INVESTIGATION ON THE LARGE SCALE PREPARATION OF HIGH PURITY ZINC CHLORIDE AND BATTERY ACTIVE MANGANESE DIOXIDE

Abstract

Zinc chloride1*1 la a white granular powder, consisting of small hexagonal crystals. Tt has an unpleasant metallic taste end a strongly corrosive action. It Is extremely toxic and highly hygroscopic. When the solution of sine chlorifle8 is evaporated to syrup and mixed with a little concentrated hydrochloric add, deliruescent crystals of Znda«HaO are obtained. If the solution is evaporated to dryness the oxychlorides, 3i(0H) aL,3iaO&8 are also foxmed to some extent, but if the dry mass is distilled at red heat, the anhydrous zinc chloride paasea over or sublimes in the form of white hexagonal needles. It Is one of the most soluble substances known (solubility 65.5 moles per 100 moles of water at 25°C ). It forms several hydrates*. The composition and transition points of the compound are as followat -60° C -30*C 6°C Ice *- 3»Cla.4Ha0 ^^ a»Cla.3HaO -^-^ 2nCla.2—-H«0 z 11.6CC , 26°C 28°C •^—^- 2nC3L a.l «±- HaO ^=^ ?nci a. HaO ^=±: ZnCl a 2 Anhydrous The ao-called anhydreus form of sine chloride contains an appredable amount of water and passes very rapidly to the hydra ted fbrm at room temperature. Aqueous solutions of sine chloride are only slightly hydrolysed. When the solution of zinc chloride Is evaporated or boiled with zinc oxide or zinc carbonate, a number of oxyehlorides such as 4 aa(OH) a and 3iCla»33i(0H ) a are formed. These basic chlorides8 have structures corresponding to the layer lattices of the hydroxides in which certain OH groups are replaced by d" ions. High purity zinc chloride finds an important application in the fabrication of dry cells** In the dry cell industry, it is used as on© of the components of the electrolyte known as *sal ammoniac', the aecond main component being ammonium chloride* The resistivity of zinc chloride solution ia,in general,higher than that of ammonium chloride. The 'aal ammoniac' solution with the addition of a small amount of mercuric chloride or chromate is used to inhibit local action at the zinc can| it Is used in making the cereal paste and with or without inhibitor as vetting solution fbr the preparation of the*black mix*. 'Sal ammoniac* is corrosive and when the cell discharges, it attacks the zinc can of the cell. It serves many useful purposes when used in the dry cell*. Firstly, zinc chloride lessens the corrosion of zinc can during the 'shelf life' of the cell. Secondly, It gelatinizes the starch flour paste. ThirHy,being hygroscopic helps In lowering the vapour pressure and to retain the moisture of the contents of the dry cell.Fburthly, zinc ions form a complex' with ammonia,7hCla* &TH8, thereby, removing excess of ammonia. It hydrolyses during the operation of the cell to give an add which helps to sustcin the working voltage of the cell. Fifthly, it lowers the freezing point thereby checking deposition of the electrolyte material* Cells intended for light drains and longer 'shelf life* usually contain a higher percentage of zinc chloride thsn cells designed fbr heavier service. Zinc chloride, on the other hand, Is a product of cell reaction. The presence of some zinc chloride initially decreases the attack of the 'sal ammoniac' on the zinc can* If the cells are made without zinc chloride in the electrolyte, the initial working voltage falls rapidly to a relatively low value,and this continues to the end of discharge. Hydrolysis of zinc chloride helps to maintain a favourable r>U of the electrolyte and imr roves the reltage of the cell. Dry cells deteriorate on standing. This deterioration is due to the local action*** caused by impurities more electropodtLve than zinc such as Fe,Cu,Cd,Pb,Ag,As,Co,Ni etc., which plate out on the inner surface of the zinc container wherever they are near to zinc metal. These impurities as a matter of fact should be below 0.02 %* In such cases exposed zinc near such a plated metal acts as anode and the more electropositive metal aa cathode,the metal of the can supplies the short drcuit, and the electrolyte of the cell serves aa the electrolyte of the local couple. In the continuous discharge of such local couples,the zinc goes into solution and the hydrogen Is plated out on the other metal. Fbrtunately, the overvoltage of hydrogen on these cathodic areas is higher than the voltage of the couple, and the action is arrested* In other cases, the polarization reduces the local action or the action may form Insoluble products which incrust on the surface, Even in the absence of Impurities,the structure of zinc mey cause local action, zinc chloride is used to reduce this action. Pecause zinc chloride is so very soluble in water and deliquescent it is usually made into a standardised solution*. ThepHof the solution Is 4. The ehnrnetcristies of a»d8 solution are given in Table 1. Apart from its use In dry cell Industry,chemically pure zinc chloride is also used in textile industry3** fbr crimpling wool, cotton and silk, fbr mercerizing cotton ?*oode,as a iRordant and resist in dyeing and printing fabrics, fbr sizing cotton goods, as a carbonizing agent In treating wool and for fire proofing woollen goods. Tt is used in the rayon industry as a catalyst or process material and fbr various miscellaneous - purposes,such as swelling and softening agent. It la also used as a solvent9 in the polymerization of acrylonitrlle and as a coagulant for the polymers.