Process for reducing the content of oxygen in metallic copper

Abstract

The content of oxygen in molten metallic copper is reduced during the copper refining process by adding pure carbon monoxide, produced by electrolysis of carbon dioxide in a solid oxide electrolysis cell (SOEC), thereby removing oxygen through the reduction of CuO to Cu. This way, the purity of the metallic copper is increased.

Claims

1. A process for reducing the content of oxygen, present as copper oxide, in molten metallic copper, which is subjected to a copper refining process, the process comprising: feeding a fed gas comprising carbon dioxide to a solid oxide electrolysis cell (SOEC) stack to produce a product gas comprising carbon monoxide and carbon dioxide; injecting the product gas into the molten metallic copper through an injection system that disperses the product gas in bubbles throughout the molten metallic copper, wherein the carbon monoxide removes the oxygen from the molten metallic copper according to the equation:
CuO+CO.fwdarw.Cu+CO.sub.2 thereby increasing the purity of the molten metallic copper, wherein the product gas from the SOEC stack is compressed and wherein the injection system disperses the product gas in bubbles throughout the molten metallic copper to enhance a contact surface between the product gas and the molten metallic copper.

2. The process according to claim 1, wherein the product gas from the SOEC stack consists essentially of CO, CO.sub.2, and optionally an inert gas, wherein the product gas is at least 10% CO.

3. The process according to claim 2, wherein the product gas from the SOEC stack consists essentially of CO, CO.sub.2, and optionally an inert gas, wherein the product gas is at least 20% CO.

4. The process according to claim 3, wherein the product gas from the SOEC stack consists essentially of CO, CO.sub.2, and optionally an inert gas, wherein the product gas is at least 30% CO.

5. The process according to claim 4, wherein the product gas from the SOEC stack consists essentially of CO, CO.sub.2, and optionally an inert gas, wherein the product gas is at least 40% CO.

6. The process according to claim 1, wherein the product gas from the SOEC stack consists of CO and CO2.

7. The process according to claim 1, wherein the feed gas consists of: carbon dioxide; optionally trace amounts of hydrocarbons; and optionally trace amounts of water.

8. The process according to claim 1, wherein the product gas from the SOEC stack consists of CO, CO.sub.2, and trace levels of hydrogen.

9. A process for reducing the content of oxygen, present as copper oxide, in molten metallic copper, which is subjected to a copper refining process, the process comprising: feeding a feed gas comprising carbon dioxide to a solid oxide electrolysis cell (SOEC) stack to produce a product gas comprising carbon monoxide; purifying the product gas to above 99% pure carbon monoxide; mixing the purified gas from the SOEC stack with nitrogen to form a mixed gas; and injecting the mixed gas into the molten metallic copper through an injection system that disperses the mixed gas in bubbles throughout the molten metallic copper, wherein the carbon monoxide removes the oxygen from the molten metallic copper according to the equation
CuO+CO.fwdarw.Cu+CO.sub.2 thereby increasing the purity of the molten metallic copper.

10. A process for reducing the content of oxygen, present as copper oxide, in molten metallic copper, which is subjected to a copper refining process, the process comprising: feeding a feed gas comprising carbon dioxide to a solid oxide electrolysis cell (SOEC) stack to produce a product gas consisting essentially of CO, CO.sub.2, and optionally an inert gas, wherein the product gas is at least 10% CO; purifying the product gas in favor of CO to produce a purified gas comprising CO; mixing the purified gas from the SOEC stack with nitrogen to form a mixed gas; and injecting the mixed gas into the molten metallic copper through an injection system that disperses the mixed gas in bubbles throughout the molten metallic copper, wherein the carbon monoxide removes the oxygen from the molten metallic copper according to the equation
CuO+CO.fwdarw.Cu+CO.sub.2 thereby increasing the purity of the molten metallic copper.

Description

(1) There are several advantages in using pure CO to reduce the oxygen content in copper. One significant advantage is that hydrogen is avoided, which would otherwise increase the brittleness of the produced copper. Many copper smelting plants in operation today deliberately use high purity CO from cylinders or tube trailers to avoid hydrogen and other harmful impurities.

(2) Producing carbon monoxide (CO) through electrolysis of carbon dioxide (CO.sub.2) has inherent advantages, and this method is a surprisingly good match for providing CO to reduce the oxygen content in copper. In WO 2014/154253, the Applicant has disclosed this method, which is conducted in a solid oxide electrolysis cell (SOEC) or, typically, in an SOEC stack consisting of a large number of solid oxide cells separated by interconnects. The key element in the cells is the electrolyte where an oxygen ion can be dissociated from H.sub.2O or CO.sub.2 when an electric current (e.sup.−) is applied, viz.
CO.sub.2+2e.sup.−.fwdarw.CO+O.sup.2− or H.sub.2O+2e.sup.−.fwdarw.H.sub.2+O.sup.2−

(3) In the method, CO.sub.2 is led to the fuel side of the stack with an applied current and excess oxygen is transported to the oxygen side of the stack, optionally using air or nitrogen to flush the oxygen side, and the product stream from the SOEC, containing CO mixed with CO.sub.2, is subjected to a separation process. The method further comprises heating the inlet gas on both the fuel side and the oxygen side by means of separate heating units, so as to supply heat to the SOEC, where the operation temperature of said heating units is at least equal to the operation temperature of the cell stack minus 50° C., preferably at least equal to the operation temperature of the cell stack. An apparatus for the production of high purity CO based on SOEC electrolysis of CO.sub.2 in combination with a high purity CO.sub.2 feedstock is disclosed in WO 2013/131778, also belonging to the Applicant.

(4) Thus, according to Applicant's above WO documents, CO is produced from high purity CO.sub.2 (food or beverage grade) and electrical power. The resulting gas is a pure mixture of CO and CO.sub.2, in which the only source of undesired impurities consists in traces of hydrocarbons and water present in the CO.sub.2 feed. These traces are converted to trace levels of hydrogen.

(5) In a plant for the production of high purity CO based on SOEC electrolysis of CO.sub.2, the feed CO.sub.2 is converted to a mixture of CO and CO.sub.2. Normally, the produced gas is then further purified through a set of processing unit operations, the most common of which consists in compressing of the product mixture from the electrolysis followed by a pressure swing adsorption (PSA) step and, for high purity CO, a polishing step with applied temperature swing adsorption (TSA).

(6) Importantly, however, in the case of oxygen removal from copper there is no need at all for further removal of CO.sub.2 from the gas produced in the electrolysis step, since CO.sub.2 is an inert to the copper refining process. This means that the mixture of CO and CO.sub.2 obtained by electrolysis of CO.sub.2 can be fed directly into the copper refining process, which significantly reduces cost and greatly reduces the plant complexity of the CO generating plant.

(7) So the present invention relates to a process for reducing the content of oxygen, present as copper oxide, in molten metallic copper, which has been obtained in a manner known per se and subjected to a copper refining process,

(8) wherein carbon monoxide produced from carbon dioxide in a solid oxide electrolysis cell (SOEC) stack is added during the refining process to remove the oxygen according to the equation
CuO+CO.fwdarw.Cu+CO.sub.2

(9) thereby increasing the purity of the metallic copper.

(10) Preferably the product gas from the SOEC stack is compressed and added to the copper melt through an injection system that disperses the reducing gas in the shape of small bubbles throughout the copper melt. This way, the gas/solid contact surface is maximized.

(11) It is also preferred that the gas from the SOEC stack is purified to above 99% pure carbon monoxide and mixed with nitrogen before it is injected into the copper melt.

(12) Preferably the product gas from the SOEC stack comprises at least 10% CO, the remaining part being CO.sub.2 or an inert gas. The amount of CO in the product gas may of course be much higher, such as 20%, 30%, 40% or above.