The Kori Kollo Project is located in Bolivia, approximately 220 km south of La Paz, and 40 km northwest of Oruro. It is owned and operated by Empresa Minera Inti Raymi S.A. (IRSA), an 85% owned subsidiary of Battle Mountain Gold Company. Following a feasibility study and preliminary engineering by others, Minproc was awarded a firm price contract to design and construct the 14,000 t/d sulfide ore process facility to produce gold and silver.

The comminution circuit design was developed and the equipment ordered during the preliminary phase, based on testwork consisting of:

• impact testing on 75 x 50 mm rocks,
• rod and ball mill Bond Work Index determinations,
• Bond abrasion tests,
• a MacPherson grindability test on a composite of the samples from the Bond tests.

Pendulum testwork provided ore-specific, high energy impact breakage data and tumble testing was done to obtain low energy abrasion data, both for use directly in the SAG mill model. Classification tests were run on ground ore in a 100 mm diameter cyclone to provide the necessary input to the cyclone model. The existing Bond information provided the basis on which the ball mill performance was predicted from the JKTech database.

The objectives of the work were to:

• confirm the capability of the already purchased mills to perform the design requirements,
• providean estimate of the potential ranges of circulating loads, to confirm the design of the materials handling components,
• identify the existence of circuit bottlenecks, if any,
• confirm and optimize the classification design.

Once the optimized base case model was constructed, a large number of simulations of all aspects of the circuit was performed, initially at JKTech, and subsequently in Minproc’s Denver office. The main conclusion was that the two design targets of cyclone overflow size distribution (P80 of 0.075 mm) and solids concentration (40% w/w) could not be attained simultaneously in the base case circuit, although each could be separately achieved under suitable conditions.

The plant commenced operations early in 1993, and has since consistently run at above design throughputs. The data from several circuit surveys has been used to update and improve the original plant simulation model, as well as to compare the original model predictions with circuit performance.

Significant changes have been made to the as-built SAG mill configuration and operating conditions, since the original design simulation.

These include:

• change in ball charge volume from 6% in the model to 4% and 2% actual,
• change in ball top size from 125 mm to 100 mm,
• change in grate size,
• addition of 2-inch pebble ports.

Changing the ball top size is a directly scaleable effect which can be predicted by the model. It does not have a significant effect on the breakage rate curve, but does affect the overall breakage “dynamics” of the model.

The SAG model allows for the effect of different ore types of varying impact and abrasion breakage characteristics. The ore assumed for the original design model and the ore treated during the surveys are assumed to be the same. It is possible that a softer ore than that used for pendulum testing was passing through the plant during the surveys.

The model-fit parameters are nearly identical for the 10/4/93 and the 10/8/93 surveys. These have similar operating conditions. Their similarity confirms the reliability of the survey data. However, to confirm the validity of the original model predictions, the ball mill/cyclone circuit models for 10/4/93 and 10/8/93 were modified by substituting in the original design model parameters.