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Copper Electroplating

Reticle Carbon, which is conductive, monolithic (rigid), ultra high surface area electrode material, is the optimum choice for capacitive deionization (CDI) to remove ions from aqueous solution and thereby purify water. Reticle has been able to extend CDI to electrochemical processes to directly electroplate copper from (dilute) pregnant leach solution without any requirement for preconcentration such as solvent extraction (SX) or any requirement for subsequent energy-squandering electrowinning (EX). Reticle Carbon aspires to displace SX and EW by a simple electrochemical cell lined with ultra high surface area Reticle Carbon and in so doing reduce by one-half the present cost of copper electroplating (as compared with SX-EW).

Present "Best in Class" (SX-EW)

Todayís typical copper processing circuit is comprised of the four steps illustrated in Figure 1. In the leftmost step, copper minerals are leached (dissolved) by a sulfuric acid solution to create pregnant leach solutions (PLS). Solvent Extraction (SX)-Electrowinning (EW) for Copper ElectroplatingTypically, PLS are fairly dilute solutions (1-2 g/L Cu+2) that cannot be directly processed economically. Such dilute solutions are mixed with a solvent (i.e., LIX 64, etc.), which selectively collects the copper from the leach solution to create a "loaded organic." The "loaded organic" is then stripped using a concentrated solution of copper (approx. 35-40 g/L Cu+2) and sulfuric acid (approx. 130-150 g/L H2SO4), which exchanges the copper in the solvent for protons (H+) from the acid and thereby concentrates the strip solution to 40-50g/L Cu+2 with approx. 125 g/L sulfuric acid. Figure 1 illustrates the leach-SX-stripping process that creates concentrated copper solution.

The concentrated solution is then sent to an electrochemical cell (summarized in Figure 2), which contains 60 cathodes and 61 anodes in which the copper is removed by direct reduction (electrowinning) onto the cathodes and simultaneously oxygen is generated by the oxidation of water at the anode and acid is a byproduct of the anodic oxidation reaction. Notice that the overall potential applied to the cell must be very high (2.5V or more) in order to get EW to proceed. Importantly, absolutely huge current densities are needed to maintain a copper production rate that renders pure and uniform cathode plates.Electrowinning Cathodic and Anodic Reactions

For years, the copper production industry has focused on improving cathode efficiency and largely ignored anodic reactions, primarily seeking to increase cathodic surface area by using such materials as steel wool and other high surface area materials. These methods have all come up short, e.g., wool cathodes have only found use in gold recovery (a pound of gold is worth a lot more than a pound of steel wool).

Reticle Focuses on the Anodic Reactions

Only limited anodic work has been done to date. In particular, it has been noticed that addition of Fe+2 to the electrolyte yielded a slight improvement at the anode because the oxidation of Fe+2 to Fe+3 is a much lower energy parasite than generation of O2 gas. Unfortunately, however, the presence of Fe+3 at the cathode reduces the cathode efficiency because Fe+3 will reduce to Fe+2 and thereby be parasitic to the desired Cu+2 to Cu0 reduction.

Reticle focuses on the anodeómassively increasing the surface area of the anode by using an ultra high surface area Reticle Carbon anode. By so doing, we substitute simple, energy-preserving "filling of a capacitor" at the anode in place of energy-expensive anodic oxidation reactions that occur in SX-EW. Figure 3 shows that for each SO4= ion adsorbed onto the Reticle Carbon anode, an equal number of Cu+2 ions must move to the cathode and be reduced. Reticle combines reduction of copper at the cathode with anions being adsorbed at the anode, not oxidized. Adsorbed anions are easily removed by washing the loaded solution? with a Fe+2 solution. Stored anodic energy oxidizes Fe+2 to Fe+3, thereby releasing adsorbed ions and readying anodes for more copper plating.

We have conducted commercial tests in which we have improved standard electrowinning performance (50 g/L copper and 125 g/L acid) by dropping power consumption from the typically observed 2.2 Kw-hr/kg Cu to a mere 1.4 kW-hr/kg Cu with zero anodic oxygen generation! This is profound, a 1/3 reduction in energy cost. Equally significantly, we directly electroplate copper from raw PLS and do not need the expensive (and carcinogenic) SX preconcentration step. We plate copper directly from (dilute) PLS, thus eliminating the need for any solvent extraction and thereby cutting capital and energy costs substantially. We estimate that typical copper SX-EW costs are on the order of $0.40/lb, which Reticle reduces to approximately $0.23/lb. This is a colossal magnitude of change, offering the potential not only to substitute Reticle for every SX-EW process in the world but to encourage substitution of hydrotreatment for the standard, sulfate-emitting, economically and environmentally risky smelting process.

Reticle is presently seeking partners to engineer and commercialize its potential sea change improvement over SX-EW in the copper industry. Reticle is a truly "sustainable" process for recovering copperóno pernicious materials, no discharge of anything except pure water, low energy consumption, substantially lower cost than next best in class (SX-EW).


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