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In any grinding circuit, there are but a few basic objectives:
- Reduce the size of the incoming feed to produce a final product.
- Reduce the size of the feed material to a size where valuable minerals can be liberated and recovered in a subsequent process.
- Reject much of the material as gangue to simplify further processing.
- Accomplish the above objectives at minimum cost.
The second and third objectives listed, apply to beneficiation of metallic ores while the other two are characteristic of grinding circuits in general.
Due to the complexity of a grinding circuit, it is difficult if not impossible for an operator to manually achieve the best results from a circuit. Variables within a circuit contribute to make measurement and control of certain parameters necessary to achieve optimum performance from the circuit. These variables include ore type changes, media loading, liner wear, pump dilution, classifier feed pulp density, reagent additions, water pressure changes, pump speed and wear, sump segregation, viscosity, product particle size, etc.
Within a grinding circuit there are a number of parameters that can be measured among these parameters are the following:
- Horsepower – can be determined from standard mill amp meter measurements. Horsepower is dependent upon mill speed, media load, ore specific gravity, pulp dilution and type and condition of liners.
- Sound from the mill – Amplitude of mill sound is measured with non relevant frequencies filtered out. Sound can be an indicator of mill loading.
- Throughput – can be measured with load cells on the mill feed belt. Throughput is measured as an indication of the quantity of final product produced. Also, it is a means of determining the amount lf makeup water to add to a wet grinding application.
- Cyclone feed sump level – can be measured using a bubble tube, ultrasonic measurement, nuclear absorptions capacitance or differential pressure measurements. Cyclone sump level is a good indicator of circulating load in a grinding circuit.
- Mass flow – can be determined by a density measurement from a magnetic flow meter. Mass flow can be used to regulate reagent additions and compute mass balances.
- pH – can be measured by standard electrodes. pH can effect the formation of colloidal suspensions from metallic hydroxides. Colloidal suspensions can alter density and have a significant effect upon classification.
- Ball mill and Rod mill head water – can be measured from the pressure drop across an orifice plate. Mill head water controls pulp density and affects grinding efficiency.
- Density – often measured by nuclear absorption. Density is significant in classification, flotation, and grinding mills.
- Particle size of the classifier product – can be determined by screen analysis, direct measurement, or inferential technique. Particle size is significant for liberation of valuable minerals and in establishing the proper trade off between throughput and recovery to assure best economic operation and minimizing overgrinding to curtail ore beneficiation costs.
Each of these parameters has its advantages and disadvantages from a standpoint of control and ultimate metallurgical and economic performance. Each parameter has a setpoint that can be maintained by measurement and control of variables within the grinding circuit.
The sophistication of a control system depends upon available capital and upon the return that can be realized from an investment in control equipment. Different control concepts are appropriate for different concentrators depending on mill design, capital available and the sensitivity of the subsequent processes to liberation. Following are some examples:
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