Because of the particle size distribution of precious metals in used casting investment, it is mathematically impossible to obtain accurate assay numbers using conventional sampling and assaying techniques. This material contains large metallic particles, which cause the sampling problems known as "nugget effect". For this reason, methods that size the material before additional processing are far more accurate.

This study is based on mineral processing techniques in which the raw material is first screened into mesh sizes followed by a variety of other processes including gravity separation, grinding, chemical treatment, and smelting. Investment is treated in three categories: 1. Casting machine dirt and flask scrapings 2. Quench tank material 3. Wash tank material. Recovery figures are given for material tested at Vimar Corporation. However, these should only be regarded as preliminary. Because of the large number of variables involved, actual recoveries will probably vary widely from company to company.

The single most important finding is that most of the gold is contained in about 5% of the investment in the largest mesh sizes. Simple test procedures are given. Also discussed are methods of low cost recovery. This paper includes photos of the casting artifacts found in the investment.

Author: Dave Schneller