Partial extraction of metals CHIM

In the CHIM method of prospecting a direct current is introduced into the ground by means of a current electrode and an element collection electrode. This facilitates the extraction and accumulation of ions of the metals in zones near electrodes. The subsequent analysis of the extracted metals yields information about their distribution in the rocks in the zone of investigation. 

To increase the speed of ion flow, a relatively large electric field is used (greater than, for example, that used in the CPC method, described below). Under these conditions, we can neglect the diffusion and convection components of current compared to the migration component. Then the density of current j + due to cations with number n, is  

The motion of metallic ions to the cathode and their accumulation on its surface are accompanied hy interaction of metallic ions with the products of the cathodic electrochemical reactions. In particular, discharge of hydrogen ions at the cathode leads to the accumulation of the hydroxyl ions in this region, according to the activities of the hydrogen ions [H ] and hydroxyl ions [OH ]  

In order to avoid this undesirable effect and to promote metal ion accumulation in the liquid phase, Ryss and Goldberg (1973) developed a special element-collector. This consists of a vessel containing a metallic electrode and a semi-permeable membrane, on one side of which is a solution of nitric acid (Fig. 2-17). The semi-permeable membrane prevents egress of the acid solution and allows ionic exchange between the element-collector and the surrounding environment. The acid neutralises the hydroxyl ions and thereby maintains the solubility of metal ions in the vicinity of the cathode. 

The acid in the element-collector dissociates according to the equation, 

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Ryss, Yu.S. and Goldberg, I.S., 1973. Method of partial extraction of metals (CHIM) for ore deposit prospecting. In Principles and Techniques of Exploration. Rotaprint VITR, N84 5-19 (in Russian). 

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