Weak acidic leaching

Solvent extraction is applied either directly to the weakly acidic leach hquor (the method is known as the Purlex process in South Africa, and the Amex process in the USA) or to the strongly acidic eluate obtained from a preliminary ion-exchange treatment of the leach liquor (known as the Bufflex process in South Africa, and the Eluex process in the USA). In the Purlex process, a 5 /(i solution of a tertiary amine in kerosene is typically used to treat a leach liquor containing up to 1 g of uranium per litre (although concentrations as low as 0.04 g 1 may be encountered) at a pH value of 1 to 2. A modifier such as isodecanol (2-5%) is usually incorporated into the organic phase to prevent the formation of a third phase and to inhibit the formation of an emulsion. The extraction of uranium(VI) is usually considered to proceed according to reactions such as ... 

Fig. 12. Trace element/Ca ratios of foraminifera subject to different cleaning procedures, (a) Mn/Ca, (b) Mg/Ca and (c) Sr/Ca. Samples cleaned using procedure A were subject to ultrasonication in methanol, oxidation and a weak acid leach. Those cleaned using procedure B were subject to additional reductive cleaning, and those cleaned using procedure C were subject to additional reductive cleaning as well as cleaning in DTPA (diethylene triamine pentaacetic acid). DTPA removes refractory phases rich in Ba and the rare earth elements (e.g. Haley Klinkhammer 2002). Note that the introduction of the reductive cleaning step significantly lowers Mn/Ca and also lowers Mg/Ca. Reductive cleaning has no resolvable effect on Sr/Ca. DTPA has little effect on Mn/Ca, Mg/Ca or Sr/Ca. Error bars (where visible) represent the standard deviation of the mean of two (0. universa) or three (G. conglomerata) separate analyses. Data are from Hathorne (2004).

T.T. Chen, J.E. Dutrizac and C. Canoo, Mineralogical Characterization of Calcine, Neutral Leach Residue and Weak Acid-Leach Residue from the Vieille-Montagne Zinc Plant, Balen, Belgium , Trans. Instn. Min. Metall.. Vol. 102,1993, C19-C31. 

The process begins with a weak acid leach (pH 1.5-1.8) followed by a pH bump to 3.8-4.0 for iron, silica, arsenic and antimony precipitation. The slurry is thickened, and the solid residues are filtered and washed to recover water-soluble zinc in two stages of American disk filters. This filtration step is being upgraded as described in a following section. These iron-rich residues are then repulped and combed with the residue slurry from the oxide leaching circuit and are pumped to the smelter where the slurry is filtered, combined with other materials and process in the JCIVCET furnace. 

Amine systems achieve higher uranium purity than organophosphorus systems (due to the greater selectivity of amines for uranium) and have lower extractant losses due to their lower aqueous-phase solubility. SX is applied either directly to the weakly acidic leach liquor (also known as the Purlex or Amex process, Vaal River West, South Africa) or to the strongly acidic eluate from an ion-exchange preconcentration treatment of the leach liquor (the Bufflex or Eluex process, Vaal River South, South Africa) (Nicol et al. 1987). A more modem variation (such as at Southern Cross Resources Uranium One, South Africa) is to treat the ore by pressure leaching followed by SX. 

Glass offers good resistance to strong acid at high temperatures. However, it is subject to thermal shock and a gradual loss in integrity as materials such as iron and siUca are leached out into the acid. Nonmetallic materials such as PTFE, PVDC, PVDF, and furan can be used for nitric acid to a limited degree, but are mainly restricted to weak acid service at ambient to moderate temperatures. 

AH commercial copper extractants selectively extract copper from weakly acidic aqueous leach solutions by the general reaction... 

From the foregoing discussion we conclude that some sophisticated tools are now available by which the activity coefficient in hydrometal— lurgical systems can be addressed. What is lacking is the actual application of these tools by the industry. The next step in establishing the accuracy of the available approaches lies in providing a broader data base for complex multicomponent systems which can be used for parameter refinement. TTte lack of data is most serious in the weak electrolyte area, but even familiar systems such as those encountered in sulfuric acid leaching need attention. 

The possibility for transport of Dicamba in subsurface soils, resulting in subsequent groundwater pollution, is potentially high. Both Dicamba and its initial transformation product 3,6-dichlorosalicylic acid have p2Ca values of 1.95 [392]. The high solubility of these weak acids at neutral to high pH makes it feasible for them to be mobile in lime treated or neutral pH soils. In the field, Dicamba (1) has been found to leach to a depth of 1 m over a 2-month period following application in a Missouri clay pan soil [296], (2) was discovered in approximately... 

Another reported example with two metals in solution is the recovery of copper and zinc from brass mill flue dust [11]. The material contains very little iron. The solid material is leached with sulfuric acid to produce a weak acid... 

Fig. 14.4 Extraction of copper and zinc from weak sulfuric acid leach solutions.

Weak-acid partial leach - 0.5 g of ground sample material leached for 2 hours in 5 ml of 2% HNO3 then diluted 10x with weak acid. 

A simple but effective means of preparing supported metal ion catalysts is to employ ion exchange resins. For example, a cobalt-exchanged H-type resin (Dowex 50) was shown43 to be an effective solid catalyst for the autoxidation of acetaldehyde to acetic acid at 20°C. No leaching of cobalt ions from the resin was observed and the catalyst was used repeatedly (5x) without any significant loss of activity. More recently the use of weak acid resins exchanged with cobalt ions as catalysts for the autoxidation of cyclohexane... 

In the Nippon Mining Company process,163 the aqueous feed consists of a solution of cobalt, nickel and other metal sulfates obtained by the high-pressure oxidative leaching of a mixed metal sulfide ore. Following the removal of iron and copper by chemical precipitation methods, and the successive extraction of zinc and cobalt by organophosphorus acid reagents (Section 63.3.2.2) under weakly acidic conditions, the pH value of the solution is adjusted to between 9 and 9.5 with ammonia, and nickel is extracted into a 25% solution of LIX 64N in an alkane solvent. 

Due to the need for an extensive pretreatment of the feedstock mineral, those aqueous processes which use direct carbonation could in fact be considered as indirect. As an alternative to energy-intensive pretreatment, several groups worldwide have embarked on methods to either dissolve or leach the Mg or Ca from minerals or industrial byproducts and wastes, by using strong or weak acids (mineral or organic), alkali solutions, or ligands. [101-105]. An example of this is the process route developed by Kakizawa et al. [50] in Japan (see Section 14.3.3.2). 

Nalgene (8000) General laboratory use, good flexibility, nonflammable, clear. Not good with organic solvents and most oils, OK with weak acids, but best to avoid strong acids and alkalis, contains plasticizers that can leach out during operations such as distillation. 

Separation by ion exchange Strongly basic or weakly basic ion exchange resins are used to separate selectively the uranium from the weakly acidic or alkaline solutions from the leaching step. The uranium is eluted from the ion exchange resins with nitrate or chloride solutions as anionic (carbonato- or sulfato-) complexes. 

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