Zinc recovery

Zinc. The electrowinning of zinc on a commercial scale started in 1915. Most newer faciUties are electrolytic plants. The success of the process results from the abiUty to handle complex ores and to produce, after purification of the electrolyte, high purity zinc cathodes at an acceptable cost. Over the years, there have been only minor changes in the chemistry of the process to improve zinc recovery and solution purification. Improvements have been made in the areas of process instmmentation and control, automation, and prevention of water pollution. 

The key to this zinc recovery process is a two-stage precipitation,26 with the second precipitation taking place under careful pH control, using sodium hydroxide in contact with circulating slurry of zinc hydroxide crystals. All of the zinc precipitates in the second step, most of the impurities in the first. 

The conventional technique for removing zinc from the spinning acid wastestream has been direct lime precipitation to -pH 10, with no zinc recovery. The economics of this approach are compared to the American Enka zinc recycle process. 

The zinc industry employs 2200 workers at mines and mills and 1400 at primary smelters.14 The four primary zinc smelters in the U.S. are located in Illinois, Oklahoma, Tennessee, and Pennsylvania. There are ten secondary zinc recovery plants in the U.S.17... 

Mayze, R. Nickel laterite processing in Western Australia. Copper, Cobalt, Nickel and Zinc Recovery, Conference, Victoria Falls, Zimbabwe, July 16-18, 2001, F1-F12. 

Walker [9] has studied the effect of ultrasonic agitation in terms of hardness, quality of deposit and efficiency. Using spent zincate plating baths, he observed that both the efficiency of the zinc recovery process and the maximum current density which could provide for good quality deposits was significantly increased in the presence of ultrasound (Tab. 6.7). He also observed that the improvement was inversely dependent on the bath concentration the more dilute was the bath (0.5 g dm zinc) then the greater the improvement. 

Cadmium is obtained as a byproduct in zinc recovery processes. The metal volatdizes during roasting of zinc concentrates and collected as dust or fume in bag houses or electrostatic precipitators. The dusts are mixed with coal (or coke) and zinc chloride and calcined. The cadmium chloride formed volatihzes upon calcination and thus separates out from zinc. The chloride then is treated with sulfuric acid in the presence of an oxidizing agent. This converts lead, present as impurity in cadmium ore, to lead sulfate which precipitates out. Cadmium is finally separated from copper by the addition of zinc dust and... 

Zinc oxide is obtained as an intermediate in recovering zinc from minerals (See Zinc, Recovery). The oxide is prepared by vaporizing zinc metal and oxidation of the zinc vapors with preheated air (French process). The oxide can be produced by other processes. Another method involves roasting franklinite and other ores with coal and then oxidizing the product in air. 

Zinc sulfate is produced as an intermediate in recovering zinc from mineral zinc blende, ZnS (see Zinc, Recovery). The mineral is roasted at about 1,000°C to form zinc oxide and sulfur dioxide which, on prolonged heating in excess air, converts to zinc sulfate ... 

In the zinc recovery process, roasted products are leached with sulfuric acid, whereupon zinc oxide is converted to sulfate. 

Other promising processes for zinc recovery are examined in Morris (1985). 

Pertraction (PT) can be realized through a liquid membrane, but also through a nonporous polymeric membrane that was applied also industrially [10-12]. Apart from various types of SLM and BLM emulsion liquid membranes (ELM) were also widely studied just at the beginning of liquid membrane research. For example, an emulsion of stripping solution in organic phase, stabilized by surfactant, is dispersed in the aqueous feed. The continuous phase of emulsion forms ELM. Emulsion and feed are usually contacted in mixed column or mixer-settlers as in extraction. EML were applied industrially in zinc recovery from waste solution and in several pilot-plant trials [13,14], but the complexity of the process reduced interest in this system. More information on ELM and related processes can be found in refs. [8, 13-16]. 

Samaniego, H., San Roman, M.F. and Ortiz, I. (2007) Kinetics of zinc recovery from spent pickling effluents. Industrial el Engineering Chemistry Research, 46, 907. 

The first large scale application of ion exchange to effluent treatment was in the recovery of water, ammonia, and basic copper sulfate from the waste streams encountered in the cuprammonium rayon process. Originally a phenolic type condensation resin was employed, but more recently carboxylic acid acrylic-based exchangers have been introduced. A similar process exists for zinc recovery from the spinning acids of viscose rayon plants, except that in this operation a sulfonic acid resin is employed. 

Zmc. The low recovery of zinc is probably not caused by insuflBcient APDC since the concentration of APDC was increased by a factor of five, but with no increase in zinc recovery. 

To date, there have been three more industrial plants installed for zinc recovery the 700 rn Vh capacity plant at Glanzstoff AG, Austria, the 200 rn 7h capacity plant at CFK Schwarza, Germany, and the 200 m Vh capacity plant at AKZO lede, Netherlands. 

Clarified zinc sulfate solution goes to electrowinning for zinc recovery. 

Zinc Recovery. Part of the spent lye from the gas wash (a) is used to precipitate zinc (f) from the stretching bath and from the wastewater of the tow wash. By addition of H2SO4 to the precipitated zinc sulfide, zinc sulfate is recovered for the spinning process. During zinc precipitation, a small portion of zinc polysulfide is formed that cannot be converted to zinc sulfate. This portion must be discharged. 

Example 10.1 Lead-zinc recovery Lead and zinc minerals often occur together and are selectively floated and recovered. The main lead mineral is galena (PbS). 

Regeneration of Chromium-Based Passivation Baths with Zinc Recovery.. 

---