The rising costs for and the possibility for limited availability of electrical energy are emphasizing the continual conflict faced in the selection of comminution circuits used in preparing ores for concentration, namely; capital cost vs. operating cost. Frequently, the circuit with the lowest capital cost is not the most efficient in the use of electrical power and wear resistant materials.

The simplest circuit in terms of flow, pieces of equipment and capital cost is the single stage autogenous or partial autogenous circuit used to grinding primary crusher discharge or run of mine ore to the desired product size. (Fig. 1) From this circuit, complexity increases to multi-stage crushing circuits preparing feed for multi-stage grinding circuits. Fig. 2 showing a three stage rod mill ball mill arrangement is an example of a complex grinding circuit. Stage concentration further complicates the flow. Multi-stage circuits being the most expensive to install, often give the greatest operating flexibility and offer the best possibilities for the most efficient use of available power and wear resistant materials.

Within available grinding circuits, other comparisons such as high speed vs. low speed mills, grate discharge vs. overflow discharge, metal liners vs. rubber liners, high charge vs. low charge, etc. all add to the conflict between capital cost and operating cost as partially reflected by the most efficient use of electrical power and wear resistant materials.

Bench scale and pilot plant scale test work, the cost for which must be within the scope of the project, can be used in establishing the following ore characteristics needed to make an evaluation of the various alternative circuits available.

- Autogenous media characteristics of the ore.
- Suitability of the ore for:

a) Single stage autogenous grinding

b) Single stage partial autogenous grinding

c) Two stage autogenous grinding

d) Primary autogenous grinding – ball milling

e) Primary partial autogenous grinding – ball milling - Crushing Impact Work Index
- Work indices – Rod and ball mill grindabilities
- Abrasive characteristics
- Presence of size fractions, with high and low resistance to comminution.
- Particle size distribution requirements for processing steps following comminution and suitability of products produced by comminution circuits for subsequent processing steps.

The current and probably on-going energy availability and cost crisis plus financial considerations demand obtaining the best efficiency from the comminution circuits selected. This calls for designing the comminution circuits so that; instrumentation, data collection and recording and even the use of computer controls can be selected to perform the required measuring and calculating functions.

In a comminution circuit, there are four variables that influence the utilization of the power available to comminute the ore.

Feed size

Product size

Resistance to breakage

Feed rate

Beyond these four are other factors, such as:

Breakage patterns

Pulp density

Moisture content in the ore

Clay or sticky component in the ore

that influence circuit efficiency, and cause variations in operational factors, such as mass flow in the circuit, circulating load and particle size distribution which when measured reflect changes in the four key variables.

The Bond Equation (2)

W = 10 Wi/√P – 10W/√F

where

W = KwH per short ton

Wi = Work Index

F = Size of the feed in micrometers which 80% passes

P = Size of the product in micrometers which 80% passes

encompasses the four key variables.

Using the Bond Equation in the following form (3)

Wio = W ÷ (10/√P – 10/√F)

permits computation of work indices from operating data, so as to differentiate this from work indices obtained from laboratory tests, this work index is designated as Wio.

By definition “work index is the power required to comminute a short ton of a material from an infinite feed size to 80% passing 100 micrometers.” Therefore, by definition, work indices calculated from operating data take the variations in the four key variables in the circuit performance and refer these to a common range of work.

Operating work indices can be used to:

- Record performance
- Compare current performance with past performance
- Compare circuits in multi-circuit plant
- Evaluate in-plant testing
- Measure efficiency of power utilization

Unless operating work indices are to be compared to work indices obtained from the standard Bond Laboratory procedures for impact and grindability testing, it is not necessary to calculate operating work indices on the basis of short-tons when plant feed data is measured in metric tonnes or long tons.

Circuit feed rate is obtainable by weighing and totalizing the feed going to each comminution stage. Using power measuring equipment, power delivered to the motor driving the comminution machine is obtainable. These two with operating time data gives the power per tonne data needed for the operating work index equation.

Either automatic samplers or manual sampling can be used to collect samples on a regular basis for determination of pulp density and screen analyses. From these, the 80% passing size for the feed and product can be determined.

With feed rate, power and screen analysis data, operating work indices can be calculated either manually or by computer. A computer can be programmed to take the following data:

- Total Feed
- Total Kilowatt Hours
- Operating Time
- Feed Size Analysis
- Product Size Analysis

and determine the following:

- Kilowatt Hours per Tonne
- 80% Passing Size in Micrometers for the Feed
- 80% Passing Size in Micrometers for the Product
- Operating Work Index

If the scope of the operation permits the inclusion of laboratory facilities in the mill to run standard tests such as:

- Bond Rod Mill Grindability Tests
- Bond Ball Mill Grindability Tests
- Bond Impact Crushing Tests
- Autogenous Media Competency Tests
- Screen Analyses
- Surface Area
- Abrasion Index

This will help obtain data needed to obtain the maximum utilization of the power delivered to the comminution circuit, which also generally means the optimum utilization of wear resistant materials.