Process for the recovery of tin

A process is described for recovering tin from oxide or oxide/sulphide low-grade starting materials and concentrates containing little or relatively low concentrations of tin, in which such low-grade starting materials are mixed with KOH and decomposed at high temperatures. The decomposed substance is then leached with water, and metallic tin is separated out electrolytically in one or more stages from the lye. The electrolyte with reduced tin content is concentrated by evaporation, impurities being removed, and dehydrated. The potassium hydroxide solution recovered is re-used for decomposition.

BACKGROUND OF THE INVENTION 
The invention relates to a process for recovering tin from starting 
materials containing relatively low concentrations of tin. 
Supply of tin is based worldwide upon rich tin concentrates because, to be 
economically desirable, occurrences of tin are based upon rich 
concentrates of workable ores. The efficiency of the traditional 
pyrometallurgical two-stage recovery process depends strongly on the tin 
concentration in the first concentrate runnings. Even where there are 
favourable cost relationships, the boundary-line of efficient processing 
must be seen as 40 to 50% Sn and also 6% Fe in the concentrate. 
It is true that fuming processes have been developed and put to industrial 
use with the volatilization of SnS (S being a carrier in the starting 
material or subsequently added) or SnO or SnOS, for the enrichment of 
starting materials containing little to relatively low concentrations of 
tin. These fuming processes modify these tin starting materials up to a 
more concentrated tin material for subsequent further processing on 
traditional pyrometallurgical lines into metallic tin, but such fuming 
processes operate at high temperature (1000.degree. to 1400.degree. C.) 
with huge energy consumption and represent only an enrichment process. 
Chlorination processes, for reasons connected with equipment and corrosion 
engineering, have not been able to gain acceptance. 
The advances made in preparation technology make it possible, in 
particular, efficiently to enrich, from oxide and oxide/sulphide starting 
materials or ores, concentrates having up to a 10 to 25% Sn content. Such 
starting materials can be added only to a limited extent to very rich Sn 
concentrates (much greater than 60%) for the traditional two-stage tin 
recovery, where an iron content of less than or equal to 6% also has has 
to be observed. A direct tin-recovery process from starting materials with 
about 10 to 30% Sn content does not yet exist and has not not been put to 
industrial use. 
Starting materials of such low Sn concentration cannot be efficiently 
processed into metallic tin with presently conventional pyrometallurgical 
two-stage recovery process using: 
1. partial metal reduction for tin recovery with slag formation having 8 to 
greater than 25% Sn, 
2. slag reduction with hard slag production containing 40 to 80 % Sn and 20 
to 50% Fe and finishing slags containing less than 1% Sn, 
3. hard slag re-introduced into the first processing stage. 
The task of the present invention is to provide a simple process with which 
it is possible to convert the first SnO.sub.2 /SnS runnings from a poor 
concentrate by a decomposition through melting into water-soluble 
compounds from which metallic tin can recovered electrolytically. 
This problem is solved by having the decomposition by melting effected with 
potassium hydroxide under inert gas or in air.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The decomposition substance is leached at one or more stages at 283.degree. 
K. or higher temperatures in water or aqueous alkali solution, and the lye 
that is formed and filtered out after decomposition and leaching is 
subjected to a high temperature and a cathodic current density between 50 
and 500 A/m.sup.2 to conduct electrolysis for recovery of the metallic 
tin. 
In pursuance of the idea of the invention, an especially progessive aspect 
is the fact that the final electrolyte of the tin-recovering electrolysis 
can be recovered by evaporation and concentration, salting out foreign 
substances and dehydrating as decomposition agent, and then re-employed to 
decompose the starting material by melting. 
Not only low-grade ores may be regarded as starting materials in the sense 
of the invention, but also tin containing by-products or residues from tin 
recovery. 
A low-grade tin concentrate in the sense of the invention comprises, for 
example: 10 to 40% Sn, 5 to 40% Fe, 0 to 10% S, 5 to 20% SiO.sub.2, 5 to 
20% Al.sub.2 O.sub.3 and 0 to 10% TiO.sub.2. Such a mixture is mixed with 
a sufficient quantity of KOH to convert the Sn content involved in a 
decomposition by melting into a soluble form, so that potassium stannates 
are formed. Under inert gas or in air, a decomposition product results, 
containing tin in a form soluble in an aqueous solution. 
In a second processing step, the decomposition substance, in one stage or 
in several consecutive leaching steps, is leached at greater than 
283.degree. K., especially in countercurrent with water, preferably at 
high temperatures. In this leaching process the tin is quantitatively 
almost dissolved. The filter residue is then washed, and the washing water 
is passed back into the leaching process. This is possible because the 
leaching is also carried out with water. 
In a third processing step, through single or multi-stage electrolysis, tin 
is recovered from the filtered lye solution directly at the cathode of 
steel, stainless steel or nickel electrodes plated with tin. 
In a fourth processing step, the final electrolyte, stripped of tin or 
having its tin much reduced, is concentrated by evaporation so that salt 
out takes place, undesired impurities are removed, and finally it is 
dehydrated. The recovered potassium hydroxide passes back to the first 
processing step. The process in accordance with the invention is 
represented in the flow chart of the FIGURE and will now be illustrated in 
greater detail by two examples. In the FIGURE the tin concentrate 
represents possible additions, water leaching can be in one or more 
stages, fresh water can be condenser water, tin recovery electrolysis can 
be in one or more stages, and cathode tin can be formed by melting off or 
stripping and melting down (mach refining) casting. 
EXAMPLE 1 
100 parts of the low-grade tin concentrate indicated above were mixed with 
a quantity of potassium hydroxide sufficient to convert the Sn into 
water-soluble form (e.g. SnO.sub.2 : KOH=1:13) and then decomposed in the 
temperature range 713.degree.-753.degree. K. By means of subsequent hot 
water leaching at 343.degree. K., almost 100% of the early Sn runnings 
were dissolved. 
With an initial electrolyte concentration of 30 g Sn/l and temperatures of 
343.degree.-348.degree. K., cathode tin at a current density of 100 
A/m.sup.2 (greater than 99.5% Sn; 0.3% Fe; less than 100 g/t Al and traces 
of As, Pb, Sb, Zn) was produced up to Sn contents in the final 
electrolytes of approximately 0.5 g Sn/l. 
Over 90% of the potassium hydroxide was recovered for re-use for 
decomposition purposes by concentration through evaporation, salting out 
from foreign substances and dehydrating. The Sn yield is greater than 90% 
of the content of the first concentrate runnings. 
EXAMPLE 2 
100 parts of the low-grade tin concentrate indicated above were mixed with 
a quantity of potassium hydroxide sufficient to convert the Sn into 
water-soluble form such as SnO.sub.2 : KOH=1:15 and then decomposed in the 
temperature range 703.degree.-733.degree. K. Almost 100% of the early Sn 
runnings were dissolved by a subsequent leaching with hot water. 
With an initial electrolyte concentration of 70 g Snl at high temperature 
and current density of 400 A/m.sup.2, the cathode tin produced was of 
greater than 99.5% pure Sn. 
After concentration by evaporation, salting out from foreign substances, 
and de-hydrating, the final electrolyte permitted recylcing of greater 
than 90% of the potassium hydroxide into the decomposition stage. The 
total yield of Sn is greater than 90% of the content of the first 
concentrate runnings.