Carbonate leach liquor

In this process [27], the metals are extracted successively by the same extractant, employing suitable pH values for each metal. The spent catalyst is first roasted in an air stream at 300°C to remove oil and sulfur deposited on the surface of the catalyst. Then the metals are dissolved in an ammonium carbonate leach liquor at 80°C. Finally, the three metals are isolated and purified from one another using solvent extraction with an alkylmono-thiophosphoric acid (MTPA) with a structure similar to DEHPA. The following flow sheet was suggested (Fig. 14.19) ... 

The first group, minerals containing high concentrations of uranium, mostly in the tetravalent state, can be concentrated by specific gravity methods when in massive form. Frequently, however, the particle size is so small that the uranium-bearing mineral must be dissolved in sulfuric acid or sodium carbonate leach liquors. In either case, an oxidant must be added to bring uranium to the soluble, hexavalent state. 

Recovery of uranium from leach liquors. Uranium may be recovered from leach liquors by precipitation, ion exchange, or solvent extraction. Precipitation with sodium hydroxide was the recovery method used in the first uranium mills. When used on sodium carbonate leach liquors, the uranium precipitate is fairly free of other metallic contaminants, because sodium carbonate dissolves few other metals beside uranium. However, when used in sulfuric acid leach liquors, the uranium precipitate contains other metals, such as iron dissolved from the ore by the add, and is no longer commercially acceptable. Consequently, in the United States, uranium mills emfdoying add leaching now follow it with selective recovery by either solvent extraction or ion exchange. These processes are described in Secs. 8.5 and 8.6, respectively. 

I rt 1 of Table 5.19 lists mills using carbonate leaching. There is no standard process for recovery of uranium from carbonate leach liquors. The trend is away from precipitation with NaOH as crude NajUj07 toward further purification as by UO4 precipitation at Rio Algom or by ion exchange followed by precipitation as (NH4)3U207 at George West. 

In this mill, uranium is recovered from carbonate leach liquor by precipitation with sodium... 

Copper sulphate and ammonia, added to the sodium carbonate leach liquor, have been found to catalyse the oxidation by air where this is necessary. This process has been used in the large pilot plant at Grand Juntion in the United States. ... 

Ion-exchange can be used for the extraction of uranium from sodium carbonate leach liquors. However, the process economics are usually unfavourable since a fairly pure concentrate can be obtained by direct alkali precipitation from the carbonate solution. The leaching stage in the ion-exchange process also tends to be more expensive than acid leaching, except in special cases where limestone is present. Consequently, little or no commercial use is made of the process at present. 

The uranium anion present in carbonate solutions is [U02(C03)8] and this is associated with few other impurity anions. Absorption capacities as high as 100 to 200 mg/g of dried resin have been obtained on Amberlite IRA-400 2 and Dowex I , under conditions where competing anionic impurities such as phosphate and aluminate ions have only absorbed to an insignificant extent. The resin capacity, in both cases, is greatest at low sodium carbonate concentrations. Vanadate ion absorption can take place to an appreciable extent when vanadium is present in the carbonate leach liquor from the ore. It is, however, readily separated from the uranium, e.g. by a preliminary elution with a saturated solution of sulphur dioxide. This removes the vanadium from the resin by reducing it to a lower valency state. 

Uranium Is recovered from carbonate leach liquors by 2uilon exchange in industrial operations. Its anion exchange behavior is similar in carbonate solution to that in sulfate solution. 

The process making use of adsorption on carbon involves first contacting a pregnant leach liquor with the adsorbent (activated carbon) and then stripping the species adsorbed on it. Activated carbon is a widely recognized as a metallurgical reagent that has found a number of industrial applications. 

CIL [Carbon In Leach] A process for extracting gold from cyanide leach liquors using activated carbon. 

CIP (1) [Carbon In Pulp] A general name for hydrometallurgical extraction processes that use activated carbon in slurries of ground ores. One such process is used for extracting gold from cyanide leach liquors. First operated on a large scale at the Homestake lead mine in South Dakota in 1974. See also RIP. 

Recovery of Bismuth from Tin Concentrates. Bismuth is leached from roasted tin concentrates and other bismuth-bearing materials by means of hydrochloric acid. The acid leach liquor is clarified by settling or filtration, and the bismuth is precipitated as bismuth oxychloride [7787-59-9], BiOCl, when the liquors are diluted using large volumes of water. The impure bismuth oxychloride is usually redissolved in hydrochloric acid and reprecipitated by diluting several times. It is then dried, mixed with soda ash and carbon, and reduced to metal. The wet bismuth oxychloride may also be reduced to metal by means of iron or zinc in the presence of hydrochloric acid. The metallic bismuth produced by the oxychloride method requires additional refining. 

Molybdenum can also be recovered economically from some uranium leach liquors, particularly those of the USA. When uranium is stripped from amine extractants by solutions of sodium chloride, any molybdenum present remains in the organic phase, and can be subsequently recovered by being stripped into a solution of sodium carbonate. A process has been operated in which the strip liquor is acidified to a pH value of 4.5 and the molybdenum is reextracted into a solution of quaternary amine chloride in kerosene.218 The extracted metal is stripped into a solution containing sodium hydroxide and sodium chloride to produce liquors containing 30-40 g of molybdenum per litre, from which calcium molybdate can be precipitated by the addition of calcium chloride. 

The leaching reactions and the distribution of the various anionic uranyl species are very dependent on the pH value of the leach liquor and on the sulfate or carbonate concentration. Nominally, only the anionic di- and tri-sulfate or carbonate species will exchange with the functional groups of an anion-exchange resin, but the resin itself can facilitate the formation of complex anions in the resin phase because of the high concentration (approximately 0.5 M) of the co-ion on the functional group. Therefore, a complex equilibrium is established in which the resin is a participant the following reactions describe these equilibria for sulfuric acid leach liquors ... 

Uranium ores are leached with dilute sulfuric acid or an alkaline carbonate [3812-32-6] solution. Hexavalent uranium forms anionic complexes, such as uranyl sulfate [56959-61-6], U02(S04)"23, which are more selectively adsorbed by strong base anion exchangers than are other anions in the leach liquors. Sulfate complexes are eluted with an acidified NaCl or ammonium nitrate [6484-52-2], NH4N03, solution. Carbonate complexes are eluted with a neutral brine solution. Uranium is precipitated from the eluent and shipped to other locations for enrichment. Columnar recovery systems were popular in South Africa and Canada. Continuous resin-in-pulp (RIP) systems gained popularity in the United States since they eliminated a difficult and costly ore particle/leach liquor separation step. 

Carbonate leaching under ambient conditions is extremely slow with poor recoveries. Therefore, the ore is typically leached in an autoclave with air providing most of the needed oxygen. The leach liquor is separated from the solid in a countercurrent—decantation system of thickeners, and the uranium is precipitated from the clarified sodium carbonate solution with addition of sodium hydroxide (eq. 9) (23). 

Chemical Precipitation. The product of the extraction processes, whether derived from acid or carbonate leach, is a purified uranium solution that may or may not have been upgraded by ion exchange or solvent extraction. The uranium in such a solution is concentrated by precipitation and must be dried before shipment. Solutions resulting from carbonate leaching are usually precipitated directly from clarified leach liquors with caustic soda without a concentration step, as shown in equation 9. 

For direct precipitation of vanadium from the salt-roast leach liquor, acidulation to ca pH 1 without the addition of ammonia salts yields an impure vanadic acid when ammonium salts are added, ammonium polyvanadate precipitates. The impure vanadic acid ordinarily is redissolved in sodium carbonate solution, and ammonium metavanadate precipitates upon addition of ammonium salts. Fusion of the directly precipitated ammonium salts can yield high purity V2Os for the chemical industry. Amine solvent extraction is sometimes used to recover 1—3 g/L of residual V2Os from the directly precipitated tail liquors. 

Since the leach liquor becomes more alkaUne after reaction with the mineral it is advantageous for sodium bicarbonate to be present along with the sodium carbonate leaching reagent. The bicarbonate is partially converted to carbonate during the leaching process, i.e. 

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