Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iva metal
Patent
1992-02-27
1993-05-11
Morris, Theodore
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iva metal
423 89, 423 99, 423101, 423 87, 423158, 423618, C01G 1900
Patent
active
052099113
DESCRIPTION:
BRIEF SUMMARY
Divalent metal stannates are being increasingly used as fire retardant additives in polymer formulations. They generally exist in an two forms of formula MSn(OH).sub.6, and MSnO.sub.3, the latter being readily formed by heating the former to drive off water, and also in a third form of formula M.sub.2 SnO.sub.4. The standard way of making these divalent metal stannates is by mixing an aqueous solution of sodium or another stannate with an aqueous solution of a salt of the divalent metal in question, and recovering the precipitated divalent metal stannate. This process suffers from disadvantages. The precipitate tends to be colloidal and difficult to filter, and is contaminated with chloride or other anions. The byproduct is sodium chloride or other sodium salt of low value.
German Patent No. 348670 describes a process for the separation of tin from an alkaline solution by reacting the soiution with an excess of an insoluble magnesium salt such as magnesium phosphate or carbonate.
Canadian Patent No. 1,214,O45 describes a process for the recovery of indium and tin in which tin containing solids are fused with solid sodium or potassium hydroxide to produce a soluble stannate which is then leached out with water. The leached solution is then reacted with an excess of calcium hydroxide to give a precipitate from which tin is recovered.
In contrast to these prior methods, which are designed to recover tin in an unspecified state, the present invention has the object of making a metal stannate or hydroxystannate by an improved route.
The present invention provides a method of making a substantially pure metal stannate, which method comprises mixing a solid metal oxide, hydroxide or carbonate with an aqueous stannate solution and recovering a precipitated metal stannate, the solid metal oxide, hydroxide or carbonate being added in an amount which is stoichiometrically not more than the amount of tin.
The metal stannate generally has the formula M.sub.a SnO.sub.3 or M.sub.a Sn(OH).sub.6 where M is the metal and a is 2/(metal valency); or M.sub.b SnO.sub.4 where b is 4/(metal valency). Metal valency may be 1, 2, 3, 4 or higher. The nature of the metal is not very critical. Preferred divalent metals, on account of the economic importance of their stannates are Zn, Mg, Ca, Ba and Bi; most preferred are Zn and Bi. These metals are used in the form of their oxide, hydroxide or carbonate, preferably the oxide or hydroxide, which terms are used to include mixed forms such as hydrated oxide or basic carbonate. The oxide, hydroxide or carbonate is generally insoluble or substantially insoluble in water, is used in solid form, and contains substantially no non-fugitive anion which might otherwise contaminate the product. To speed reaction, this solid reagent is preferably used in comminuted form.
The stannate solution is generally an aqueous solution of sodium or potassium stannate. This is mixed with the solid reagent, in proportions which are not very critical, provided that the solid reagent is added in an amount which is stoichiometrically not more than the amount of tin. If the solid reagent is added in an excess then the metal stannate produced will be contaminated with unreacted metal oxide, hydroxide or carbonate. In order to obtain the metal stannate in substantially pure form it is preferred that the solid reagent is stoichiometrically no more than the amount of tin in the solution. Provided quantitative reaction is obtained it is possible to remove essentially all the stannate ions from solution. If the caustic soda/sodium stannate liquor is to be recycled, there is little disadvantage in use of a slight excess of stannate, a method which is preferred as it ensures complete conversion of divalent species to the hydroxystannate. Indeed a large excess of sodium stannate may help to convert less reactive oxides, hydroxides or carbonates such as magnesium. The excess stannate remains in solution and does not contaminate the product. The mixture is stirred at ambient or elevated temperature until the reaction has gone
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Chemical Abstracts, vol. 99, No. 7 99:62791m, p. 570.
Chemical Abstracts, vol. 116, No. 19 116:195630z.
Alcan International Limited
Morris Theodore
Squillante Edward
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