Co-current leaching

Fio. 2.1. Co-current leaching. Frequency distribution of residence times in a series of N vessels (U.K.A.E.A. copyright). 

NiSx has been observed to behave better than CoSx and FeSx. The decrease in overpotential can be as high as 0.3 V with respect to Ni [25, 151]. NiCo2S4 (formal composition) has been found to operate at -0.1 V vs DHE, at a current density of 1 A cm-2 and has been tested successfully for several thousand hours [444,452]. At open circuit a bluish coloration of the solution indicates that some Co is leached out since it becomes anodic with respect to Ni. A small continuous cathodic protection would be necessary. Teflon-bonded NiCoSx electrodes have been found to be immune from Fe poisoning this has been attributed to the precipitation of FeSx on the electrode surface due to the presence of sulfur in solution leached out from the surface. This condition will not be realized in a continuously flowing solution. 

It is frequently possible to design the leaching process on a continuous co-current basis, where acid, water and ore are fed into a vessel continuously, or at regular intervals, and discharged in a similar manner. In these circumstances the agitation system is usually designed to suspend the solids sufficiently in the liquid phase so as to allow them both to overflow from... 

In order to achieve a continuous co-current system in which a higher proportion of the ore is leached for a period near to the mean residence time, several vessels may be used in cascade. The ore, acid and water are fed to the first vessel as in the single-stage system but the overflowing slurry... 

The basic design of a leaching vessel for a batch, co-current or counter-current system can be identical with that used in other chemical processes involving solid-liquid reactions. A stirred vessel is commonly employed. 

An ammonium carbonate leaching pilot plant has been described which consists of five towers, each 10 ft high by 4 in. internal diameter, connected in series. The slurry of acid and ore is pumped in a co-current manner from the bottom to the top of each tower and then allowed to pass via 0-5 in. piping from the top of one tower to the base of the next. Air is injected at the base and mid-point of each tower. Typical pressures in the first and last towers are 130 psi and 90 psi. The system is fed from an agitated mixing tank, via a positive displacement piston pump located at the base of the first tower. After leaching, the gases are separated off by means of a pressure release valve and the leach liquor then removed from the spent ore in a hydrocyclone. 

Various continuous leaching processes have also been employed. Usually the powder and leaching acid are fed in a co-current manner into a rotating or stirred vessel and at some point it is necessary to separate the resulting metal powder under conditions where it is not allowed to remain static for long periods. The complexity of a safe continuous system, coupled with the relatively small scale of operation, usually leads to a preference for a batch leaching system. 

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

Co. of California. Proven orebody reserves at the end of December 1978 were 365,000 metric tons, with indicated reserves of over 3 million metric tons of rare earth oxide (REO). Current mine production capacity is 27,000 metric tons per year of bastnaesite concentrate produced in 3 grades a 60% REO unleached concentrate, a 70% REO leached concentrate (SrO and CaO removed), and a 90% REO calcined concentrate (CO2 removed). In 1977, shipments totaled 13,521 metric tons of contained REO. Polishing compounds consumed approximately 10% of this production. 

The Nippon Mining Co. refinery in Japan, which had previously used D2EHPA to extract cobalt from solutions obtained in the leaching of a mixed cobalt—nickel sulfide in sulfuric acid, changed in 1978 to the new extractant 2-ethylhexyl 2-ethylhexylphosphonate enabling a much-improved selectivity for cobalt over nickel to be obtained.8990 The process is carried out in three counter-current stages with an organic solution of phosphonic acid that has been converted to the ammonium salt by contact with aqueous ammonia ... 

The specific role and the fate of Mo in the alloy has been investigated [141]. It has been found that Mo is not at all stable but tends to be leached out, which would be the origin of the deactivation observed on cathodic load. The deactivation results in a progressive increase in the Tafel slope, which cannot be reactivated in situ by addition of molybdenum salts. On the other hand, that Mo is leachable can be inferred also from the observation that in situ deposited Co-Mo alloys are quickly dissolved as the current is interrupted [528, 529]. This seems to indicate a provisory activation of the cathode by Mo, which cannot be recovered in a simpley way once decayed [141]. However, this contrasts somewhat with the claim of long term stability and resistance to cell shut-downs for the thermally prepared Ni-Mo coating [5]. The structure of the layer may differ depending on the details of the preparation procedure. 

Current developments in combinatorial chemistry and rapid screening techniques promote new systems in the field of polymer-supported catalysts (cf. Section 3.1.3) [279]. A drawback for this technology is still the leaching tendency of the catalyst which is clearly shown in recycling experiments (cf. Section 3.1.1.3) [280]. Polycondensation of organo-functionalized silanes and polysiloxanes leads to covalent support of catalyst on the surface, also known as the sol-gel process [281-283]. Van Leeuwen and co-workers reported a phosphine ligand with a xanthene backbone which was functionalized with a propyltrialkoxysilane... 

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