Weak acid effluent

Extraction of Zinc from Weak Acid Effluents... 

Fig. 14.1 Extraction of zinc from weak acid effluents. 

Zinc concentrates contain various levels of selenium. Part of this selenium is released in the off-gas from the roasting operation. Following treatment of the roaster off-gas at an acid plant, the ultimate fate of the selenium is the weak acid effluent from the acid plant. 

The basic principle of the selected treatment method is the reduction and precipitation of the selenium present in the weak acid effluent as elemental selenium (Se (IV) -> Se°). Selenite present in the weak acid bleed is a relatively strong oxidizing agent as given in Dean (2) ... 

Storage tanks for the weak acid effluent pumped from the acid plant... 

Another alternative involves the use of a weak acid cation exchanger in the hydrogen form. This resin is not capable of removing aH cations. It removes only the amount equivalent to the bicarbonate in the influent water. The acidity in the effluent stream is carbonic acid [463-79-6] which can be eliminated by installing a degasifter. 

It is sometimes possible to improve detection by changing the pH of the eluent, or by the use of photochemical reactions. The common barbiturates used in therapy are weak acids that are easily separated in their acid (unionised) forms. Because the conjugate bases are much stronger chromophores than the acids, barbiturates have been detected by post-column mixing with a pH 10 borate buffer followed by uv absorption at 254 nm. An example of the second approach is the detection of cannabis derivatives in body fluids involving the conversion of cannabis alcohols to fluorescent derivatives on subjecting the column effluent to intense uv radiation. 

The first example describes the extraction of zinc from weak acid solutions. In the manufacture of rayon, rinse waters and other zinc-containing liquid effluents are produced. The total liquid effluent in a rayon plant may amount to several per minute with a zinc concentration of 0.1-1 gdm and pH normally 1.5-2. In addition to zinc, the effluent contains surface-active agents and dirt (organic fibers and inorganic sulfide solids). The use of both precipitation (OH and S ) and ion exchange has been reported to remove zinc from such effluents. In addition, solvent extraction has successfully been used to recycle the zinc back to the operation. 

Water vapor from the evaporator passes to a water-cooled condenser, and the weakly acidic condensate is discharged as liquid effluent. The extractive distillation section of the plant is maintained under slight negative pressure, and the condenser off-gases are scrubbed with water and discharged as gaseous effluent. 

The European titanium dioxide producers have developed different effluent treatment processes to meet the environmental requirements [2.47]. The most important processes are the precipitation of gypsum (CaS04) from the weak acid [2.48] and the concentration and recovery of the free and bound acid. 

AN ABSORPTION column is required to absorb the nitrogen oxide components (NOx) from the gaseous reactor effluent by contacting this gas with an aqueous medium. Three incoming streams are handled by the column. The first is the gas stream of 32 570 kg/h with 12% (wt./wt.) nitrogen oxides. It enters at a temperature of 65°C and a pressure of 960 kPa. The second stream contains weak-acid condensate from the oxidation unit. It is 42% (wt.) aqueous nitric acid solution, at 50°C and 970 kPa. The final stream is deionized water at 7°C and 950 kPa. 

A mixture of l-(2,3,5-tri-0-benzoyl-p-D-ribofuranosyl)-4-methylthio-l,2-dihydro-l,3,5-triazin-2-one (0.5875 g), absolute methanol (5 ml) and a normal methanolic sodium methoxide solution (1.2 ml) is stirred at room temperature with the exclusion of atmospheric moisture (a guard tube filled with potassium hydroxide pellets is fitted to the reaction vessel). The starting compound passes into solution in the course of 5 min. The resulting solution is allowed to stand at room temperature for 45 min and then the cations are removed by passage of the solution through a column packed with 10 ml of a weakly acidic cation exchange resin in the H+ form prewashed with water and methanol. The methanolic effluent (60 ml) is evaporated under reduced pressure at 30°C, the residue is dissolved in methanol (20 ml) and the solution once again is evaporated and the l-p-D-ribofuranosyl-4-methoxy-l,2-dihydro-l,3,5-triazin-2-one was obtained. 

The cation exchange step is exactly as described for the Strong Acid Cation (SAC)-Strong Base Anion (SBA) process but now the acidic cation column effluent passes down a column of weakly basic anion exchange resin. The strong mineral acids are taken up by the anion resin through addition to form the acid salt forms, whilst the too weakly acidic dissolved carbon dioxide and silica pass through unaffected (Chapter 4). 

The phenate ion has been found to be quite satisfactory. Not only does it have a more favorable selectivity coefficient on Dowex-2 (0.14 for phenate vs. 1.5 for OH ) but an acid resin as the suppressor would convert it to a phenol, which is a very weak acid that would be dissociated only feebly and contribute little to the conductivity of the effluent from the suppressor. A disadvantage of the phenate ion is that it forms oxidation products that tend to poison the column and shorten its life. Table 24-1 shows several eluents for anions. 

The ionic forms of many substances will be affected by the pH of the effluent solution. Hydrolysis of metal ions and of salt of weak acids and bases is controlled by adjusting the pH. Weak acids will not dissociate in high acid concentrations and will not exchange, and the same is true for weak bases in high alkaline concentrations. Control of pH is especially important in the separation of amino acids, which are amphoteric (can act as acids or bases). There are three possible forms ... 

An environmental chemist analyzed the effluent (the released waste material) from an industrial process known to produce the compounds carbon tetrachloride (CCI4) and benzoic acid (HC7H5O2), a weak acid that has one acidic hydrogen atom per molecule. A sample of this effluent weighing 0.3518 g was shaken with water, and the resulting aqueous solution required 10.59 mL of 0.1546 M NaOH for neutralization. Calculate the mass percent of HC7H5O2 in the original sample. 

In typical zinc operations, the source of selenium in liquid effluents is the weak acid bleed from the acid plant. Selenium is most often found as a selenide solid solution impurity in a sulfide ore, as is the case for the zinc sulfide concentrates being used in the CEZinc plant in Valleyfield. During roasting of the zinc sulfide concentrate, the sulfur is oxidized to sulfur dioxide ... 

The scrubbing liquor is recirculated at the scrubber. A bleed of the scrubbing liquor is required to control the build-up of sulfuric acid, selenium, mercury and other species. This is the so-called weak acid bleed. The volume of this bleed is relatively small. However, it is concentrated since it results from a recirculation loop at the scrubber. Historical data from the Valleyfield operation show that the average concentration of selenium in the weak acid bleed is in the order of 50 mg/1 with peaks up to 180 mg/1 the average mercury concentration is 30 mg/1 (SNC-Lavalin (3)). The concentration of selenium in such a bleed first depends on the specific selenium content of the zinc concentrate being proeessed. It is known that some zinc ore deposits are characterized by high contents of selenium. Unless adequate treatment technologies are available for the removal of selenium fi-om the acid plant effluent, future development of such deposits may encounter environmental obstacles. 

Dilution is not an alternative either. Mixing of the weak acid bleed with other effluents from the zinc plant (for the joint treatment at the central HDS facility) would result in lower concentrations of selenium by dilution. At the Valleyfield operation, the acid bleed only... 

As part of a larger program to eliminate the toxicity of the plant liquid effluents, the CEZinc Division had the objective of treating the weak acid bleed effluent at the source, subject to the feasibility of developing and implementing a new treatment method capable of achieving a high removal performance for selenium. 

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