Circuit extraction

The initial requirement in the development of a solvent extraction process for the recovery or separation of metals from an aqueous solution is knowledge of the solution composition, pH, temperature, and flow rate. Both pH and temperature can be adjusted, within certain economic limits, before feeding to the solvent extraction circuit, but only in a few cases can the leaching or dissolution conditions be dictated by the extraction process. Consequently, no serious development work on the extraction process can be carried out before the leaching conditions or the type of feed solution are established. 

Measuring the transfer of acid between the strip and extraction circuits and establishing methods for control... 

Here again, few data are available on the solubility of the various diluents, in either its use or being studied for use in solvent extraction circuits. Although this is expected to be low (<2ppm), what few data are available apply to water and not to solvent extraction raffinates (see, however Chapter 2). 

Tertiary amines are susceptible to oxidation (RsNiO), and some evidence has been found to suggest that oxidation does indeed occur after several months recycling in a uranium solvent extraction circuit [A. W. Ashbrook, unpublished data]. 

Common to all or most solvent extraction operations in the mining industry is the problem of stable formation of cruds. The crud can constitute a major solvent loss to a circuit and thereby adversely alfect the operating costs. Because there can be many causes of crud formation, each plant may have a crud problem unique to that operation. Factors such as ore type, solution composition, solvent composition, presence of other organic constituents, design and type of agitation all can adversely alfect the chemical and physical operation of the solvent extraction circuit and result in crud formation [32-34]. 

The nature of the feed composition can be a major determining factor as to whether crud will form in the subsequent extractive operations [33,34]. The presence and concentration of certain cations, such as Fe, Si, Ca, Mg, or Al, with sufiicient shear in the mixing process can produce stable cruds [32,34]. Solids must be absent from most solvent extraction circuits, and clarification is usually aimed at achieving about 10 ppm of solids. One of the major causes of crud is the lack of good clarification of the feed solution, with the result that solids get through to the solvent extraction circuit. The presence of... 

Solids in the feed were mentioned as one of the major causes of subsequent crud formation in the solvent extraction circuit. Good clarification, therefore, is necessary to minimize crud and thereby operating costs. Table 7.4 itemizes some benefits of good clarification [48]. In North America and South Africa, the objective is to obtain approximately 10 ppm suspended solids in the feed to extraction. This is usually obtained by the use of sand filters after a countercurrent decantation (CCD) circuit. At least one plant has reported that crud quickly developed when the sand filters were not in... 

Young, W. Crud in Gulf Minerals Rabbit Lake solvent extraction circuit. Paper at AIME Annual Meeting, New Orleans, February 1979. 

The zinc circuit consists of a similar pH-controlled leach at 60°C under oxidizing conditions. Zinc fine dust with low copper content is leached with the copper barren raffinate and with part of the zinc raffinate. The zinc leaching operation is maintained at about pH 2 for most of the leaching time and then slowly raised to a final pH of 4.5, reducing the iron level to below 10 ppm. The leach solution is filtered and cleaned from impurity metals such as Cu, Ni, and Cd by an ordinary cementation procedure, again filtered and finally fed to a solvent extraction circuit. Zinc is extracted in three stages with... 

The residue after the water wash is leached at an elevated temperature by sulfuric acid, part of which is the recycled raffinate from the vanadium extraction. The leaching yield of vanadium (mainly IV-valent) is about 55% and of nickel about 95%. A final (post) leach with sodium hydroxide dissolves the remaining vanadium (mainly V-valent). The resulting leach solution, containing practically all the vanadium ( 25gdm ) and nickel ( 12gdm ) is fed to the solvent extraction circuit. 

The anolyte, containing about 75gdm CD at pFI 2, is fed to a solvent extraction circuit for the separation of Fe—Co and Fe—Ni with a tertiary alkyl amine, dissolved in kerosene. The separation is based on the tendency of the metals to form metal-chloride-amine complexes (Fig. 11.2). At low chloride ion concentration (about 75gdm ), Fe(III) is extracted to the organic solvent, while Co(II) and Ni(II) remain in the aqueous raffinate. If the chloride ion concentration is then increased to about 250 gdm, cobalt is extracted, leaving nickel behind in the raffinate. 

The precipitated zinc(II) hydroxide is removed by filtration, and the zinc-free organic phase is recycled to the extraction circuit after conversion to the chloride form with hydrochloric acid. 

In the various solvent-extraction circuits employed in this process, use is made of a solution of D2EHPA in kerosene as the extractant. The selective recovery of the various metals is achieved by careful control of the equiUbrium pH value of the aqueous phases in the multistage extraction and stripping operations. After the leach liquor has passed through two separate circuits, each of which comprises five stages of extraction and four of stripping, the europium product is obtained initially as a solution of europium(III) chloride. Further purification of the product is accomplished by reduction with amalgamated zinc to Eu +, which is by far the most stable of the divalent lanthanide ions with respect to the reduction of water cf. the redox potentials of the Eu /Eu and Sm /Sm + couples, which are —0.43 and —1.55 V respectively ). Sulfuric acid is added to the... 

Fig. 2.26 Extraction circuit A-G in the t,s-diagram of carbon dioxide CP critical point, V isochors with density indication, g gas, I liquid, f supercritical...

Generally, reactive extraction processes represent efficient (and smart) technologies for the separation and concentration of ionic or molecular species in solution and are frequently used in both research and industry [16, 18, 19]. The industrial adaptation of the two-step extraction scheme discussed above leads to a closed extraction circuit of the type shown in Fig. 4.3. 

Fig. 4.3. Typical flowsheet of an industrial solvent extraction circuit.

FIGURE 8.6-1 Solvent extraction circuit of the Falconbridge Nikkelvert A/S matte leach plant for separation of iron, copper, and cobell. From Ref. 6, with permission. 

Product solutions leaving the two solvent extraction circuits at a concentration around 30 g UsOg/Uter are combined and flow through four stirred precipitation tanks 8 ft (2.4 m) in diameter by 12 ft (3.7 m) high in series. Steam is added to the first tank to heat its contents to 60°C. A mixture of two to four volumes of air and one volume of ammonia is added to the last three tanks to raise the pH to 7.0. This precipitates uranium as mixed sodium and ammonium diuranate. 

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