Patent Number: 045499853
Section: description

DETAILED DESCRIPTION OF THE INVENTION This invention comprises a method for processing an acid solution containing dissolved metal compounds such as spent acid electrolyte from an electrolytic bath. The method is specifically directed to the solidification, separation and consolidation of soluble compounds of metals including uranium and radioactive components from phosphoric acid solutions for disposal or recovery of the respective constituents. The invention is capable of dealing with phosphoric acid solutions including soluble forms of metals such as iron, nickel, molybdenum, copper, zinc, chromium, aluminum, cobalt and manganese, and radio-nuclides including plutonium, uranium, radium, cobalt, strontium, and americium. Uranium may be present in the phosphoric acid solution in both soluble and insoluble compounds comprising UO.sub.2, U.sub.3 O.sub.8, UO.sub.4, (NH.sub.4).sub.2 U.sub.2 O.sub.7, CaU.sub.2 O.sub.7, UF.sub.4 and UO.sub.2 F.sub.2. Referring to the diagram of the drawing, an electroplating system is illustrated comprising an electrolytic bath containing phosphoric acid as the electrolyte for the electropolishing of metal in accordance with the techniques of the art. This invention deals with the spent electrolyte from such a system, comprising phosphoric acid solution containing metals dissolved therein and retained as soluble compounds in the acid medium. The metal-containing acid solution effluent from the electrolytic bath system can be subjected to a preliminary treatment of its contact with an anion exchange medium when it is appropriate to increase the concentration ratio of the dissolved metals to the acid of the solution, and to reclaim phosphoric acid for return and reuse within the electrolytic bath of the system. The effluent from the electrolytic bath can be diluted if its viscosity is so high (for example a specific gravity of greater than about 1.5) as to retard effective percolation through a particulate mass of the ion exchange medium and a preliminary filtering for the purpose of excluding any solids from obstructing flow through the ion exchange material and polluting the exchange material. Contact with an anion exchange material removes a substantial portion of the phosphoric acid by preferential diffusion into the exchange material from the electrolyte solution effluent, which results in an increased proportion of dissolved metals to free phosphoric acid in solution free of the remaining exchange material. The phosphoric acid absorbed by the anion exchange material can be recovered and recycled to the electrolytic bath for reuse by washing the resin free of acid with water. The acid solution or the effluent from the ion exchange material, or unit containing same, consisting of a high dissolved metal-to-acid solution is treated for the initial metal precipitation by the addition of a solution of a carbonate of an alkali metal. The carbonate solution is introduced in an amount sufficient to lower the pH of the acid solution to about 5 to 6. Heat and agitation can be applied to the solution following the carbonate addition to expel carbon dioxide therefrom. Sodium carbonate (Na.sub.2 CO.sub.3) is preferred for this precipitation, but other useful carbonates comprise sodium bicarbonate, potassium carbonate, and potassium bicarbonate. The reaction of the preferred sodium carbonate with the phosphoric acid solution is shown in the equation: EQU 4H.sub.3 PO.sub.4 +3Na.sub.2 CO.sub.3 .fwdarw.2Na.sub.2 HPO.sub.4 +2NaH.sub.2 PO.sub.4 +3H.sub.2 O+3CO.sub.2 [ 1] Typical metals in the solution form mixed hydroxides and phosphates of generally low solubilities whereby the bulk of the initial soluble metals are precipitated out of solution at this stage of the process. However, when such a solution contains soluble uranium compounds, a portion thereof remain soluble as a tri-carbonate complex. The solids precipitated from the acid solution by the addition of the carbonate are separated and removed from the liquid portion by conventional means, such as filtration, settling or centrifuging. The remaining filtrate or supernate solution is passed to the next operation while the solids recovered are retained for a suitable disposal. When the soluble uranium compound content of the solution is significant, (for example about 5 or more parts per million by weight), it is preferred to subject the solution to an intermediate precipitation treatment of the addition thereto of sodium hydrosulfite (Na.sub.2 S.sub.2 O.sub.4) solution. The reaction of the sodium hydrosulfite in the phosphoric acid solution produces uranous bi-phosphate having the following formula: EQU U(HPO.sub.4).sub.2 To maximize the uranous precipitation produced by the addition of sodium hydrosulfite, the phosphoric acid solution should be neutralized with a carbonate solution such as sodium carbonate with an adjusted concentration which leaves the resultant neutralized supernate at the optimum phosphate concentration at about 1.1.+-.0.2 moles per liter. If the resultant phosphate concentration is too high, it can be reduced by water dilution, or if it is too low, it can be increased by evaporation. Also, to maximize the precipitation, it is preferred that the sodium hydrosulfite be introduced into the solution while at a temperature of about 75.degree. C. and with agitation. The solids precipitated from the solution by the addition of the sodium hydrosulfite are separated and recovered from the liquid portion by conventional means, including filtration, settling or centrifuging. The remaining filtrate or supernate solution is passed to the next operation while the solids recovered are retained for a suitable disposal. The final precipitation of the sequence comprises the addition of a soluble calcium salt, comprising calcium nitrate or calcium chloride, and also an alkali metal hydroxide to the filtrate or supernate solution from the former precipitation and solids separation. The preferred calcium salt is calcium nitrate, which is added first. The pH of the solution is subsequently adjusted to a basic condition with the metal hydroxide, preferably sodium hydroxide. The reaction mechanism for this precipitation is shown in the equations: EQU 2Na.sub.2 HPO.sub.4 +3Ca(NO.sub.3).sub.2 .fwdarw.Ca.sub.3 (PO.sub.4).sub.2 +4NaNO.sub.3 +2HNO.sub.3 [ 2] EQU 2HNO.sub.3 +2NaOH.fwdarw.2NaNO.sub.3 +2H.sub.2 O [3] As indicated, following the addition of calcium nitrate, a pH reversal occurs due to the formation of nitric acid. The fromed nitric acid reverses the reaction of equation 2 allowing for some phosphate solubility. The hydroxide is added to reverse this occurrence of solubility, and also to minimize the solubility of any contained residual metal ions. At a pH of about 10, the individual metal hydroxide ions' solubilities are at or near their minimums. All metals are present below 2 parts per million in the resultant solution filtrate or supernate and copper, molybdenum, cobalt, chromium and uranium are below 1 part per million. The calcium nitrate can be added at about 10 up to about 50 percent excess phosphate stoichiometry with good results. The precipitation reaction is preferably carried out in a hot solution of about 50.degree. C. The solids precipitated from the solution by the addition of the calcium salt and the alkali metal hydroxide are separated and recovered from the liquid portion by conventional means, including filtration, settling or centrifuging. The precipitated solids from each of the foregoing solidification and separation operations can be dehydrated to reduce their volume and combined if appropriate, for disposal or salvage. The filtrate or supernate solution from the foregoing sequence of solidification and separation operations, consisting primarily of a solution of sodium nitrate, and essentially free of radio-nuclides, can be safely disposed of in a waste retention lagoon or in other apt waste repositories. The procedures of the invention provides for the separation of potentially radioactive materials from a liquid medium and their solidification and reduction to a minimum volume for isolation and storage in a safe and efficient manner.