Category: Blog

Gold ore plant grinding media size
Several operations have experimented with progressively larger ball sizes in efforts to improve SAG throughput, particularly with hard, coarse ores. While the energy of individual impacts increases with larger balls, the number of impacts for a given load (or volume filling of ball charge) decreases. The optimum ball size for a given operation is a function of feed size, ore size, and competency, as well as mill operation conditions (speed, steel charge, and total mill volume). There is little consensus in this area. This may be a function of the difficulty in conducting and evaluating plant trials with ore of various feed characteristics (a relatively small performance benefit), or relate to the ore-specific nature of the optimum steel size.

Mill relines
No discussion of milling, and particularly SAG milling, would be complete without some mention of relining. Unlike a concentrator with multiple grinding lines, conducting SAG mill maintenance shuts down an entire concentrator, so there is rightly a tremendous focus on minimizing required maintenance time; reline time represents the majority of scheduled maintenance requirements.

Reline times are a function of the number of pieces to be changed, and the time required per piece. Advances in casting and development of progressively larger lining machines have allowed larger and larger liners. Like many operations, PTFI has devoted substantial resources to a liner development program to reduce the total number of liner pieces and fasteners.

SAG mill discharge classification
There are two primary mechanisms for classifying SAG mill discharge: screens and trommels. Due to a reduction in the required capital costs and space required for screens, trommels enjoyed a period of popularity, but the most favourable method of preparing SAG circuit oversize for pebble crushing is screening. All things being equal in terms of ore character and pulp density, screening produces a cleaner, drier oversize with much less fines carryover than does a trommel. This is particularly true for large SAG installations. As mill diameter increases, the volumetric throughput increases substantially faster than the effective screening area of a trommel. Trommel oversize like pebble-crusher feed has caused problems with pebble crushing at a number of operations. This is due to carryover of fines and moisture, which results in crusher chamber packing and ring bounce.

As most SAG circuits are now designed for the inclusion or subsequent addition of a pebble-crushing circuit, a design that incorporates screens allows maximum future flexibility. The method of returning the classified oversize to the mill requires mention. Some large SAG operations have been designed and built with water cannon return. With this system, the trommel oversize is returned directly to the mill with a water jet. Of course, the ability for these plants to retrofit pebble crushing into the circuits is significantly more complicated than if an external method of recycling oversize had been employed. It goes without saying that the oversize from the SAG mill classifier oversize must be returned to the mill via a belt system to use a pebble-crushing circuit. The use of screens is not without complications. Attaining even feed distribution to multiple screens can be challenging, the maintenance requirements of screens requires that stand-by units (either installed stand-by, for use in a rotating spare program, or both) be used.

Reflecting the importance of primary-crushing performance on the milling process, many operations have transferred primary crushers from the operational control of the mine to the mill. Regardless of who operates the equipment, primary crushing has two customers: serving the mine to maximize load and haul equipment utilization, and also serving the mill to maximize overall comminution efficiency. Many mines select primary crushers based on top size of the designed or anticipated run-of-mine (ROM) ore. This can result in a substantial excess of primary-crushing capacity for smaller operations.

Frequently for large operations, though, primary-crushing capacity becomes an issue not only for milling operation (in terms of capacity and unit power input), but also for mine productivity. As primary crushing becomes taxed, issues that are conventionally dealt with in secondary, tertiary, or pebble-crushing roles become more critical. Management of gap setting,power draw, predicting and extending liner life within a specified performance envelope and productivity issues all become important to the overall comminution process. As with other crushing operations, employing stockpiles(or ore pockets), reclaim, or bins can maximize equipment productivity and efficiency. Most large gyratory installations are designed to accept direct dumping. While this minimizes the capital of the primary-crusher installation, it is inevitable that there is a trade-off with reduced primary-crusher utilization and crusher efficiency. This is a result of feeding the primary crushers based on the frequency that haul trucks present themselves to the crusher, instead of based on optimum crusher operation. 

Management of the mill stockpile is critical to maintaining consistent throughput. Most operations reclaim from a stockpile of primary-crushed material. There is generally some degree of stockpile segregation, with coarse material preferentially accumulating towards the outside of stockpiles. Maintaining a live stockpile and balancing multiple reclaim feeders result in the highest average (and most stable) throughput. Maintaining the stockpile at reasonable levels minimizes the effect of load and haul equipment shift changes on downstream operations. The size of the stockpile should be based on anticipated fluctuations in production of primary-crushed material as a result of primary-crusher maintenance, load and haul asset maintenance, mill maintenance downtime requirements, and normal fluctuations due to mine planning and sequencing. Natural stockpile segregation can also offer opportunities to improve overall circuit operation, either via balancing the SAG mill and ball mill circuits, or in cases, by preferential milling of ore from different feeders.

1. Solids feed control

Most gold plants have some potential problems with the control of solids feeds. These can include the adverse effects of ore segregation, non-linear or erratic actuator dynamics (especially vibratory feeders), moisture content, conveyor-belt dynamics and blockages. All of these issues are readily solved by appropriate advanced technologies that produce the best possible results. When there are multiple withdrawal points from an ore silo, blending can be maximized, or sometimes segregation can be exploited to optimize short- to medium-term grinding conditions. Invariably, a fast and noise-free response of the actual solids feed rate to its setpoint is required for good higher-level control, e.g. for the control mill load.

2. Crushing stage control
Crushers have relatively fast dynamics. Screens, too, have quite fast dynamics. As a result, these two processes can be essentially regarded as instantaneously responding units, connected by conveyor belts to other units or solids storage bins. Control can be applied in a strategy that allows particularly for the dynamics of conveyor belts and for constrained control and optimization. Attention should also be given to start-up and shutdown sequences and their control.

3. Mill circuit control
In the early 1980s, successful multivariable control of industrial milling brought recognition to the fact that milling circuits have complex interactive dynamics that can be modelled and controlled very well by advanced methods. For operation away from the influences of process limits, good multivariable control is easily implemented by many of the advanced methods that are now standard. Because the optimum operation of milling circuits is almost invariably near process limits, optimum control requires a multivariable controller that has switching control strategies, for use when various combinations of process inputs and outputs change between being constrained and unconstrained. Model predictive control (MPC) is a good general technology for this, but it is complex, cumbersome and less reliable in its general form, so specialist adaptation should be applied to exploit specific characteristics of milling circuits. A switching control strategy has been developed to allow for easy specification and tuning. It provides for controlling process outputs to setpoints, minima or maxima, or combinations of these. A typical list of setpoints for a simple closed milling circuit would be setpoints for sump level, product size, circulating load density or underflow angle and cyclone feed density or flow. Minima and maxima could be set for mill power, sump level, product size and cyclone feed density. Priorities can be set for the various setpoints and limits for when not all of them can be satisfied. Besides handling the process limits properly, the controller should optimize the operation of a milling circuit while controlling product size to a setpoint. The setpoint used for product size itself might need to be adjusted to give a required long-term solids throughput. In autogenous milling, conditions inside the mill might need to be made such that preferential grinding of either the fine material or the coarse ore takes place.

4. Thickener control
The most common process controllers used in thickening are those for thickener underflow density and flocculant addition. The thickener underflow density is controlled to a setpoint by a variable-speed pump. The setpoint can be manipulated in a plant-wide optimization scheme that minimizes gold losses and prevents any undesirable bottlenecks. The flocculent is usually added in ratio to the flow of solids to the thickener. If unknown, this flow of solids can sometimes be estimated from a smart sensor based on measurements of an upstream milling circuit.

5. Carbon-in-pulp and carbon-in-leach control
Carbon-in-pulp (CIP) and carbon-in-leach (CIL) are widely used for the extraction of gold by the use of cyanide. Pulp levels are easily controlled, usually mechanically, by means of overflow weirs. Carbon transfer is done in batches or, in some cases, continuously. The long time constants in CIP and CIL plants make it possible for carbon-related measurements from hand samples to be used. However, cyanide concentrations are best determined and controlled automatically and with minimum measurement delays, as they reflect faster process variations and disturbances, and these need to be reacted to quickly. The carbon and cyanide inventories should be optimized to give the optimum financial performance of the plant as a whole. The associated index of performance should include the income from gold recovered minus the costs of cyanide, carbon and any other reagents used.

6. Flotation control
A flotation plant should have advanced stabilizing control of its pulp levels if it has three or more flotation banks or columns operating in a cascade. This is because conventional PI control operates poorly on the related third- or higher-order dynamics of flotation plants and simply transmits disturbances between stages instead of eliminating them. Where there are cleaner flotation stages, flow rates and residence times
should also be optimized dynamically, while the levels are being controlled.

Some of the most important variables measured on gold plants are as follows

1.Solids flow on a conveyor belt

Nuclear meters and weightometers are standard and in common use for measuring solids flows on conveyor belts. Weightometers are often more accurate and give less ‘noisy’ signals. Nuclear meters are non-mechanical and can be more reliable and less costly. The calibration of these meters is typically done or checked by belt cuts, in conjunction with measurements of belt speeds. The positioning of the meters on conveyor belts is important, because the delay while the solids are on the belt can have an adverse impact on process control. The dynamics of feed belts can result in poor control ability of the solids feed, and the need for advanced control methods for good control.

2. Water flowrate

Magnetic-induction flow meters are the most common and reliable means of measuring the flow of plant water. Most of the meters are suitable only for cleaner waters, because of the problems of scale build-ups.

3. Slurry flowrate

Magnetic-induction flow meters are the most common and reliable means of measuring the flow of slurries. Calibration of these meters is often done by the physical measurement of water flows where this is possible. Sometimes this calibration is done off-site. If just trends of flow and not absolute values are important, measurements of pressure – e.g. at the feed to a hydrocyclone– can sometimes be used as a less costly substitute for a magnetic-induction flow meter. Pressure and flow usually correlate very well.

4. Density of slurry in a pipe

Slurry density is generally measured by a nuclear meter. It is important for it to be installed on a vertical pipe, and that the normal slurry flow through the pipe should be above 2 m/s, to minimize the adverse effects of settling. For the purposes of calibrating the meter, the pipe should conveniently be able to be filled with bubble-free water (e.g. there should be an isolating valve somewhere below the meter). A calibration can be done by

a. noting themeter reading with water in the pipe;

b. obtaining the same reading with air in the pipe and inserting lead plates of appropriate thickness in the radiation path; and

c. noting the reading (corresponding to a density of 2,000 kg/m3) with the lead plates still in place and water in the pipe too.

5. Mill power

Mill power is a good indication of the rate of grinding in a mill. Measurements of mill power generally contain unusually large components of noise. Their use is therefore often unnecessarily constrained to quite long term control strategies. Actually, advanced electrical and digital processing of the signals from mill power meters can eliminate unwanted noise, leaving a fast-responding signal with good process information for control.

6. Slurry levels

Where there is little or no froth present, direct ultrasonic meters are often used reliably for level measurements. Problems can arise beyond extremes of measurement ranges if the meter gives unexpected outputs. Ultrasonic meters are also used quite commonly for flotation cells, but with floats and connected platforms located above the froth to reflect the ultrasound.

7. Angle of hydrocyclone underflow spray

A hydrocyclone fulfils two important functions: separating fine from coarse particles and producing an underflow slurry of a consistency that induces efficient grinding in the mill to which the slurry is recycled. Generally, the most efficient operation of a standard closed milling circuit requires the underflow of the hydrocyclone to have a narrow-angle underflow spray nearly at the point of roping, but not actually roping. An ultrasonic meter for measuring this angle has been used successfully in industry for control and optimization.

8. Particle size of milled product

Industrial online size measuring devices operate on principles such as ultrasound attenuation, laser diffraction or imaging and physical probing. These systems can give good results if installed, calibrated and maintained well. Their accuracy and reliability are very dependent on having representative and well-maintained sampling systems. Soft sensors that use flows, densities and other measurements can sometimes give accurate and more reliable derived measurements of size, but they do require regular calibrations. These soft sensors also eliminate the time lag between real and measured size changes that result with the physical meters. This time lag can be a severe limitation to fast and effective control. The best system uses a combination of both a soft sensor and a physical meter, with the use of the best features of both.

9. Grade

Online measurements of gold concentrations in various process streams would be good to have for control, but are generally not practicably possible for gold. This is especially true in respect of solids in tailings streams, where the measurement of grade would be most useful.

10. Cyanide concentration

Titration and electrochemical methods have been applied with success to measure cyanide concentrations in absorption and leaching vessels. They can give good indications of the leaching strength of solutions that contain multiple cyanide complexes, because they tend to measure the concentrations of those complexes that tend to be available for leaching gold.