An Improved Process of Treating Zinc Bearing Refractory Ores and Apparatus to be used therein Zinc Bearing Products.
Year: 1894

page_text: The decision of the Chief Examiner, acting on behalf of the Comptroller, of the
21st day of September 1895; allowed; under Section 18-of the Patents, &o.
Act, 1883 an, Amendment of this. Specification.
Date of Application, 18th July, 1894-Accepted, 25th Aug.,
1894
COMPLETE SPECIFICATION (AMENDED).
An Improved Process of Treating Zinc Bearing Refractory Ores and
Apparatus to be used therein Zinc Bearing Products.
I, EDGAR ARTHUR ASHCROFT, of Himalaya, Thomas Street, Broken Hill, in
the Colony of New South Wales, Electrical Engineer, 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 method of and apparatus-to-be-nsed-for,
economically extracting the inetals contained in refractory ores, particularly those
refractory ores characterised by containing admixtures of zinc and lead in the
form of sulphides, with or without silver, gold, copper or other constituents, and
having special reference to the refractory ores occuring at Broken Hill, New
South Wales, Nevada U.S.A. and elsewhere (which consist for the most part of
mechanical mixtures of argentiferous zinc blende and argentiferous galena with
more or less silica and other constituents) and are known as refractory sulphide
ores. My invention is also suitable for the treatment of zinc oxide, or of
admixtures of zinc oxide with any matrix which has no objectionable influence
upon the various operations, or for the treatment of ores or products containing
zine in any form which can be readily converted to zinc oxide.
The special features of my process consist in first oxidising the ore; second,
leaching with one or more of the materials hereinafter described; third, electro-
lysing the solutions in the manner and with the materials hereinafter described
whereby the application of electrolysis is secured at a cost very much below
anything heretofore obtained, the leaching materials are regenerated and great
economy is effected in the materials employed or required for leaching electrolysing
or treating the ore and solutions.
In carrying my improved process into effect the ore (or product) may be either
treated by the herein described process without previous mechanical separation,
and at the most economically suitable locality, or it may first be subjected to
mechanical separation or concentration whereby more suitable products for
operating on may be produced. For instance in the case of some of the Broken
Hill ores concentration may be employed to produce two products, one high in
lead and low in zinc, and one high in zinc and low in lead and both containing
silver. The former product may then be smelted or otherwise treated on the
mining field and the latter product treated by my process herein described at the
most economically suitable locality. (In the case of Broken Hill ores, probably
Newcastle in-New South Wales, or some similarly situated place or a place where
water power is available, such as Launceston in Tasmania:)

page_text: contents of the resiune avent may
then be recovered by amalgamation or hypo-sulphite leaching,
The residues which it is intended to smelt together with the due complement of
iron oxide flux for the same are preferably dried into small blocks, bricks or other
convenient shape for handling in the furnace either with or without the addition of
other substances or the aid of pressure. The iron oxide will form a valuable
binding agent whereby the masses are held together which prevents the formation.
of excessive flue dust and the choking of the furnace.
After all the iron in the solution has been precipitated and left with the ore
residues the solution now containing nothing but zinc sulphate (together with a
little of the silver contents of the ore which may have gone into solution as
sulphate of silver and any soluble impurities. formed from the ore) may be treated
for the recovery of the dissolved silver by the action of metallic zinc or by any
alkaline reagent such as lime, soda or magnesia. The solution is then, after
cooling and filtering, returned to the electrolysis vats.
Here the solution of-neutral zinc sulphate is subjected to a double electrolysis.
process whereby a portion of the zinc equivalent to that dissolved from the ore is
recovered in the metallic, form and an amount of iron sufficient to absorb the acid.
and oxygen set free from the. zinc is first dissolved in the solution in the state of
ferrous sulphate and afterwards raised to the form of ferric sulphate..
t. This electrolysis may be conducted in any number of vats arranged in sets of
three. Of each of these sets-two vats may have soluble iron anodes and the third
vat insoluble carbon anodes. All the cathodes may be alike of sheet brass, sheet
copper, or sheet zinc and all the wats in each set of three may have the same
surface exposed to the liquid and. may be traversed by the same current in-series
whereby each vat is traversed by the same quantity of current.
The first two vats (those with .iron anodes) may have the cathodes separated
from the anodes by à suitable number of diaphragms consisting of any suitable
porous material, not too porous (to allow too free diffusion of the liquids) and not
too dense (to offer too high a resistance to the passage of heavy currents)!
A practicable material may be secured in certain textured cloths, prepared asbestos,
or prepared parchment paper. These diaphragms together with the anodes and
cathode plates do not extend to the bottom of the rat or cell but allow a free space
of a few inches at the bottom of the eell out for the passage of liquid. A continuous
circulation of the liquid is then kept up while the electrolysis is in progress, by
pouring neutral sulphate of zinc solution (free from iron) continuously into the top
of the cathode compartments whence it passes down the eell put and up the anode
compartments and is continuously drawn off at the top of the latter. The solution
descending past the cathodes is depleted of a portion of its zinc and that ascending
ipast the anodes is charged with a portion of ferrous sulphate an equivalent weight
of iron being dissolved to that of the zinc deposited in these two eells vats
according to the equation. In two vats
1
f
2 ZnSO, + 2 Fe=2 FeSO, + 2 Zu.
21.9
The third vat of the series differs from the first two in that it has anodes of
carbon (or other insoluble conducting material) and the diaphragms are extendel
to the bottom of the cell and are selected with a view to minimising the diffusion
of the liquids. The solution of zinc sulphate containing. ferrous sulphate which
leaves the anodes of the two previously described cells or, vats is then circulated
continuously past the anodes of the third cell while fresh neutral zinc sulphate
solution, (free from iron) is circulated in a separated current past the cathodes.
The anode solution in this third cell may enter at the top and leave at the
bottom while the cathodes solution may enter at the bottom and be caused to
overflow into the anode compartments at the top. By this means any solution
containing iron which may have passed by diffusion through the diaphragms' an'l
become reduced from ferric to ferrous sulphate at. the cathode is again allowed
to pass by the anodes and become persulphatised by the acid and oxygen set
.

page_text: It will be seen that the whole of the iron dissolved as ferrous sulphate in the
first two, eelle vats of the series, will be converted by the same current into ferric
sulphate in the last or third eell vat of the set. As zinc is deposited tin all three
eelle vats and iron is only dissolved in two the proportion of iron used to that of
zinc obtained will be as follows
ג
3]
The use of the iron anodes, folloiced by the use of carbon anodes besides affording
a most handy means of obtaining solution-free from iron for the électrolysis and
solution charged with ferric sulphate for the leaching, very materially reduces the
electro-motive-force required to decompose the zinc sulphate and produce metallic
zinc, which means that both the consumption of fuel and the usually somewhat
large first cost of the plant are proportionately reduced whilst the oxidizing of the
ore before leaching yields solution free from iron for the electrolysis. In the third bath
also the electro-motive force required is less than would be the case were no
depolarising liquid present. The theoretical electro-motive-force required by
these baths would be 5 for, each of the first two and 1.25 for the third, that is to
say not more than a third of that required' for the decomposition of zinc sulphate
alone. In practice more than the theoretical value of the voltage must always
be employed in order to force the heavy currents through the resisting medium of
the solution and the diaphragms. It is best to employ as high current density as
possible in these baths as a better quality of zinc is secured thereby and a higher
per centage of iron in the solution which leaves the vats for the leaching apparatus.
The deleterious heating of the solution by heavy currents is also much lessened
by the employment of iron anodes. These advantages are many times greater
than the cost of the iron dissolved while the iron is all recovered in the form of
oxide in the leaching process.
For the soluble anodes, cast, wrought or scrap iron may be used or anodes may
be made from ferric oxide by the spongy iron process whereby after heating the
oxide in a close reverberatory furnace a spongy mass of metallic iron is reduced
produced and the iron in this form has a large surface and is readily acted upon by
the acid in electrolysis.
For the insoluble anodes, gas retort carbon; 'platinum or (if sufficiently permanent)
manufactured; carbon may be employed.
For the cathodes almost any metal may be used but I prefer to use stout plates
of brass or copper polished (or dipped) bright and varnished with a fine film of
beeswax dissolved in turpentine. The zinc when sufficiently thick can then be
readily removed and the cathode plate redipped and used again and again. By
using plates of small size in large numbers and constructing the bath so that each
plate can be removed without interupting operations the labour of attending to the
vats can be reduced to a minimum.
The ferric chloride process is very similar to the above except 'that the leuching
solution should not be heated too strongly. Alternate leaching and double electrolysis
are employed in precisely similar apparatus (except that the vessels, diaphragms,
cells, etc., must be made capable of resisting chloride solutions and nascent
chlorine). A solution of zinc chloride is employed which in the double electrolysis
becomes, charged first with ferrous and then with ferric chloride and depleted more
for less of zinc. In the leaching the ferric chloride acts upon the zinc oxide and
the iron is precipitated as hydroxide while the zinc.(and sometimes: a portion of
the lead) goes into solution.as; chloride.

page_text: The reactions on the zinc may be written thus
Electrolysis in two vats with-iron anodes
3+
The lead oxide in the roasted ore will also be partially converted to lead chlori le
by a precisely similar reaction and as lead chloride is slightly soluble in cold and
soluble in hot water a quantity will go into solution and must be removed before
returning the solution to the zinc electrolysis vats. This may be simply done by the
action of metallic zinc, which if placed in hot solution, will quickly throw down all
the lead present as metallic lead. A quantity of zine equal to approximately one
third weight of the lead thrown down will be dissolved as chloride of zinc and. will
enrich the solution and be again recovered in the electrolysis vats. The whole of
the lead and any soluble silver may be leached from the residues in this way by
boiling them with metallic zine and any silver or other-valuable product-remaining
with the residues may be recovered by known and suitable methods such as hypo
sulphite leaching or amalgamation or if preferred the residues containing the
undissolved lead chloride may be smelted for the recovery of the lead and silver.
The solution-after being freed from lead and silver in this or any other suitable
way is returned to the electrolysis vats as before.
It will be seen that both of these methods are continuous + ore, metallic iron,
mechanical power, and heat being supplied to the severally described apparatus and
the metals-to-be-recovered are received therefrom.
In
In either of the two methods described there will be a loss of the combining
medium employed. In the ferric sulphate process SO, and oxygen will be lost in
small quantities by leakage of solution, combination with lead and with impurities
in the ore and by loss with zinc sulphate not recovered from the wet residues.
the ferric chloride process the same causes produce loss of Cl especially if the
ehloride-of lead residues are smelted in place of being dissolved-and-decomposed
by zine These losses can be made good, and sufficient SO, + O or Cl produced to
admit of a periodical renewal of the solution in either of the following ways.
(1.) In the roasting for the ferric sulphate treatment by choosing a suit-
able furnace, temperature and duration of roast, a portion of the zinc and
lead will be produced as zinc and lead sulphates and some basic zinc sulphate,
the SO, in which salts will serve to reduce and to make good the loss
referred to.
In the roasting for the ferric chloride process a small per centage of chloride
of sodium may be added to the ore whereby a portion of the zinc and lead
will be in the form of chlorides the Cl in which will serve to reduce and make
good the loss. referred to.
(2.) In leaching with ferric-sulphate a small quantity of H₂SO. may be added
to the solution and in leaching with ferric chloride a small quantity of HCI may
be added. These acids may be obtained, H₂SO, by collecting a portion of the
SO, gases from the roasting in suitable apparatus and oxidising them, or HCl by.
acting on chloride of sodium with H,SO, so obtained.
I have found experimentally that in the ferric chloride process an improve-
ment in the quality of the deposited zinc and a very important increase in the
conductivity of the bath may be secured by the addition of a strong solution of.
chloride of sodium to the electrolyte. Now as it is well substantiated that-no
conduction of electricity can take place through a liquid without a definite

page_text: chemical decomposition proportional to the current passing occuring, it would
at first sight appear that as the chloride of sodium is not used up or permanently
altered-conduction could not be assisted in this way, A-study of the following
Penetions-which-no-doubt take place will however shew that an important-inerense
of conductivity and consequently of the rate at which the-zine can be economically
thrown down is secured
1
The total electro-motive force of the cell is unaltered the recombinations
returning the power required to effect the decompositions. The sodium chloride is
regenerated, and the current passing, and the quantity of zine reduced at the
enthodes, with a given area of plate, is greatly increased without excessive rise of
potential-at-the-vat terminals. An analogous effect could doubtless be produced
by the addition of potassium, ammonium, magnesium or other similar alkaline
chlorides to the liquid and in the electrolysis of zinc sulphate solution no doubt the
addition of potassium, sodium, ammonium etc. etc.-sulphates to the liquid would
produce a like advantage. I have not yet had the opportunity of experimentally
verifying the last named effects.
:
A modification of the foregoing ferrio-sulphate-process may in-certain-enses-be
employed-When it is desired to leach the ore directly with H,SO,, the third
or persulphatising cell of each set is replaced by an iron-anode-sell exactly-similar
to the first two. Ferrous sulphate is then formed in all the electrolysis-cells and
is afterwards separated from the liquid by crystallisation and the H,SO-recovered
therefrom-by-retorting and collecting the neid in the same manner as when ferrous
sulphate is employed for-the-commercial-maunfacture of fuming balphuric acid-
The iron-oxide then remains in the retorts. The electrical power required for
this method would be less for a given-weight of sins but the iron-dissolved-would
be greater by 50% und the expense of recovering the neid, would also be added.
A-modification of the ferrie chloride process may also in certain-cases-be-employed
namely, the raw unroasted ore may be dissolved in hydrochlorie neid and the
sulphuretted hydrogen gas given off may be burned with a limited supply of oxygen for
the production of brimstone and flowers of sulphur as is sometimes done in connection
with other chemical operations.-The-sulphur-may be collected and sold or may
-be-burned-for the production of sulphuric acid-if-desired.-The-solation-of-zin
nehloride and lead, chloride together with the residues-of-lead-chloride-may-then be
boiled with metallic-zine whereby the lend-und-any silver-in-solution-will-be thrown
down in the metullie-state and the zine chloride-solution-strengthened-by-the-zine
so-dissolved will-be-ready for electrolysis. As in the last mentioned modification
the electrolysis is carried on in-cells having all iron anodes and ferrous-chloride is
produced. —The liquid is then trented for the regeneration of the HCI for further
use from the ferrous chloride. This can be done by any of the known-chemical
methods one of which would be to treat the solution-with-H.SO, and collect-the
liberated HC in coke towers afterwards. crystalizing out the FeSO,-and-using it
as before for the manufacture by retorting of freeh H, SO, for further use, This
method has the advantages of obrinting the necessity of, roasting the ore, of
prodneing, free culeable sulphur (or if the sulphur- or the HS is burned to
prednee H₂SO, free-ferrous sulphate) and of requiring less: electrient. power thun
the ferrie chloride process already described. It has however the disadvantages of
requiring more, iron (by 60%), and the complicated and troublesome-process of
regaining the hydro-chiorie-noidai y
Therapparatus by which my improved, leaching and electrolytic treatment,is
carried onto consists essentially of first,rleaching apparatus-preferably in sora
"
: N

page_text: series-with-connections from one to the other; second, an apparatus where the
solution is filtered and cooled if necessary; third, a series of electrolytic cells or vats
through which electricity is applied to the solutions in two stages, first two
eelle outs with soluble anodes and then one eell vat with insoluble anodes; fourth,
an apparatus where the resulting solution is heated by convenient means and
fifth a special system of pipe and other connections through and between the
above apparatus providing for a continuous round of the solution employed or
portions thereof into and through the vessels named in the manner hereinbefore
described.
-2
It will be manifest that a great variety of different forms of leaching machines
apparatus, vats, cells, tanks, etc., may be used and it is not necessary to show the
details of construction as they can be supplied by the mining engineer and electrician
from common knowledge and form no part of my invention.
The accompanying sheet however represents in diagramatic form a set of
apparatus such as may be used for carrying out the ferric chloride and ferrie
sulphate processes described herein.
In the accompanying diagrams Figure 1 is a general diagramatic view of the
apparatus which may be employed.
A B C represent three vats or other apparatus used for leaching and so arranged
that the solution descends from vat to vat while the ore passes upwards being
first charged into the bottom vat and then removed step by step upwards. This
may of course be effected mechanically or the ore may be made to descend and
the solution pumped upwards.
}
D represents a reservoir where the solution is filtered and cooled if necessary after
all the iron is removed as hydroxide.
E and F represent two vats with iron anodes and G represents a vat with
insoluble anodes.
HHH represent a system of pipes or channels arranged to continuously supply
solution into all the cathode divisions from D.
K represents a system of pipes leading from the anodes of E and F and
conveying the solution (charged with ferrous sulphate) to the anodes of G.
J represents the pipes conveying the solution from the anode compartments
of G and running it into a tank L where it is first heated by any convenient source
of heat such as a steam pipe P.
M is a pump to return the solution through pipes 000 to the leaching vats.
Figure 2 is a plan of one of the vats shewing iron or carbon anodes A A,
separated from cathodes C C by diaphragms D D. The dotted line X represents
the wire from dynamo and the dotted line Y the wire to next vats in series and
thence to dynamo. The only difference in the vats is that in the first two E and F
Figure 1 iron anodes are used and the diaphragms do not extend to the bottom of
the cell while in the third vat G insoluble anodes are used and the diaphragms
go right down. The circulation of the liquid is also different as herein before
described.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what I
.claim is.
First. A process for the treatment of refractory ores (particularly those ores
named and described in the preamble hereto) characterized by the employment of
a solution of sulphate or chloride of zinc which is caused to circulate continuously
and to undergo alternately a double electrolysis and leaching, the former being
carried on, first in vats with soluble irou anodes (whereby ferrous sulphate or
chloride is imparted to the solution in small quantities relatively to the zinc present,
in place of the zine sulphate or chloride decomposed) and secondly in vats having
insoluble anodes (whereby the ferrous salts so produced are raised to the ferric
state by combination with an additional proportion of SO, or chlorine);
-solution being then heated more or less and caused to come into intimate contact
the

page_text: .
1
་
with oxidised zinc. ores. or products, whereby, the, iron, salts, are decomposed, the
iron precipitated as hydroxide and the zinc dissolved: as chloride or sulphate of zinc ;
the solution after- the: leaching, being freed from lead. (in the case of chloride
solution) or silver by metallic,zine or alkaline reagents as. described and returned
to the electrolysis vats; the silver lead residues being then either smelted, trented
with metallic zine or otherwise utilised, substantially as described and according to
the reactions herein. set. forth.
Second. A modification of the process, characterised in Claim 1 whereby the
oxidised-zine bearing ores are leached with H,SO, solution and the resulting zine
sulphate solution electrolysed in vats all having iron unodes; the H,SO, being
recoverable for further use from the ferrous sulphate formed; br retorting after
the ordinary commercial method, and the iron oxide may be used if desired as a flux
for smelting the silver lead residues or other useful purpose.
Third: Another modification of the process characterised in Claim 1 whereby
the raw zine bearing culphide ores are dissolved in hydro chloric noid (the H, gus
given-off may be-burned with a limited supply of air for the recovery of sulphur)
and the liquid and residues are boiled with metallio zive for the purpose of throwing
down the lead and silver in a metallic-state the resulting colution of zine chloride
being then electrolysed with iron anodes and the resulting ferrous-chloride-may be
treated by any of the known chemical methods for recovery of the hydrochloric
acid for further use or the hydrochloric acid may be produced by acting upon
chloride of sodium-with-sulphuric-neid.
Fourth. In electrolysis of zine chloride or zinc sulphate-solutions the addition
of alkaline chloride or sulphate to the liquid such as (in the case of zine-chlorides)
ehloride of sodium for the purpose of increasing the conducting capacity of the
solution by means of the decomposition of the alkaline salt at the anode and its
recomposition at the cathode substantially as described and according to the
reactions set forth.
Fifthecond. In the electrolysis of zinc sulphate or chloride solutions in
combination with the leaching of zine bearing ores with ferric salts the employinent
of the herein described double system of electrolysis whereby in each set of three
baths vats traversed by the same current, two have soluble anodes and produce
ferrous salts in the liquid while the third bath vat has insoluble anodes and raises
the ferrous salts produced in the first two baths vats to the ferric state the liquid
passing in its course (after being freed from iron and charged with zinc in the
leaching process) first into the cathode compartments and theace downward and up
the anode compartment of the first two cells and from the anode compartments of
these to the anode compartments of the third cell whilst in the cathode compartment
of the third cell is circulated a independent current of fresh liquid charged with
zinc and free from iron which again overflows into the anode compartment and
passes down the same and thence out of the bath vat to the leaching apparatus.
Sixth. Turd. In the leaching of zinc bearing ores with ferric salts in combination
with electrolysis the employment of the ore in a completely (or nearly completely)
oxidised state for the purpose first of trecing the solution entirely from iron at
every round enabling the solution to be used in a pure state for the deposition of
zinc and secondly of adding the resulting iron hydroxide to the residues.
Seventh Fourth. In the electrolysis of zinc sulphate or zinc chloride solutions
in combination with leaching with ferric salts the use of solutions containing a
small relative proportion of these salts periodically or continuously renewed by the
passage round of the solution thereby minimising the diffusion of iron into the
eathode compartments and preventing the contamination of the deposited zine and
waste of power consequent upon such diffusion and also enabling zine to be
recovered with the use of less dense diaphragms and thus lessening the useless
resistance of the-apparatus substantially as described.
Eighth In the treatment of refractory ores containing both lead and zine
the method of separating the lead from the zino by treating the oxidised oros with
: solution-containing ferric chloride or ferric sulphate whereby iron oxide is

page_text: precipitated and sine oxide converted to soluble zine sulphate- or chloride
substantially as described:
Ninth. In processes using the operation characterised in Claim S-or-Claim 3 the
method of throwing down the silver, or (in case of ferrie-chloride-being used as a
solvent for the oxidised ore) the method of throwing down silver and lead, in the
metallic state by means of heating the solution with metallic-zine.
Tenth The general form and construction of the electrolytic apparatus-as
fully set forth in this Specification and illustrated in the drawings herewith.
Eleventh. Fifth. In the treatment of zinc bearing sulphide ores the combination
of the following operations.
(a) Oxidising the ore.
(6) Leaching with zinc sulphate or zinc chloride solution containing ferric
sulphate or ferric chloride; the leaching being preferably conducted in two
or more stages.
(c) Electrolysing zinc sulphate or zinc chloride solution (being the product of
operation (6) first with iron anodes and then with insoluble anodes and whereby
the leaching solution is regenerated for the purpose of returning the solution to
the leaching apparatus for use again in operation (b).
Twelfth. In electrolytic treatment of ores the production and use of spongy
iron anodes formed by reducing iron oxide in a close reverberatory furnace
substantially as described:
L10th 1 T... 1004
EDGAR ARTHUR ASHCROFT,
By A. M. & Wm. Clark,
53, Chancery Lane, London, Chartered Patent Agents.