hardness changes on SAG operation. These effects were investigated using a combination of historical data analysis and in-plant testing. Results from these analyses are presented below.
SAG mill water addition, as characterized by the percent solids content of the mill discharge, exerts a very strong influence on SAG mill performance. Each line of constant volumetric loading is made up of data of similar ore type and SAG ball charge levels.
The beneficial effects of increased SAG mill discharge percent solids on SAG mill throughput may be observed along all lines of constant volumetric loading. One explanation for the increase in throughput under hard ore conditions, with increasing discharge density is related to the increased ability of the mill charge to suspend more and larger particles as the discharge density increases.
The variable speed drives on the SAG mills have proven to be useful tools for compensating for the large ore hardness swings associated with the Chino ore body. When ore conditions are soft, the SAG mills tend toward grind-out conditions.
The volumetric level at which the SAG mill is operated is a very important control parameter strongly affecting grinding rates and transport action within the SAG mill. Unfortunately, the volumetric load level is not directly measurable via instrumentation, and inferential measures of volumetric loading such as SAG mill bearing pressure or power draw must be utilized.
SAG mill stabilizing and optimizing process control activities are subjected to many challenges when ore types possessing markedly different grinding characteristics must be processed. These challenges result from the wide variations in SAG performance associated with differing ore types and to the different grinding mechanisms at work within the SAG mill.
