In many cases the grading of samples of slime, mill pulps and crusher products is performed in the reduction works, more frequently the assay department sends assayers for especial training and courses on grading tests. The most important samples to be sized are the mill feed, gold concentrate and mill tails. Whether they are taken by hand or by automatic samplers, it is common to cover eight hours shift and then three of each type are collected daily. It is also general practice for a confirmatory twenty-four hour sample both of the pulp and residue to be taken as well. In this way a total of eight slime samples have to be graded each day through 300, 150 and 75 microns sieves. All samples as received are moist and have to be dried either on hot plates or ovens. Each dried sample is thoroughly mixed on a rubber or plastic sheet and if necessary is reduced in quantity by passing it through a Jones Riffle Sampler. After final mixing an amount of 100-110 grams is measured out, preferably on a direct reading balance and is sized through 200 mm diameter, 50 mm deep, testing sieves.
The separation of the screen sizes can be conducted either with water or in the dry state. In the former procedure the 100 g sample is tipped into a bucket sieve with a 75 microns screen forming the base and washed with water to remove the minus 75 microns fraction. The plus 75 microns material remaining on the screen is removed, dried and put into the top screen of a nest of three testing sieves which are almost invariably 300, 150, and 75 microns. If desired, other sizes such as 212 or 106 microns may be interpolated. The first screen, 300 microns, is fitted with a lid and the last screen, 75 microns screen, fits into a bottom pan. The nest is then placed in a mechanical shaker which is automatically timed to operate for eight minutes. After the shaker has stopped, the mass of the contents of each screen is determined and recorded. Then, it will be determined the percentage of minus 75 microns.
In the dry method the 100-110 g sample is placed on the top of the 300 microns screen and after the lid has been fitted, the nest of screen is agitated in the shaker for ten minutes. The contents of each screen are removed and their mass determined. The mass of the contents of the bottom pan is seldom determined to obtain the percentage of minus 75 microns, as usually this aspect is calculated by difference as in the case of the wet procedure. The minus 75 microns material can be employed to determine the plus and minus 45 microns percentages if these are wanted.
When gold determinations of graded fractions are required, separate nests of screens are used for testing pulverized samples and residues. Also the screens are not employed for the grading of any other samples to avoid the possibility of gold contamination. The various fractions are retained in separate containers until sufficient of the smallest quantity (plus 300 microns) is available for assay. Each product is then assayed separately to determine its gold content.