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.
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.