Thermochemical process for recovering Cu from CuO or CuO2
BACKGROUND OF THE INVENTION 
     This invention was made in the course of, or under, a contract with the United States Department of Energy. It relates to the thermochemical production of hydrogen. 
    
    
     This is a division of application Ser. No. 934,664, filed Aug. 17, 1978 U.S. Pat. No. 4,169,884. 
    
    
     For a number of years hydrogen has been produced by the electrolysis of water. Electrolysis is highly inefficient due to the inefficiencies present in electricity production coupled with the efficiency of about 80% for electrolysis. The wide-spread use of hydrogen as a primary energy source will likely depend upon the efficient production of hydrogen by thermochemical processes or by electrolytic processes which have a significantly lower demand for electrical energy than that required by electrolysis of water. Thermochemical techniques involve the decomposition of water into hydrogen and oxygen through a series of chemical reactions not involving the use of fossil fuels. Preferably, the series of reactions is carried out in a closed cyclic manner, such that all products except hydrogen and oxygen are reused as reactions in the other reactions. Typical of such processes are those described in commonly assigned U.S. Pat. Nos. 3,490,871 and 4,005,184. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a chemical reaction flowsheet depicting the sequence of reactions for three embodiments of the cyclic production of hydrogen according to the process of this invention. 
     FIG. 2 is a chemical reaction flowsheet depicting the sequence of reaction for the recovery of Cu from CuO. 
    
    
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a process for producing hydrogen from copper and barium compounds. 
     It is a further object to provide a closed thermochemical process for producing hydrogen which requires no input other than water and thermal energy. 
     It is a further object to provide a hybrid process involving both thermochemical and electrolytic steps whereby the consumption of electricity is significantly reduced over that needed for the electrolysis of water. A portion of the process of this invention is also useful for recovering copper from CuO. 
     These and other objects are achieved according to this invention in a process for producing hydrogen comprising the step of (a) reacting Cu with Ba(OH) 2  in the presence of steam to produce hydrogen and BaCu 2  O 2 . The BaCu 2  O 2  can be reacted with H 2  O to form Cu 2  O and a Ba(OH) 2  product which can be recycled to the initial reaction step. Cu can be obtained from the Cu 2  O product by several methods. In one embodiment the Cu 2  O is reacted with aqueous HF solution to provide CuF 2  and Cu. The CuF 2  is reacted with H 2  O to provide CuO and HF. CuO is decomposed is Cu 2  O and O 2 . The HF and the Cu 2  O are recycled and the Cu is recovered as a product. In another embodiment Cu 2  O is reacted with aqueous H 2  SO 4  solution to provide CuSO 4  and Cu. The CuSO 4  is decomposed to form CuO and SO 3 . The CuO is decomposed to form Cu 2  O and O 2 . The SO 3  is recycled to form H 2  SO 4  and the Cu 2  O is recycled for reaction with H 2  SO 4 . In another embodiment Cu 2  O is decomposed electrolytically to Cu and O 2 . In its copper recovery aspects the process of this invention comprises recovering Cu from CuO by the steps of decomposing CuO to form Cu 2  O and O 2 , reacting the Cu 2  O with aqueous HF solution to produce Cu product and CuF 2 , reacting with CuF 2  with H 2  O to form CuO and Hf, and recycling the CuO and HF to previous reaction steps. 
     DETAILED DESCRIPTION 
     An aspect of this invention is the discovery of integrated chemical reaction cycles which produce H 2  or Cu in high yields without difficult separation steps. The reaction conditions for the individual reactions can be varied somewhat from the specific temperatures, pressures, times, etc. described herein and still provide the desired products. All that is required according to this invention is that the reactants, or material containing the reactants, be reacted to produce the specified products which are then separated and/or recycled to other steps in the process, regardless of the specific conditions under which the reactions are conducted. 
     The cyclic production of hydrogen according to the present invention proceeds according to FIG. 1. FIG. 1 illustrates two reaction steps common to each of the hydrogen production cycles and three alternative methods for regenerating the Cu reactant for the initial reaction. The first reaction is the reaction of metallic Cu with Ba(OH) 2  in the presence of steam to provide a BaCu 2  O 2  reaction product and H 2 . The reaction of Cu with Ba(OH) 2  is performed by contacting Cu, preferably in the form of powder or turnings, with Ba(OH) 2  in the presence of steam at a temperature of 600-800° C. An H 2  O pressure of 1-10 atmosphere is suitable. The reaction should be carried out in copper equipment or equipment resistant to corrosion by Ba(OH) 2 . The reaction can be easily carried out on a laboratory scale in a horizontal tube furnace. The Ba(OH) 2  is liquid at the reaction temperature and the copper readily reacts with the liquid to yield hydrogen as illustrated by reaction (1). ##EQU1## H 2  is carried out of the reaction zone with the steam and the steam is condensed from the gaseous phase, providing a gaseous H 2  product. Since Cu and Ba(OH) 2  are obtained upon treatment of the BaCu 2  O 2  product in subsequent steps in the cycle, it is not necessary at this point to separate unreacted Cu and Ba(OH) 2  from the solid BaCu 2  O 2  containing product. 
     In the second step in the cycle, the solid product of reaction (1) is reacted with H 2  O to provide Ba(OH) 2  and Cu 2  O according to the following reaction: 
     
         BaCu.sub.2 O.sub.2 +H.sub.2 O →Ba(OH).sub.2 +Cu.sub.2 O (2) 
    
     This reaction is easily conducted by contacting the solid product of reaction (1) with water at about 25-100° C. at atmosphere pressure. Since the Ba(OH) 2  product is partially water soluble, the reaction and separation can be performed simultaneously be repeated boilings and filtration of the solid product of reaction (1), to provide a Ba(OH) 2  solution and a solid product containing Cu 2  O. An inert atmosphere, e.g. N 2  or Ar, should be used to prevent formation of BaCO 3  from CO 2  present in air. In the laboratory, this reaction is easily conducted in a Soxhlet apparatus with the solid reactant on a glass frit and H 2  O in the reservoir. The H 2  O is heated to cause boiling or evaporation and subsequent condesation. The condensate contacts the solid reactant causing the formation of Ba(OH) 2  which dissolves in the excess condensed water. The Ba(OH) 2  solution is evaporated and the solid Ba(OH) 2  is recycled to reaction (1). Since the Cu-BaO-steam system is essentially equivalent to the Cu-Ba(OH) 2  -steam system, it will be apparent that BaO can be used in reaction (1) instead of Ba(OH) 2 . If BaO is to be recycled, the Ba(OH) 2  product of reaction (2) can be heated, e.g. to 900° C., to decompose the Ba(OH) 2  to BaO prior to recycle. 
     Cu can be recovered from the Cu 2  O product of reaction (2) in several ways. In one embodiment (represented by the solid lines in FIG. 1) Cu 2  O is reacted with aqueous HF solution according to the following reaction: 
     
         2Cu.sub.2 O+4HF(aq)→2CuF.sub.2 +2Cu+2H.sub.2 O      (3a) 
    
     This reaction can be performed by contacting Cu 2  O with an aqueous HF solution (15 to 48 wt. %) at a temperature of 25 to 80° C. The reaction goes essentially to completion. The reaction product mixture is an aqueous CuF 2  solution containing metallic copper which can be separated by filtration, sedimentation, etc. The separated Cu is recycled to reaction (1). To facilitate Cu separation, sufficient water should be present in the HF solution to maintain the CuF 2  product in solution. CuF 2  is recovered from solution by evaporation and the resulting CuF 2  powder is reacted with steam at temperatures of 250-500° C. by the following reaction: 
     
         CuF.sub.2 +2H.sub.2 O→2CuO+4HF(g)                   (4a) 
    
     This reaction is conducted by passing excess steam either over or through the CuF 2  powder. The H 2  O pressure is not critical, with 1-10 atm. being suitable. The reaction proceeds essentially to completion. HF is recovered from the reaction mixture by condensing the gaseous product containing excess steam to provide an aqueous HF solution. The aqueous HF solution is adjusted to the appropriate concentration for recycle to reaction (3a). CuO which remains as a solid product, is thermally decomposed according to the following reaction: ##EQU2## This reaction is carried out by heating the solid product of reaction (4a) to about 950 ° to 1050° C. in the presence of steam. The preferred H 2  O pressure is about 1 atm. The presence of steam significantly lowers the reaction temperature and produces Cu 2  O particles rather than a frozen liquid. Excess H 2  O can be condensed from the reaction atmosphere leaving a gaseous O 2  product. The solid Cu 2  O is recycled to reaction (3a) for treatment with HF. 
     In a second embodiment (represented by dashed lines in FIG. 1) Cu 2  O from reaction 2 is reacted with aqueous sulfuric acid according to the following reaction: 
     
         2Cu.sub.2 O+2H.sub.2 SO.sub.4 (aq)→2CuSO.sub.4 +2Cu+2H.sub.2 O (3b) 
    
     This reaction is carried out by contacting the solid product of reaction (2) with aqueous H 2  SO 4  solution (about 20 to 60 wt.%) at a temperature of 25° to 80° C. The reaction goes essentially to completion. The CuSO 4  product is recovered in the form of an aqueous solution. The Cu precipitates and can be separated by filtration, sedimentation, etc. for recycle to reaction (1). The H 2  SO 4  solution should contain sufficient H 2  O to keep the resulting CuSO 4  in solution thereby facilitating recovery of Cu precipitate. The CuSO 4  product is recovered from solution by evaporation and thermally decomposed to CuO by heating to 600°-900° C., according to the following reaction: 
     
         2CuSO.sub.4 →2CuO+2SO.sub.3                         (4b) 
    
     to yield solid CuO and SO 3 . The reaction can be conducted in air and SO 3  can be recovered from the reaction mixture by passage through an aqueous solution. The CuO product is decomposed by heating at from 950° to 1050° C. in steam atmosphere at about 1 atmosphere H 2  O pressure according to reaction (5a) described earlier. The Cu 2  O product is recycled to reaction (3b). The SO 3  product of reaction (4b) is dissolved in water to provide H 2  SO 4  solution by the following reaction: 
     
         SO.sub.3 +H.sub.2 O→H.sub.2 SO.sub.4                (5b) 
    
     The H 2  SO 4  is recycled to reaction (3b). The preferred method of recovering SO 3  is to bubble the gaseous product through a concentrated H 2  SO 4  solution, e.g. 37 M, to form oleum, which can then be diluted with water to be appropriate concentration for reaction (3b). 
     In a third embodiment (represented by dotted lines in FIG. 1) Cu is recovered from the Cu 2  O product of reaction (2) by electrolysis according to the following reaction: ##EQU3## The electrical energy necessary for this step is about 60% less than that required for direct electrolysis of water. The electrolysis can be performed in molten Cu 2  O or at a temperature below the Cu 2  O melting point in a molten salt solution. Suitable electrodes are a Cu cathode and an anode of a metal more noble than Cu, e.g. Pt, for evolving O 2 . Those skilled in the art of electrochemistry can carry out the electrolysis according to the well-established principles for carrying out electrolysis from melts, such as those described for molten salt mixtures and molten salt mixtures containing dissolved Al 2  O 3  in Potter, Electrochemistry, Principles and Aplications, Cleaver-Hume Press Ltd., London, (1956) pp. 316-327, which is incorporated herein by reference. 
     The following examples demonstrate several of the reactions of the present invention. 
     EXAMPLE I 
     Reaction 1 was carried out in a copper boat or in a single crystal MgO crucible positioned in a silica tube within a horizontal furnace. The boat contained 1.6 grams Cu turnings and 5 grams Ba(OH) 2 . Steam at 500 ml water per hour was preheated and introduced into the reactor. The steam was mixed with argon at about 100 cc/min. to facilitate subsequent H 2  analysis of the product gas stream. The exiting gases were passed through a water-cooled condenser followed by a drying column and a hydrogen monitoring assembly. Upon heatup and Ba(OH) 2  melted. The evolution of hydrogen started at about 875° K. (692° C.) and reached a maximum at about 1070°±20° K. The reaction product, BaCu 2  O 2 , was recovered from the boat and identified by x-ray diffraction and chemical analysis. In the absence of steam the yield of hydrogen was about 18% while in the presence of steam the yield of hydrogen was about 100% based upon the oxidation to Cu + . Steam prevents the decomposition of Ba(OH) 2  and thereby provides the source for H 2  production. 
     EXAMPLE II 
     The solid product of Example I was reacted with H 2  O in a Soxhlet extractor at about 345° K. H 2  O in the reservoir of the apparatus was evaporated and condensed over the solid BaCu 2  O 2  whereupon the Ba(OH) 2  was formed and dissolved. The resulting Ba(OH) 2  solution was collected in the reservoir. After reaction ceased, the residue remaining was identified as Cu 2  O. 
     EXAMPLE III 
     A known amount of Cu 2  O was dissolved in an aqueous HF solution (about 20 wt.%) in a polytetrafluoroethylene beaker with magnetic stirring. After about 30 minutes, finely divided Cu was separated by filtration from the blue CuF 2  solution. The yield of Cu as 86% of the stoichiometric amount. 
     EXAMPLE IV 
     The reaction of CuF 2  and H 2  O is performed by heating the CuF 2  powder in a HF-resistant container, i.e. polytetrafluoroethylene in a steam atmosphere (about 1 atm. pressure) at a temperature of about 250° C. HF solution is recovered as a condensate, leaving a solid CuO product. The recovered HF solution is adjusted to the appropriate concentration for use in reaction (3a). 
     EXAMPLE V 
     CuO was heated in a stream of steam containing argon. About 100% of the oxygen produced according to reaction (5a) was evolved upon heating to 1000° C. Under similar conditions in argon atmosphere, only about 3-4% of the oxygen was evolved upon heating to 1000° C. and less than 65% had been evolved after heating to nearly 1200° C. 
     Another aspect of this invention is the recovery of Cu from CuO as illustrated in FIG. 2. The reactions are conducted in the same manner as described above. CuO, obtained for example from scrap material or naturally occurring ores such as tenorite and paramelaconite, is decomposed, preferably in the presence of steam according to reaction (5a). The CuO is heated at from 950-1050° C. in steam, e.g. at 1 atm. H 2  O pressure, it provide Cu 2  O and O 2 . The solid Cu 2  O product is reacted according to reaction (3a) with aqueous 15-48 wt.% HF solution at 25-80 ° C. to provide a CuF 2  solution and a metallic Cu precipitate which is recoverably by filtration, sedimentation, etc. The HF solution should contain sufficient H 2  O to keep the resulting CuF 2  in solution, thereby facilitating recovery of Cu precipitate. The CuF 2  is recovered from solution by evaporation and reacted with steam at 250-500° C. according to reaction (4a) to provide solid CuO and gaseous HF. The HF is easily recovered from the gas phase by condensation into water to provide an HF solution. The HF concentration of the solution is adjusted for recycle to reaction and solid CuO is recycled to reaction (5a) for further treatment. 
     It can be seen that Cu can also be recovered from Cu 2  O such as naturally occurring cuprite ores by the same reaction cycle. Rather than adding CuO to reaction (5a), Cu 2  O is added to reaction (3a) as indicated by the dashed line FIG. 2 and the sequence proceeds by reactions 3a, 4a, and 5a, respectively. By appropriate mass balance, mixtures of Cu 2  O and CuO can be processed according to FIG. 2. 
     It will be apparent to those skilled in the art that all separations in the processes of this invention need not be complete and all reagents need not be directly recycled. For example, fresh H 2  O can be added to any step where it is needed and can be allowed to escape as steam or used to provide heat for other portions of the process. Other products such as HF or H 2  SO 4  may be diverted to other uses as economics might dictate, and be replaced with fresh material. These and other such obvious modifications are contempleted as equivalents of the processes described herein.