Improvements in the Separation of Tin from Tin Scrap.
Year: 1892

page_text: Date of Application, 27th Feb., 1892
Complete Specification Left, 26th Nov., 1892--Accepted, 7th Jan., 1893
PROVISIONAL SPECIFICATION.
Improvements in the Separation of Tin from Tin Scrap.
I, THOMAS TWYNAM of 10 Blandford Road Bedford Park Chiswick Metal-
lurgical Chemist do hereby declare the nature of this invention to be as follows:-
My invention relates to improvements in the separation of tin from tin plate
or tin sheet scrap or cuttings.
When the scrap is heated in air a film forms upon the surface consisting
chiefly of the tin in an oxidized state; this oxide is not readily removed by
mechanical means from the surface of the metal.
I find however that when the scrap or cuttings are heated with some fusible
salt, or plunged into a bath of such salt in a fused state, the film or scale formed
is in such a condition that it can be readily separated from the surface of the
metal either by boiling or heating in water, or by well agitating by mechanical
means.
The salt I prefer to use and the one which I find gives the best results is
chloride of calcium, but good results may be obtained however with other salts,
notably the chlorides of sodium, potassium, magnesium and iron. The scrap or
cuttings should be well covered by the salt used, and exposed to a red heat until
the wished for film forms on the surface.
I do not find that the tin goes into solution when boiled in water, but that it
falls off as a heavy insoluble powder or compound to the bottom of the containing
vessel.
Very little iron need be removed with it, and the scrap remaining may be used
up for metallurgical purposes.
Dated this 96th day of February 1892
THOMAS TWYNAM.
COMPLETE SPECIFICATION.
Improvements in the Separation of Tin from Tin Scrap.
I, THOMAS TWYNAM of 10 Blandford Road Bedford Park Chiswick Metal-
lurgical Chemist do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained
in and by the following statement :-
My invention relates to an improved process for the recovery of tin from tin
plate scrap or cuttings, and from old tin scrap which has been worked up into tin
cans etc. By my process the iron, after the tin has been separated from it, can be
reused in metallurgical operations, more especially for use as scrap in the Siemens
process of steel making.
When tin scrap is heated in air a scale forms upon the surface consisting chiefly
of the tin in an oxidized state; this oxide is not readily removed by mechanical
means from the surface of the iron.
I find however that when the scrap or cuttings are heated in the air with some
fusible salt; or plunged into a bath of such salt in a fused state; or preferably, are
coated with a film of such salt by dipping the scrap into a strong aqueous solution
of such fusible salt, and, after withdrawing therefrom, heated to a red or nearly red
heat, the film or scale which forms on the surface of the scrap is in such a condition
that it can be readily separated from the surface of the metal by throwing the
scrap, preferably whilst still hot, into water, or by well agitating by mechanical
means.

page_text: The salt I prefer to use, and the one which I find to give the best results, is
chloride of calcium, but good results may be obtained however with other salts,
notably the chlorides of the alkalies and alkaline earths & the chloride of iron.
I find the best way to coat the surface of the scrap is to form a strong aqueous
solution of the salt, which solution I prefer to thicken by the addition of one or
two per cent, of some glutinous substance, such as glue or similar body. Into this
solution the scrap is dipped, and when thoroughly wetted with the solution, is
withdrawn, carrying on its surface a film of the salt employed. The coated scrap
is then ready for heating in a suitable furnace.
When handling large quantities of scrap I prefer to charge it into a crate or
cage which can be lowered into the solution contained in a tank. From three to
five minutes is usually sufficient time for the scrap to remain in the solution. The
cage, when withdrawn, and after allowing the excess of solution to drain off, is
either lowered or placed in a suitably heated furnace or brick chamber, or the
scrap is removed from the cage into such a heated chamber. From five to fifteen
minutes heating at a dull or full red heat is usually long enough. A slightly
oxidizing atmosphere should be maintained in the furnace, but the heat should not
be too high, nor should a very strongly oxidizing atmosphere be maintained, as
otherwise a large quantity of iron will oxidize with the tin, and some tin would be
lost by volatilisation.
The scrap being heated sufficiently is withdrawn from the furnace, and without
cooling, allowed to fall into a tank of water, in which the scale, which has formed
on the surface of the scrap, at once falls off sinking to the bottom of the tank.
The surface of the iron scrap is left practically free from tin. The stripped scrap
is withdrawn from the washing tank, and can be used in many metallurgical
operations.
The insoluble scale which is left in the washing tank will be found to consist
chiefly of oxide of tin, mixed with some oxide of iron, & a small percentage of
lime. This scale may be smelted direct for tin, by mixing with carbon and some
silicious matter to remove the iron oxide.
I always prefer however to treat it with sufficient acid (preferably hydrochloric)
to dissolve out the oxide of iron; the oxide of tin, being practically insoluble in
acid, is left in a nearly pure state, and can then be readily run down into metal.
Or the mixed oxides may be fused with alkali whereby the tin can be removed as a
soluble stannate. The lime may be in either case first separated by agitation in
water, whereby it can be washed away.
Having now particularly described and ascertained the nature of my said inven-
tion & in what manner the same is to be performed I declare that what I
claim is: --
1. The improvements herein described for recovering the tin from tin plate
scrap, and for separating the tin from the iron, which consist in causing the scrap
to be coated with a film of chloride of calcium or similar fusible salt, heating to
redness & subsequently separating the resulting scale substantially as described.
2. In the recovery of tin from tin scrap, the method herein described for forming
a scale on the surface of the scrap which will separate from the scrap on treatment
with water, substantially as described.
Dated this Twenty-fifth day of November 1892.