Artificial diamond used as raw material for tool implements is mostly synthesized by the flux method. The final products of the synthetic reaction contain various kinds of impurities, such as ceramic brick, unreacted graphite and fluxes of Ni and Fe. In operation plants, the impurities are removed by chemical means. Residual fluxes are removed by dissolving with aqua regia, unreacted graphite with perchloric acid, and ceramic brick by sodium hydroxide fusing. As such methods for removing impurities are expensive and harmful to the environment, it is essential to establish more economical, safer and more efficient methods; froth flotation being one example of such attempts.
As part of the studies on the applicability of flotation to the removal of the impurities from the synthesized diamond products, flotation behaviors of unreacted graphite and synthesized diamond in the presence of depressants were investigated. Firstly, a series of flotation tests was conducted on raw synthesized diamond in order to determine the optimum conditions for removing unreacted graphite using sodium silicate, sodium hexametaphosphate and sodium pyrophosphate as depressants. When kerosene or Aerofloat #25 was used as a collector, both sodium silicate and sodium pyrophosphate were effective to depress the floatability of the synthesized diamond, but the depressing effect of sodium hexametaphosphate was more inferior than the above two because of less hydrolysis. Secondly, to assess the efficacy of separation flotation of the 1 to 1 mixture of the unreacted graphite and synthesized diamond, a series of flotation tests was carried out using sodium pyrophosphate and sodium hexametaphosphate as depressants and kerosene as a collector. It was found that these depressants were effective in separating the unreacted graphite from the synthesized diamond in an alkaline region of around pH 9.