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Comparison of ore hardness measurements for grinding mill design for the Tenke Project

J. Starkey

August 19,2009

As part of the ongoing project development for the Tenke Fungurume Copper-Cobalt Project in the Democratic Republic of Congo, Phelps Dodge Corporation (now FMI) has examined methodologies for determining the size of grinding mills to support a 7,000 mtpd operation. Six samples were tested by Hazen Research, Inc. to obtain the JK parameters using the JK Drop Weight Test, standard Bond crushing, and rod mill work indices, abrasion indices, and by Dawson Metallurgical Laboratories, Inc. for Bond Ball Mill Work Index tests using crushed feed, and SAGDesign Tests, patented by Outotec.

The comparison of these six results gives con text to how the various measurements relate to each other and how they can be used to obtain an accurate design for the grinding mills required for the Tenke Project. This is the first published direct comparison between JK Drop Weight, standard Bond work indices, and SAGDesign test results. The SAGDesign test includes a SAG test followed by a Bond BM Wi test done on SAG round material.

The test work was done in a normal and standard manner at each of the laboratories noted above. In addition, unconfined compressive strength measurements were made at the Advanced Terra Testing, Inc. laboratories and specific gravity measurements of the six samples was done at Terra, Hazen, and Dawson (as part of the SAGDesign tests). The following graph (Figure 3) shows the relative position on the SAGDesign database, for the Tenke samples. They were soft, with the majority of them falling at or below the 10th percentile of hardness variability and the hardest, at the 15th percentile. Similar trends were observed for the standard crushed Bond Wi, the SAGDesign Bond Wi on SAG ground ore and the JK Tech database. In short, each of the methods used correctly identified the hardness level when compared to previous measurements made on other ores using the same test method.

The SAGDesign test data was judged to give the same relative hardness as any of the other methods tested. Since the reproducibility of the SAGDesign tests is excellent, these results were used for the mill design. SAGDesign results have the added advantage of giving the required pinion energy to grind the ore to 80% passing 1.7 mm (SAG test) and the energy to grind from T80 1.7 mm to 80% passing 200 microns, the selected grind. Hardest measurements were used to allow any blend of feed to be treated at design t/h. A total of 9 kWh/t of grinding energy was indicated and this could be provided either in a single stage SAG mill to produce the P80 200 micron product, or a two stage circuit with equal power on the SAG and ball mills using the design Bond Wi of 11.5 kWh/t.

A number of conclusions were drawn from this work. Some are presented here. The SAG part of the SAG Design test measures much more than impact breakage energy. It measures the design energy needed to grind from F80 152 mm to T80 1.7 mm.
The lack of reproducibility in many comminution test measurements is a problem that demands many more measurements be made for good design. The best and most cost
effective way is to use the most accurate test. That test is the SAGDesign test.

High testwork costs and lack of samples for testing have, and will in the future discourage the use of more samples and ‘usual’ grinding tests, especially for small mining companies. The use of Bond equations to calculate pinion energies in ranges where the mill feed size exceeds normal rod mill feed is a mistake and leads to lower than required SAG pinion energy. Fred Bond did not envision the use of these equations for other sizes than he had data to support.

It is important that we all understand the fundamentals of SAG mill design. Otherwise mistakes will continue to happen.