Laboratory Grinding Tests

 

Laboratory grinding tests are performed basically to determine operating parameters required to establish the appropriate operating conditions of full scale mills. In this way, the metallurgist determines the grind size required for adequate liberation of valuable minerals. These tests provide information on gold liberation, sulphide – gangue mineral associations as well as sulphide – sulphide, sulphide – precious metals and precious metals-gangue associations. Other important information obtained is related to estimation of the grinding energy required to produce a particle fineness of primary grind.

A simple test series of three or four grind times followed by a standard rougher flotation stage will result in much useful information when the grind size versus gold recovery or other valuable metal is graphically graphed. Also, visual examination of the rougher tailing products using a microscope will help to reveal the extent of mineral losses due to inappropriate grinding. In general, laboratory grinding tests will help to reduce the ore particle size and liberate mineral species prior to ant testing program, determine optimum grind size – time relationships on ore samples to estimate the relative hardness of the ore or just work index determination.

The degree of liberation is determined by examining several particle size fractions of the ground ore using a microscope or any special device such Qemscan or MLA. In this way the metallurgist will have information on the modes of occurrence and level of locking of the valuable minerals. In some cases, when the metallurgist needs a preliminary and quick estimation of the liberation size, it can be determined from particle size vs. gold recovery curves. The recovery processes are gravity, flotation or cyanidation.

In order to determine the laboratory grind time for any specific size, the usual procedure is to plot the cumulative weight percent passing of several sizes versus the lab grinding time. The curve is obtained by plotting at least three points. More than three points generates a more reliable curve, which can be used to interpolate the values required for the testing program. The slurry density for laboratory grinding is usually in the range of 60% to 75% solids, depending on slurry viscosity and mineralogy of the material. the ground product is wet screened using a 38 µm sieve and the oversize is filtered, dried and dry screened from 600 µm through 38 µm. Since the wet screening is not perfect, the fines minus 38 µm will be added to the amount determined in wet screening. The weights of the several fractions are used to determine the particle size distribution.

Laboratory grinding equipment