Several economic comparisons have been made between carbon systems and Merrill-Crowe systems. One of these, a systematic comparison completed by DeMent and King (1982) is summarized below.
Five separate conventional mills involving both Merrill-Crowe and the CIP processes were compared from an economic viewpoint. The gold concentration on the feed leach liquor ranged from 0.428 to 0.054 oz of gold per ton (14.6 to 1.8 g/mt) of solution.
Capital costs at the front end of a CIP plant are based on incoming flow rate, and are not a function of the gold grade. However, the capital costs of carbon stripping recovery portions do vary, as they are dependent on the grade of gold in the solution. Carbon regeneration is also dependent on the amount of carbon processed. On the other hand, with a Merrill-Crowe process, the total capital cost is solely a function of the initial flow rate.
Operating costs vary in the CIP plant according to the grade of gold in the leach liquor. This cost does not vary with the Merrill-Crowe except in the case of zinc usage which increases with higher input concentrations.
Gold losses in conventional CIP plants result from pregnant values which are not adsorbed onto carbon and are therefore discarded to tails. Another gold loss occurs whenever gold is loaded onto very fine carbon particles. This fine carbon passes all screens and also ends up in tailings. The main gold loss in a Merrill-Crowe circuit has a much different source (i.e., it results from incomplete washing of the filter cake on the precipitate presses).
DeMent and King’s results are based on estimated costs of a CIP and Merrill-Crowe plant, bearing in mind that costs are dependent on the particular characteristics of each ore. However, it seems pertinent to point out that within one operation using Merrill-Crowe precipitation, the author’s personal experience showed that for a lower grade leach liquor, Merrill-Crowe was selected. In the plant in question, the decision was made to use zinc precipitation based on the varying grade of pregnant liquor (from 0.075 ounces per ton to 0.010 ounces per ton (2.6 to 0.3 g/mt) gold). As operating costs were negligibly affected by this variation, the design decision proved profitable.
It is worth noting that operations do exist which combine carbon adsorption with zinc precipitation. Usually a zinc precipitation plant is followed by a CIC set up. This is done to enhance the removal of gold from barren solution coming from the zinc circuit. Very frequently, if no clear distinction is evident as to whether zinc or carbon is the best choice, the final decision is based on the particular experience of the project personnel who will be operating the proposed plant.