Concentration processes precipitation

Lime Soda. Process. Lime (CaO) reacts with a dilute (10—14%), hot (100°C) soda ash solution in a series of agitated tanks producing caustic and calcium carbonate. Although dilute alkaH solutions increase the conversion, the reaction does not go to completion and, in practice, only about 90% of the stoichiometric amount of lime is added. In this manner the lime is all converted to calcium carbonate and about 10% of the feed alkaH remains. The resulting slurry is sent to a clarifier where the calcium carbonate is removed, then washed to recover the residual alkaH. The clean calcium carbonate is then calcined to lime and recycled while the dilute caustic—soda ash solution is sent to evaporators and concentrated. The concentration process forces precipitation of the residual sodium carbonate from the caustic solution the ash is then removed by centrifugation and recycled. Caustic soda made by this process is comparable to the current electrolytic diaphragm ceU product. 

Nickel and cobalt are recovered by processes that employ both pressure leaching and precipitation steps. The raw materials for these processes can be sulfide concentrates, matte, arsenide concentrates, and precipitated sulfides. Typically, acidic conditions are used for leaching however, ammonia is also effective in leach solutions because of the tendency for soluble cobalt and nickel ammines to form under the leach conditions. 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

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 ia 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 Hquors with caustic soda without a concentration step, as shown ia equation 9. 

The system or process termed rhizofiltration is the use of hydroponically cultivated plant roots of several terrestrial plants to absorb, concentrate, or precipitate toxic metals from polluted effluents... 

Electrokinetic remediation is limited by the type of contaminant, heterogeneities or anomalies in the soil, extreme pHs, pore water chemistry, lack of pore water, contaminant and noncontaminant ion concentrations, metals precipitation, and reduction-oxidation changes induced by the process electrode reactions. It may be difficult to estimate the time that will be required to remediate a site using this technology. Laboratory treatability testing may provide a false indication of the applicability of electrokinetic remediation at a specific site. Further research is required to determine the technology s limitations and ramifications. 

The EPOC microfiltration process is based on the ability of a proprietary woven polyester filter to retain particles and allow water to permeate through the filter. The technology uses a three-step process in which (1) reagents are added to the wastewater to precipitate metals and/or sorb other contaminants (2) the microfiltration unit (known as EXXFLOW) removes and concentrates the precipitates, while allowing water (permeate) to pass and (3) the concentrated precipitate is dewatered (by a module called EXXPRESS) to produce a semidry filter cake containing the metal precipitates and other filtered solids. 

Formation of mineral deposits occurs through geochemical reactions which often concentrate and precipitate specific compounds quite locally and separately fc un others of sufficiently different chemical properties. In such cases, natural processes do much of the work of concentration and separation for us. If, however, the elements of interest have closely similar chemistries,... 

Proteins that remain in whey after removing casein from milk are recovered as whey protein concentrates by precipitation with added polyphosphate or other polyvalent anionic compounds, ultrafiltration, ion exchange adsorption, gel filtration, or a combined acid and heat precipitation process. Whey protein concentrates are also manufactured by a combined process involving electrodialysis, concentration, lactose crystallization, and drying (Richert 1975 Morr 1979 Marshall 1982 Anon. 1982 Muller 1982B). 

FIGURE 8.29 At low concentrations of solute, dissolving is accompanied by a decrease in free energy so it is spontaneous. At high concentrations, dissolving is accompanied by an increase in free energy so the reverse process, precipitation, is spontaneous. The concentration of a saturated solution corresponds to a state of lowest free energy at the temperature of the experiment. 

Interest in trace element speciation studies in natural waters has increased considerably during the last decade. It has become apparent that data on total concentrations of any element rather than on individual well defined chemical entities, are often inadequate to identify transport mechanisms, ultimate fate and toxicity of particular elements to organisms. A study of the different trace metal species and their relative distribution will assist in understanding the chemical processes that take place in the highly reactive estuarine zone and in the open sea. These processes include the rate at which chemical processes take place, the participation in geochemical processes (precipitation/dissolution, adsorption/desorption). 

Although all potentiometric measurements (except those involving membrane electrodes) ultimately are based on a redox couple, the method can be applied to oxidation-reduction processes, acid-base processes, precipitation processes, and metal ion complexation processes. Measurements that involve a component of a redox couple require that either the oxidized or reduced conjugate of the species to be measured be maintained at a constant and known activity at the electrode. If the goal is to measure the activity of silver ion in a solution, then a silver wire coupled to the appropriate reference electrodes makes an ideal potentiometric system. Likewise, if the goal is to monitor iron(UI) concentrations with a platinum electrode, a known concentration of... 

After flotation or other concentration processes, the crucial chemical steps are cyanide leaching and zinc precipitation, for example,... 

In the distal part of the nephron, urine is concentrated and the likelihood of crystalline precipitation increases substantially, particularly if urinary pH favors decreased solubility. As the urinary concentrating process also involves the counter-current mechanism, solute concentrations in the medullary interstitium can reach values several times higher than tissues elsewhere in the body. Finally during the process of renal excretion, a particular drug may undergo bioactivation resulting in reactive metabolites [2]. 

Thus, three steps — solution, restricted flow, and localized precipitation — are necessary, and 2ilthough brines are common in the crust, the delicate timing of events and the chemistry needed to form lead and zinc ores coincide very rarely. Similarly complex and therefore infrequent events lead to the formation of deposits of the other scarce metals. Just how the secondary concentration processes modify the distribution curve of geochem-ically scarce metals is not known with certainty, but the end result is, I suggest, the bimodal distribution depicted in Fig. 10.2. 

Notice that the equilibrium constant is an exhemely small number compared to the hydroxide Ksp. This shows that for the same copper ion concentration, an immeasurable amount of sulhde ions is needed to achieve saturahon. In other words, it will take exhemely low sulhde concentration to precipitate more of the copper than just a pH change. As we have discussed previously, the sulhde ion concenhahon is a function of the soluhon pH. That is, the sulhde ion concentra-hon will be considerably lower at low pH (where metal ions are more stable), and become higher as the pH is raised. So like the hydroxide precipitation process, the pH can be an important variable. 

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