LEAD REFINING
THE REFINING OF LEAD BULLION[49]
BY F. L. PIDDINGTON
(October 3, 1903)
In presenting this account of the Parkes process of desilverizing and
refining lead bullion no originality is claimed, but I hope that a
description of the process as carried out at the works of the Smelting
Company of Australia may be of service.
_Introductory._—The Parkes process may be conveniently summarized as
follows:
1. Softening of the base bullion to remove copper, antimony, etc.
2. Removal of precious metals from the softened bullion by means of
zinc.
3. Refining the desilverized lead.
4. Liquation of gold and silver crusts obtained from operation No. 2.
5. Retorting the liquated alloy to drive off zinc.
6. Concentrating and refining bullion from No. 5.
_Softening._—This is done in reverberatory furnaces. In large works two
furnaces are used, copper, antimony, and arsenic being removed in the
first and antimony in the second. The size of the furnaces is naturally
governed by the quantity to be treated. In these works (refining about
200 tons weekly) a double set of 15-ton furnaces were at work. The
sides and ends of these furnaces are protected by a jacket with a 2-in.
water space, the jacket extending some 3 in. above the charge level and
6 in. to 9 in. below it. The furnace is built into a wrought-iron pan,
and if the brickwork is well laid into the pan there need be no fear of
lead breaking through below the jacket.
The bars of bullion (containing, as a rule, 2 to 3 per cent. of
impurities) are placed in the furnace carefully, to avoid injuring the
hearth, and melted down slowly. The copper dross separates out and
floats on top of the charge, which is stirred frequently to expose
fresh surfaces. If the furnace is overheated some dross is melted into
the lead again and will not separate out until the charge is cooled
back. However carefully the work is done some copper remains with the
lead, and its effects are to be seen in the later stages. The dross is
skimmed into a slag pot with a hole bored in it some 4 in. from the
bottom; any lead drained from the pot is returned to the charge. The
copper dross is either sent back to the blast furnace direct or may
be first liquated. By the latter method some 30 per cent. of the lead
contents of the dross is recovered in the refinery.
Base bullion made at a customer’s smelter will often vary greatly in
composition, and it is, therefore, difficult to give any hard and fast
figures as to percentage of metals in the dross. As a rule our dross
showed 65 to 70 per cent. lead, copper 2 to 9 per cent. (average 4 per
cent.), gold and silver values varying with the grade of the original
bullion, though it was difficult to detect any definite relation
between bullion and dross. It was, however, noticed that gold and
silver values increased with the percentage of copper.
Immediately the copper dross is skimmed off the heat is raised
considerably, and very soon a tin (and arsenic, if present) skimming
appears. It is quite “dry” and may be removed in an hour or so. It is a
very small skimming, and the tin, not being worth saving, is put with
the copper dross.
The temperature is now raised again and antimony soon shows in black,
boiling, oily drops, gathering in time into a sheet covering the
surface of the lead. When the skimming is about ½-inch thick, slaked
lime, ashes, or fine coal is thrown on and stirred in. The dross soon
thickens up and may be skimmed off easily. This operation is repeated
until all antimony is eliminated. Constant stirring of the charge is
necessary. The addition of litharge greatly facilitates the removal
of antimony; either steam or air may be blown on the surface of the
metal to hasten oxidation, though they have anything but a beneficial
effect on the furnace lining. From time to time samples of the dross
are taken in a small ladle, and after setting hard the sample is broken
in two. A black vitreous appearance indicates plenty of antimony yet
in the charge. Later samples will look less black, until finally a few
yellowish streaks are seen, being the first appearance of litharge.
When all antimony is out the fracture of a sample should be quite
yellow and the grain of the litharge long, a short grain indicating
impurities still present, in which case another skimming is necessary.
The analysis of a representative sample of antimony dross was as
follows:
PbO = 78.11 per cent.
Sb₂O₄ = 8.75 ” ”
As₂O₃ = 2.18 ” ”
CuO = 0.36 ” ”
CaO = 1.10 per cent.
Fe₂O₃ = 0.42 ” ”
Al₂O₃ = 0.87 ” ”
Insol. = 4.10 ” ”
Antimony dross is usually kept separate and worked up from time to
time, yielding hard antimonial lead, used for type metal, Britannia
metal, etc.
_Desilverization._—The softening being completed the charge is tapped
and run to a kettle or pan of cast iron or steel, holding, when
conveniently full, some 12 or 13 tons. The lead falling into the
kettle forms a considerable amount of dross, which is skimmed off and
returned to the softening furnace. By cooling down the charge until
it nearly “freezes” an additional copper skimming is obtained, which
also is returned to the softener. The kettle is now heated up to the
melting point of zinc, and the zinc charge, determined by the gold
and silver contents of the kettle, is added and melted. The charge
is stirred, either by hand or steam, for about an hour, after which
the kettle is allowed to cool down for some three hours and the first
zinc crust taken off. When the charge is skimmed clean a sample of the
bullion is taken for assay, and while this is being done the kettle is
heated again for the second zinc charge, which is worked in the same
way as the first; sometimes a third addition of zinc is necessary. The
resulting crusts are kept separate, the second and third being added
to the next charge as “returns,” allowing 3 lb. of zinc in returns as
equal to 1 lb. of fresh zinc. An alternative method is to take out gold
and silver in separate crusts, in which case the quantity of zinc first
added is calculated on the gold contents of the kettle only. The method
of working is the same, though subsequent treatment may differ in that
the gold crusts are cupeled direct.
As to the quantity of zinc required:
1. Extracting the gold with as little silver as possible, the following
figures were obtained:
Total Gold— Au.
In kettle 300 oz. │ 1 lb. zinc takes out 1.00 oz.
” ” 200 ” │ ” ” ” ” 1.00 ”
” ” 150 ” │ ” ” ” ” 0.79 ”
” ” 100 ” │ ” ” ” ” 0.59 ”
” ” 60 ” │ ” ” ” ” 0.45 ”
2. Silver zinking gave the following general results with 11-ton
charges:
Total Silver—
In kettle 1,450 oz. │ 1 lb. zinc takes out 5.6 oz.
” ” 1,200 ” │ ” ” ” ” 4.1 ”
” ” 930 ” │ ” ” ” ” 3.8 ”
” ” 755 ” │ ” ” ” ” 3.5 ”
” ” 616 ” │ ” ” ” ” 3.4 ”
” ” 460 ” │ ” ” ” ” 2.6 ”
3. Extracting gold and silver together:
───────────────────────────┬──────────────────────
TOTAL CONTENTS OF KETTLE │ 1 LB. ZINC TAKES OUT
AU. OZ. │ AG. OZ. │ AU. OZ. │ AG. OZ.
────────────┼──────────────┼─────────────┼────────
494 │ 3,110 │ 0.59 │ 3.60
443 │ 1,883 │ 0.64 │ 2.80
330 │ 2,417 │ 0.45 │ 3.34
204 │ 1,638 │ 0.36 │ 2.86
143 │ 1,330 │ 0.28 │ 2.65
123 │ 1,320 │ 0.23 │ 2.54
────────────┴──────────────┴─────────────┴────────
It will be noticed that in each case the richer the bullion the greater
the extractive power of zinc. Experiments made on charges of rich
bullion showed that the large amount of zinc called for by the table in
use was unnecessary, and 250 lb. was fixed on as the first addition of
zinc. On this basis an average of 237 charges gave results as follows:
───────────────────┬───────────┬──────────────────────
TOTAL CONTENTS │ ZINC USED │ 1 LB. ZINC TAKES OUT
AU. OZ. │ AG. OZ. │ LBS. │ AU. OZ. │ AG. OZ.
────────┼──────────┼───────────┼───────────┼──────────
520 │ 1,186 │ 507.5 │ 1.27 │ 2.91
────────┴──────────┴───────────┴───────────┴──────────
The zinc used was that necessary to clean the kettle, added as follows:
1st, 250 lb.; 2d (average), 127 lb.; 3d (average), 57 lb. In 112 cases
no third addition was required. From these figures it appears that in
the earlier work the zinc was by no means saturated.
_Refining the Lead._—Gold and silver being removed, the lead is
siphoned off into the refining kettle and the fire made up. In about
four hours the lead will be red hot, and when hot enough to burn zinc,
dry steam, delivered by a ¾ in. pipe reaching nearly to the bottom of
the kettle, is turned on. The charge is stirred from time to time and
wood is fed on the top to assist dezinking and prevent the formation
of too much litharge. In three to four hours the lead will be soft and
practically free from zinc. When test strips show the lead to be quite
soft and clean, the kettle is cooled down and the scum of lead and
zinc oxides skimmed off. In an hour or so the lead will be cool enough
for molding; the bar should have a yellow luster on the face when set;
if the lead is too cold it will be white, if too hot a deep blue. The
refining kettles are subjected to severe strain during the steaming
process, and hence their life is uncertain—an average would be about 60
charges; the zinking kettles, on the other hand, last very much longer.
Good steel kettles (if they can be obtained) are preferable to cast
iron.
_Treatment of Zinc Crusts._—Having disposed of the lead, let us
return now to the zinc crusts. These are first liquated in a small
reverberatory furnace, the hearth of which is formed of a cast-iron
plate (the edges of the long sides being turned up some 4 in.) laid on
brasque filling, with a fall from bridge to flue of ¾ in. per foot and
also sloping from sides to center. The operation is conducted at a low
temperature and the charge is turned over at intervals, the liquated
lead running out into a small separately fired kettle. This lead rarely
contains more than a few ounces of silver per ton; it is baled into
bars, and returned to the zinking kettles or worked up in a separate
charge. In two to three hours the crust is as “dry” as it is advisable
to make it, and the liquated alloy is raked out over a slanting
perforated plate to break it up and goes to the retort bin.
_Retorting the Alloy._—This is carried on in Faber du Faur tilting
furnaces—simply a cast-iron box swinging on trunnions and lined with
firebrick. Battersea retorts (class 409) holding 560 lb. each are
used; their average life is about 30 charges. The retorts are charged
hot, a small shovel of coal being added with the alloy. The condenser
is now put in place and luted on; it is made of ⅛ in. iron bent to
form a cylinder 12 in. in diameter, open at one end; it is lined with
a mixture of lime, clay and cement. It has three holes, one on the
upper side close to the furnace and through which a rod can be passed
into the retort, a vent-hole on the upper side away from the furnace,
and a tap-hole on the bottom for condensed zinc. In an hour or so the
flame from the vent-hole should be green, showing that distillation has
begun. When condensation ceases (shown by the flame) the condenser is
removed and the bullion skimmed and poured into bars for the cupel. The
products of retorting are bullion, zinc, zinc powder and dross. Bullion
goes to the cupel, zinc is used again in the desilverizing kettles,
powder is sieved to take out scraps of zinc and returned to the blast
furnace, or it may be, and sometimes is, used as a precipitating
agent in cyanide works; dross is either sweated down in a cupel with
lead and litharge, together with outside material such as zinc gold
slimes from cyanide works, jeweler’s sweep, mint sweep, etc., or in
the softening furnace after the antimony has been taken off. In either
case the resulting slag goes back to the blast furnace. The total
weight of alloy treated is approximately 7 per cent. of the original
base bullion. The zinc recovered is about 60 per cent. of that used
in desilverizing. The most important source of temporary loss is the
retort dross (consisting of lead-zinc-copper alloy with carbon, silica
and other impurities), and it is here that the necessity of removing
copper in the softening process is seen, since any copper comes out
with the zinc crusts and goes on to the retorts, where it enters the
dross, carrying gold and silver with it. If much copper is present the
dross may contain more gold and silver than the retort bullion itself.
In this connection I remember an occasion on which some retort dross
yielded gold and silver to the extent of over 800 and 3000 oz. per ton
respectively.
_Cupellation._—Retort bullion is first concentrated (together with
bullion resulting from dross treatment) to 50 to 60 per cent. gold and
silver in a water-jacketed cupel. The side lining is protected by an
inch water-pipe imbedded in the lining at the litharge level or by a
water-jacket, the inner face of which is of copper; the cupel has also
a water-jacketed breast so that the front is not cut down. The cupel
lining may be composed of limestone, cement, fire-clay and magnesite
in various proportions, but a simple lining of sand and cement was
found quite satisfactory. When the bullion is concentrated up to 50 to
60 per cent. gold and silver, it is baled out and transferred to the
finishing cupel, where it is run up to about 0.995 fine; it is then
ready either for the melting-pot or parting plant. The refining test,
by the way, is not water-cooled.
Re-melting is done in 200-oz. plumbago crucibles and presents no
special features. In the case of doré bullion low in gold, “sprouting”
of the silver is guarded against by placing a piece of wood or charcoal
on the surface of the metal before pouring, and any slag is kept back.
The quantity of slag formed is, of course, very small, so that the bars
do not require much cleaning.
The parting plant was not in operation in my time, and I am therefore
unable to go into details. The process arranged for was briefly as
follows: Solution of the doré bullion in H₂SO₄; crystallization
of silver as monosulphate by dilution and cooling; decomposition of
silver sulphate by ferrous sulphate solution giving metallic silver and
ferric sulphate, which is reduced to the ferrous salt by contact with
scrap iron. The gold and silver are washed thoroughly with hot water
and cast into bars.
In conclusion, some variations in practice may be noted. The use of
two furnaces in the softening process has already been mentioned; by
this means the drossing and softening are more perfect and subsequent
operations thereby facilitated; further, the furnaces, being kept at
a more equable temperature, are less subject to wear and tear. Zinc
crusts are sometimes skimmed direct into an alloy press in which
the excess of lead is squeezed out while still molten; liquation is
then unnecessary. Refining of the lead may be effected by a simple
scorification in a reverberatory, the soft lead being run into a kettle
from which it is molded into market bars.
THE ELECTROLYTIC REFINING OF BASE LEAD BULLION
BY TITUS ULKE
(October 11, 1902)
Important changes in lead-refining practice are bound to follow, in my
opinion, the late demonstration on a large scale of the low working
cost and high efficiency of Betts’ electrolytic process of refining
lead bullion. It was my good fortune recently to see this highly
interesting process in operation at Trail, British Columbia, through
the kindness of the inventor, A. G. Betts, and Messrs. Labarthe and
Aldridge, of the Trail works.
A plant of about 10 tons daily capacity, which probably cost about
$25,000, although it could be duplicated for perhaps $15,000 at the
present time, was installed near the Trail smelting works. It has been
in operation for about ten months, I am informed, with signal success,
and the erection of a larger plant, of approximately 30 tons capacity
and provided with improved handling facilities, is now completed.
The depositing-room contains 20 tanks, built of wood, lined with tar,
and approximately of the size of copper-refining tanks. Underneath the
tank-room floor is a basement permitting inspection of the tank bottoms
for possible leakage and removal of the solution and slime. A suction
pump is employed in lifting the electrolyte from the receiving tank and
circulating the solution. In nearly every respect the arrangement of
the plant and its equipment is strikingly like that of a modern copper
refinery.
The great success of the process is primarily based upon Betts’
discovery of the easy solubility of lead in an acid solution of lead
fluosilicate, which possesses both stability under electrolysis and
high conductivity, and from which exceptionally pure lead may be
deposited with impure anodes at a very low cost. With such a solution,
there is no polarization from formation of lead peroxide on the anode,
no evaporation of constituents except water, and no danger in its
handling. It is cheaply obtained by diluting hydrofluoric acid of
35 per cent. HF, which is quoted in New York at 3c. per pound, with
an equal volume of water and saturating it with pulverized quartz
according to the equation:
SiO₂ + 6HF = HSiF₆ + 2H₂O.
According to Mr. Betts, an acid of 20 to 22 per cent. will come
to about $1 per cu. ft., or to $1.25 when the solution has been
standardized with 6 lb. of lead. One per cent. of lead will neutralize
0.7 per cent. H₂SiF₆. The electrolyte employed at the time of my
inspection of the works contained, I believe, 8 per cent. lead and 11
per cent. excess of fluosilicic acid.
The anodes consist of the lead bullion to be refined, cast into plates
about 2 in. thick and approximately of the same size as ordinary
two-lugged copper anodes. Before being placed in position in the tanks,
they are straightened by hammering over a mold and their lugs squared.
No anode sacks are employed as in the old Keith process.
The cathode sheets which receive the regular lead deposits are thin
lead plates obtained by electrodeposition upon and stripping from
special cathodes of sheet steel. The latter are prepared for use by
cleaning, flashing with copper, lightly lead-plating in the tanks, and
greasing with a benzine solution of paraffin, dried on, from which the
deposited lead is easily stripped.
The anodes and cathodes are separated by a space of 1½ to 2 in. in the
tank and are electrically connected in multiple, the tanks being in
series circuit. The fall in potential between tanks is only about 0.2
of a volt, which remarkably low voltage is due to the high conducting
power of the electrolyte and to some extent to the system of contacts
used. These contacts are small wells of mercury in the bus-bars, large
enough to accommodate copper pins soldered to the iron cathodes or
clamped to the anodes. Only a small amount of mercury is required.
Current strengths of from 10 to 25 amperes per sq. ft. have been used,
but at Trail 14 amperes have given the most satisfactory results as
regards economy of working and the physical and chemical properties of
the refined metal produced.
A current of 1 ampere deposits 3.88 grams of lead per hour, or
transports 3¼ times as much lead, in this case, as copper with an
ordinary copper-refining solution. A little over 1000 kg., or 2240
lb., requires about 260,000 ampere hours. At 10 amperes per sq. ft. the
cathode (or anode) area should be about 1080 sq. ft. per ton of daily
output. Taking a layer of electrolyte 1.5 in. thick, 135 cu. ft. will
be found to be the amount between the electrodes, and 175 cu. ft. may
be taken as the total quantity of solution necessary, according to Mr.
Betts’ estimate. The inventor states that he has worked continuously
and successfully with a drop of potential of only 0.175 volt per tank,
and that therefore 0.25 volt should be an ample allowance in regular
refining. Quoting Mr. Betts; “260,000 ampere hours at 0.25 volt works
out to 87 electrical h.p. hours of 100 h.p. hours at the engine shaft,
in round numbers. Estimating that 1 h.p. hour requires the burning of
1.5 lb. of coal, and allowing say 60 lb. for casting the anodes and
refined lead, each ton of lead refined requires the burning of 210 lb.
of fuel.” With coal at $6 per ton the total amount of fuel consumed,
therefore, should not cost over 60c., which is far below the cost of
fire-refining base lead bullion, as we know.
In the Betts electrolytic process, practically all the impurities
in the base bullion remain as a more or less adherent coating on
the anode, and only the zinc, iron, cobalt and nickel present go
into solution. The anode residue consists practically of all the
copper, antimony, bismuth, arsenic, silver and gold contained in the
bullion, and very nearly 10 per cent. of its weight in lead. Having
the analysis of any bullion, it is easy to calculate with these data
the composition of the anode residue and the rate of pollution of the
electrolyte. Allowing 175 cu. ft. of electrolyte per ton of daily
output, it will be found that in the course of a year these impurities
will have accumulated to the extent of a very few per cent. Estimating
that the electrolyte will have to be purified once a year, the amount
to be purified daily is less than 1 cu. ft. for each ton of output.
The amount of lead not immediately recovered in pure form is about
0.3 per cent., most of which is finally recovered. As compared with
the ordinary fire-refined lead, the electrolytically refined lead is
much purer and contains only mere traces of bismuth, when bismuthy
base bullion is treated. Furthermore, the present loss of silver in
fire refining, amounting, it is claimed, to about 1½ per cent. of the
silver present, and covered by the ordinary loss in assay, is to a
large extent avoided, as the silver in the electrolytic process is
concentrated in the anode residue with a very small loss, and the loss
of silver in refining the slimes is much less than in treating the
zinc crusts and refining the silver residue after distillation. The
silver slimes obtained at Trail, averaging about 8000 oz. of gold and
silver per ton, are now treated at the Seattle Smelting and Refining
Works. There the slimes are boiled with concentrated sulphuric acid and
steam, allowing free access of air, which removes the greater part of
the copper. The washed residue is then dried in pans over steam coils,
and melted down in a magnesia brick-lined reverberatory, provided
with blast tuyeres, and refined. In this reverberatory furnace the
remainder of the copper left in the slimes after boiling is removed by
the addition of niter as a flux, and the antimony with soda. The doré
bars finally obtained are parted in the usual way with sulphuric acid,
making silver 0.999 fine and gold bars at least 0.992 fine.
Mr. Betts treated 2000 grams of bullion, analyzing 98.76 per cent. Pb,
0.50 Ag, 0.31 Cu, and 0.43 Sb with a current of 25 amp. per square
foot in an experimental way, and obtained products of the following
composition:
Refined Lead: 99.9971 per cent. Pb, 0.0003 Ag, 0.0007 Cu, and 0.0019 Sb.
Anode Residue: 9.0 per cent. Pb, 36.4 Ag, 25.1 Cu, and 2.95 Sb.
Four hundred and fifty pounds of bullion from the Compania Metalurgica
Mexicana, analyzing 0.75 per cent. Cu, 1.22 Bi, 0.94 As, 0.68 Sb, and
assaying 358.9 oz. Ag and 1.71 oz. Au per ton, were refined with a
current of 10 amp. per square foot, and gave a refined lead of the
following analysis: 0.00027 per cent. Cu, 0.0037 Bi, 0.0025 As, 0 Sb,
0.0010 Ag, 0.0022 Fe, 0.0018 Zn and Pb (by difference) 99.9861 per cent.
Although the present method for recovering the precious metals and
by-products from the anode residue leaves much room for improvement,
the use of the Betts process may be recommended to our lead refiners,
because it is a more economical and efficient method than the
fire-refining process now in common use. I will state my belief, in
conclusion, that the present development of electrolytic lead refining
signalizes as great an advance over zinc desilverization and the fire
methods of refining lead as electrolytic copper refining does over the
old Welsh method of refining that metal.
ELECTROLYTIC LEAD-REFINING[50]
BY ANSON G. BETTS
A solution of lead fluosilicate, containing an excess of fluosilicic
acid, has been found to work very satisfactorily as an electrolyte
for refining lead. It conducts the current well, is easily handled
and stored, non-volatile and stable under electrolysis, may be made
to contain a considerable amount of dissolved lead, and is easily
prepared from inexpensive materials. It possesses, however, in common
with other lead electrolytes, the defect of yielding a deposit of lead
lacking in solidity, which grows in crystalline branches toward the
anodes, causing short circuits. But if a reducing action (practically
accomplished by the addition of gelatine or glue) be given to the
solution, a perfectly solid and dense deposit is obtained, having very
nearly the same structure as electrolytically deposited copper, and a
specific gravity of about 11.36, which is that of cast lead.
Lead fluosilicate may be crystallized in very soluble brilliant
crystals, resembling those of lead nitrate and containing
four molecules of water of crystallization, with the formula
PbSiF₆,4H₂O. This salt dissolves at 15 deg. C. in 28 per cent. of
its weight of water, making a syrupy solution of 2.38 sp. gr. Heated
to 60 deg. C., it melts in its water of crystallization. A neutral
solution of lead fluosilicate is partially decomposed on heating, with
the formation of a basic insoluble salt and free fluosilicic acid,
which keeps the rest of the salt in solution. This decomposition ends
when the solution contains perhaps 2 per cent. of free acid; and the
solution may then be evaporated without further decomposition. The
solutions desired for refining are not liable to this decomposition,
since they contain much more than 2 per cent. of free acid. The
electrical conductivity depends mainly on the acidity of the solution.
My first experiments were carried out without the addition of gelatine
to the fluosilicate solution. The lead deposit consisted of more or
less separate crystals that grew toward the anode, and, finally, caused
short circuits. The cathodes, which were sheet-iron plates, lead-plated
and paraffined, had to be removed periodically from the tanks and
passed through rolls, to pack down the lead. When gelatine has been
added in small quantities, the density of the lead is greater than can
be produced by rolling the crystalline deposit, unless great pressure
is used.
The Canadian Smelting Works, Trail, B. C. , have installed a refinery,
making use of this process. There are 28 refining-tanks, each 86 in.
long, 30 in. wide and 42 in. deep, and each receiving 22 anodes of
lead bullion with an area of 26 by 33 in. exposed to the electrolyte
on each side, and 23 cathodes of sheet lead, about 1/16 in. thick,
prepared by deposition on lead-plated and paraffined iron cathodes. The
cathodes are suspended from 0.5 by 1 in. copper bars, resting crosswise
on the sides of the tanks. The experiment has been thoroughly tried of
using iron sheets to receive a deposit thicker than 1/16 in.; that is,
suitable for direct melting without the necessity of increasing its
weight by further deposition as an independent cathode; but the iron
sheets are expensive, and are slowly pitted by the action of the acid
solution; and the lead deposits thus obtained are much less smooth and
pure than those on lead sheets.
The smoothness and the purity of the deposited lead are proportional.
Most of the impurity seems to be introduced mechanically through the
attachment of floating particles of slime to irregularities on the
cathodes. The effect of roughness is cumulative; it is often observed
that particles of slime attract an undue amount of current, resulting
in the lumps seen in the cathodes. Samples taken at the same time
showed from 1 to 2.5 oz. silver per ton in rough pieces from the iron
cathodes, 0.25 oz. as an average for the lead-sheet cathodes, and only
0.04 oz. in samples selected for their smoothness. The variation in
the amount of silver (which is determined frequently) in the samples
of refined lead is attributed not to the greater or less turbidity of
the electrolyte at different times, but to the employment of new men in
the refinery, who require some experience before they remove cathodes
without detaching some slime from the neighboring anodes.
Each tank is capable of yielding, with a current of 4000 amperes,
750 lb. of refined lead per day. The voltage required to pass this
current was higher than expected, as explained below; and for this
reason, and also because the losses of solution were very heavy until
proper apparatus was put in to wash thoroughly the large volume of
slime produced (resulting in a weakened electrolyte), the current used
has probably averaged about 3000 amperes. The short circuits were
also troublesome, though this difficulty has been greatly reduced by
frequent inspection and careful placing of the electrodes. At one time,
the solution in use had the following composition in grams per 100
c.c.: Pb, 6.07; Sb, 0.0192; Fe, 0.2490; SiF₆, 6.93, and As, a trace.
The current passing was 2800 amperes, with an average of about 0.44
volts per tank, including bus-bars and contacts. It is not known what
was the loss of efficiency on that date, due to short circuits; and
it is, therefore, impossible to say what resistance this electrolyte
constituted.
Hydrofluoric acid of 35 per cent., used as a starting material for the
preparation of the electrolyte, is run by gravity through a series of
tanks for conversion into lead fluosilicate. In the top tank is a layer
of quartz 2 ft. thick, in passing through which the hydrofluoric acid
dissolves silica, forming fluosilicic acid. White lead (lead carbonate)
in the required quantity is added in the next tank, where it dissolves
readily and completely with effervescence. All sulphuric acid and any
hydrofluoric acid that may not have reacted with silica settle out
in combination with lead as lead sulphate and lead fluoride. Lead
fluosilicate is one of the most soluble of salts; so there is never
any danger of its crystallizing out at any degree of concentration
possible under this method. The lead solution is then filtered and run
by gravity into the refining-tanks.
The solution originally used at Trail contained about 6 per cent. Pb
and 15 per cent. SiF₆.
The electrical resistance in the tanks was found to be greater than
had been calculated for the same solution, plus an allowance for
loss of voltage in the contacts and conductors. This is partly, at
least, due to the resistance to free motion of the electrolyte, in
the neighborhood of the anode, offered by a layer of slime which may
be anything up to ½ in. thick. During electrolysis, the SiF₆ ions
travel toward the anodes, and there combine with lead. The lead and
hydrogen travel in the opposite direction and out of the slime; but
there are comparatively few lead ions present, so that the solution
in the neighborhood of the anodes must increase in concentration and
tend to become neutral. This greater concentration causes an e.m.f. of
polarization to act against the e.m.f. of the dynamo. This amounted
to about 0.02 volt for each tank. The greater effect comes from the
greater resistance of the neutral solution with which the slime is
saturated. There is, consequently, an advantage in working with rather
thin anodes, when the bullion is impure enough to leave slime sticking
to the plates. A compensating advantage is found in the increased ease
of removing the slime with the anodes, and wiping it off the scrap in
special tanks, instead of emptying the tanks and cleaning out, as is
done in copper refineries.
It is very necessary to have adequate apparatus for washing solution
out of the slime. The filter first used consisted of a supported
filtering cloth with suction underneath. It was very difficult to
get this to do satisfactory work by reason of the large amount of
fluosilicate to be washed out with only a limited amount of water.
At the present time the slime is first stirred up with the ordinary
electrolyte several times, and allowed to settle, before starting to
wash with water at all. The Trail plant produces daily 8 or 10 cu. ft.
of anode residue, of which over 90 per cent. by volume is solution.
The evaporation from the total tank surface of something like 400 sq.
ft. is only about 15 cu. ft. daily; so that only a limited amount of
wash-water is to be used—namely, enough to replace the evaporated
water, plus the volume of the slime taken out.
The tanks are made of 2 in. cedar, bolted together and thoroughly
painted with rubber paint. Any leakage is caught underneath on sloping
boards. Solution is circulated from one tank to another by gravity, and
is pumped from the lowest to the highest by means of a wooden pump. The
22 anodes in each tank together weigh about 3 tons, and dissolve in
from 8 to 10 days, two sets of cathodes usually being used with each
set of anodes. While 300 lb. cathodes can be made, the short-circuiting
gets so troublesome with the spacing used that the loss of capacity is
more disadvantageous than the extra work of putting in and taking out
more plates. The lead sheets used for cathodes are made by depositing
about 1/16 in. metal on paraffined steel sheets in four of the tanks,
which are different from the others only in being a little deeper.
The anodes may contain any or all of the elements, gold, silver,
copper, tin, antimony, arsenic, bismuth, cadmium, zinc, iron, nickel,
cobalt and sulphur. It would be expected that gold, silver, copper,
antimony, arsenic and bismuth, being more electronegative than lead,
would remain in the slime in the metallic state, with, perhaps, tin,
while iron, zinc, nickel and cobalt would dissolve. It appears that tin
stands in the same relation to lead that nickel does to iron, that is,
they have about the same electromotive forces of solution, with the
consequence that they can behave as one metal and dissolve and deposit
together. Iron, contrary to expectation, dissolves only slightly, while
the slime will carry about 1 per cent. of it. It appears from this that
the iron exists in the lead in the form of matte. Arsenic, antimony,
bismuth and copper have electromotive forces of solution more than 0.3
volt below that of lead. As there is no chance that any particle of
one of these impurities will have an electric potential of 0.3 volt
above that of the lead with which it is in metallic contact, there is
no chance that they will be dissolved by the action of the current. The
same is even more certainly true of silver and gold. The behavior of
bismuth is interesting and satisfactory. It is as completely removed by
this process of refining as antimony is. No other process of refining
lead will remove this objectionable impurity so completely. Tin has
been found in the refined lead to the extent of 0.02 to 0.03 per cent.
This we had no difficulty in removing from the lead by poling before
casting. There is always a certain amount of dross formed in melting
down the cathodes; and the lead oxide of this reacts with the tin in
the lead at a comparatively low temperature.
The extra amount of dross formed in poling is small, and amounts to
less than 1 per cent. of the lead. The dross carries more antimony and
arsenic than the lead, as well as all the tin. The total amount of
dross formed is about 4 per cent. Table I shows its composition.
The electrolyte takes up no impurities, except, possibly, a small part
of the iron and zinc. Estimating that the anodes contain 0.01 per
cent. of zinc and soluble iron, and that there are 150 cu. ft. of the
solution in the refinery for every ton of lead turned out daily, in
one year the 150 cu. ft. will have taken up 93 lb. of iron and zinc,
or about 1 per cent. These impurities can accumulate to a much greater
extent than this before their presence will become objectionable. It
is possible to purify the electrolyte in several ways. For example,
the lead can be removed by precipitation with sulphuric acid, and
the fluosilicic acid precipitated with salt as sodium fluosilicate.
By distillation with sulphuric acid the fluosilicic acid could be
recovered, this process, theoretically, requiring but one-third as much
sulphuric acid as the decomposition of fluorspar, in which the fluorine
was originally contained.
The only danger of lead-poisoning to which the workmen are exposed
occurs in melting the lead and casting it. In this respect the
electrolytic process presents a distinct sanitary advance.
For the treatment of slime, the only method in general use consists in
suspending the slime in a solution capable of dissolving the impurities
and supplying, by a jet of steam and air forced into the solution, the
air necessary for its reaction with, and solution of, such an inactive
metal as copper. After the impurities have been mostly dissolved, the
slime is filtered off, dried and melted, under such fluxes as soda, to
a doré bullion.
The amount of power required is calculated thus: Five amperes in 24
hours make 1 lb. of lead per tank. One ton of lead equals 10,000
ampere-days, and at 0.35 volts per tank, 3500 watt-days, or 4.7
electric h.p. days. Allowing 10 per cent. loss of efficiency in the
tanks (we always get less lead than the current which is passing would
indicate), and of 8 per cent. loss in the generator, increases this to
about 5.6 h.p. days, and a further allowance for the electric lights
and other applications gives from 7 to 8 h.p. days as about the amount
per ton of lead. At $30 per year, this item of cost is something like
65c. per ton of lead. So this is an electro-chemical process not
especially favored by water-power.
The cost of labor is not greater than in the zinc-desilverization
process. A comparison between this process and the Parkes process, on
the assumption that the costs for labor, interest and general expenses
are about equal, shows that about $1 worth of zinc and a considerable
amount of coal and coke have been done away with, at the expense
of power, equal to about 175 h.p. hours, of the average value of
perhaps 65c., and a small amount of coal for melting the lead in the
electrolytic method.
More important, however, is the greater saving of the metal values by
reason of increased yields of gold, silver, lead, antimony and bismuth,
and the freedom of the refined lead from bismuth.
Tables II, III, and IV show the composition of bullion, slimes and
refined lead.
Tables V, VI, VII, and VIII give the results obtained experimentally in
the laboratory on lots of a few pounds up to a few hundred pounds. The
results in Tables VI and VII were given me by the companies for which
the experiments were made.
TABLE I.—ANALYSES OF DROSS
For analyses of the lead from which this dross was taken, see Table II
───┬──────┬─────────┬─────────┬─────────┬─────────┬─────
│NO. IN│ │ │ │ │
NO.│TABLE │ CU. │ AS. │ SB. │ FE. │ZN.
│ II. │PER CENT.│PER CENT.│PER CENT.│PER CENT.│
───┼──────┼─────────┼─────────┼─────────┼─────────┼─────
1 │ 2 │ 0.0005 │ 0.0003 │ 0.0016 │ 0.0016 │none
2 │ 3 │ 0.0010 │ 0.0008 │ 0.0107 │ 0.0011 │ “
───┴──────┴─────────┴─────────┴─────────┴─────────┴─────
TABLE II.—ANALYSES OF BULLION
───┬─────────┬─────────┬─────────┬─────────┬────────
NO.│ FE. │ CU. │ SB. │ SN. │ AS.
│PER CENT.│PER CENT.│PER CENT.│PER CENT.│PER CENT.
───┼─────────┼─────────┼─────────┼─────────┼─────────
1 │ 0.0075 │ 0.1700 │ 0.5400 │ 0.0118 │ 0.1460
2 │ 0.0115 │ 0.1500 │ 0.6100 │ 0.0158 │ 0.0960
3 │ 0.0070 │ 0.1600 │ 0.4000 │ 0.0474 │ 0.1330
4 │ 0.0165 │ 0.1400 │ 0.7000 │ 0.0236 │ 0.3120
5 │ 0.0120 │ 0.1400 │ 0.8700 │ 0.0432 │ 0.2260
6 │ 0.0055 │ 0.1300 │ 0.7300 │ 0.0316 │ 0.1030
7 │ 0.0380 │ 0.3600 │ 0.4030 │ │ tr.
───┴─────────┴─────────┴─────────┴─────────┴─────────
───┬─────────┬─────────┬─────────┬─────────┬─────────
NO.│ AG. │ AU. │ PB. │ AG. │ AU.
│PER CENT.│PER CENT.│PER CENT.│OZ. P. T.│OZ. P. T.
───┼─────────┼─────────┼─────────┼─────────┼─────────
1 │ 1.0962 │ 0.0085 │ 98.0200 │ 319.7 │ 2.49
2 │ 1.2014 │ 0.0086 │ 97.9068 │ 350.4 │ 2.52
3 │ 1.0738 │ 0.0123 │ 98.1665 │ 313.2 │ 3.6
4 │ 0.8914 │ 0.0151 │ 97.9014 │ 260.0 │ 4.42
5 │ 0.6082 │ 0.0124 │ 98.0882 │ 177.4 │ 3.63
6 │ 0.6600 │ 0.0106 │ 98.2693 │ 192.5 │ 3.10
7 │ 0.7230 │ 0.0180 │ 98.4580 │ 210.9 │ 5.25
───┴─────────┴─────────┴─────────┴────────────────────
TABLE III.—ANALYSES OF SLIMES
─────────┬─────────┬─────────┬─────────┬─────────┬─────┬────┬─────
FE. │ CU. │ SB. │ SN. │ AS. │ PB. │ZN. │BI.
PER CENT.│PER CENT.│PER CENT.│PER CENT.│PER CENT.│ │ │
─────────┼─────────┼─────────┼─────────┼─────────┼─────┼────┼─────
1.27 │ 8.83 │ 27.10 │ 12.42 │ 28.15 │17.05│none│none
1.12 │ 22.36 │ 21.16 │ 5.40 │ 23.05 │10.62│ “ │ “
─────────┴─────────┴─────────┴─────────┴─────────┴─────┴────┴─────
TABLE IV.—ANALYSES OF REFINED LEAD
───┬───────┬───────┬───────┬───────┬──────┬───────┬──────┬──────┬─────
│ │ │ │ │ │ │ │ NI, │
│ CU. │ AS. │ SB. │ FE. │ ZN. │ SN. │ AG. │CO, CD│ BI.
NO.│ PER │ PER │ PER │ PER │ PER │ PER │ OZ. │ PER │ PER
│ CENT. │ CENT. │ CENT. │ CENT. │ CENT.│ CENT. │ P. T.│ CENT.│CENT.
───┼───────┼───────┼───────┼───────┼──────┼───────┼──────┼──────┼─────
1 │0.0006 │0.0008 │0.0005 │ │ │ │ │ │
2 │0.0003 │0.0002 │0.0010 │0.0010 │ none │ │ │ │
3 │0.0009 │0.0001 │0.0009 │0.0008 │ ” │ │ 0.24 │ │
4 │0.0016 │ │0.0017 │0.0014 │ │ │ 0.47 │ none │
5 │0.0003 │ │0.0060 │0.0003 │ │ │ 0.22 │ │
6 │0.0020 │ │0.0010 │0.0046 │ │ │ 0.22 │ none │
7 │0.0004 │ none │0.0066 │0.0013 │ none │0.0035 │ 0.14 │ │
8 │0.0004 │ │0.0038 │0.0004 │ ” │0.0035 │ 0.25 │ │
9 │0.0005 │ │0.0052 │0.0004 │ ” │0.0039 │ 0.28 │ │
10 │0.0003 │ none │0.0060 │0.0003 │ ” │0.0049 │ 0.43 │ │
11 │0.0003 │ ” │0.0042 │0.0013 │ ” │0.0059 │ 0.32 │ │
12 │0.0005 │ ” │0.0055 │0.0009 │ ” │0.0049 │ 0.22 │ │
13 │0.0005 │ ” │0.0055 │0.0007 │ ” │0.0091 │ 0.11 │ │
14 │0.0004 │ ” │0.0063 │0.0005 │ ” │0.0012 │ 0.14 │ │
15 │0.0003 │ ” │0.0072 │0.0003 │ ” │0.0024 │ 0.24 │ │
16 │0.0006 │ ” │0.0062 │0.0012 │ ” │0.0083 │ 0.22 │ │
17 │0.0006 │ ” │0.0072 │0.0011 │ │0.0080 │ 0.23 │ │
18 │0.0006 │ ” │0.0057 │0.0010 │ │0.0053 │ 0.34 │ │
19 │0.0005 │ ” │0.0066 │0.0016 │ │0.0140 │ 0.38 │ │
19 │0.0005 │ ” │0.0044 │0.0011 │ │0.0108 │ 0.35 │ │
20 │0.0004 │ ” │0.0047 │0.0015 │ │0.0072 │ 0.22 │ │
20 │0.0004 │ ” │0.0034 │0.0016 │ │ trace │ 0.23 │ │
21 │0.0022 │ ” │0.0010 │0.0046 │ none │0.0081 │ 0.38 │ none │ none
───┴───────┴───────┴───────┴───────┴──────┴───────┴───────────────────
TABLE V.—ANALYSES OF BULLION AND REFINED LEAD
──────────────┬───────────┬───────────┬───────────┬──────────
│ AG. │ CU. │ SB. │ PB.
│ PER CENT. │ PER CENT. │ PER CENT. │ PER CENT.
──────────────┼───────────┼───────────┼───────────┼───────────
Bullion │ 0.50 │ 0.31 │ 0.43 │ 98.76
Refined lead │ 0.0003 │ 0.0007 │ 0.0019 │ 99.9971
──────────────┴───────────┴───────────┴───────────┴───────────
TABLE VI.—ANALYSES OF BULLION AND REFINED LEAD
────────┬──────┬──────┬──────┬──────┬──────┬──────┬─────┬──────┬──────
│ CU. │ BI. │ AS. │ SB. │ AG. │ AG. │ AU. │ FE. │ ZN.
│ PER │ PER │ PER │ PER │ PER │ PER │ PER │ PER │ PER
│ CT. │ CT. │ CT. │ CT. │ CT. │ CT. │ CT. │ CT. │ CT.
────────┼──────┼──────┼──────┼──────┼──────┼──────┼─────┼──────┼──────
Bullion │0.75 │1.22 │0.936 │0.6832│358.89│ │1.71 │ │
Refined │ │ │ │ │ │ │ │ │
lead │0.0027│0.0037│0.0025│0.0000│ │0.0010│none │0.0022│0.0018
────────┴──────┴──────┴──────┴──────┴──────┴──────┴─────┴──────┴──────
TABLE VII.—ANALYSES OF BULLION, REFINED LEAD AND SLIMES
────────────┬─────┬──────┬───────┬───────┬───────┬───────┬──────┬─────
│ │ │ │ │ │ │FE,ZN,│
│ PB. │ CU. │ AS. │ SB. │ AG. │ AG. │NI,CO.│ BI.
│ PER │ PER │ PER │ PER │ OZ. │ PER │ PER │
│CENT.│ CENT.│ CENT. │ CENT. │ Per T.│ CENT. │ CENT.│
────────────┼─────┼──────┼───────┼───────┼───────┼───────┼──────┼─────
│ │ │ │ │about │ │ │
Bullion │96.73│0.096 │0.85 │ 1.42 │275[51]│ │ │
Refined lead│ │0.0013│0.00506│ 0.0028│ │0.00068│0.0027│trace
Slimes (dry │ │ │ │ │ │ │ │
sample) │ 9.05│1.9 │9.14 │29.51 │9366.9 │ │0.49 │trace
────────────┴─────┴──────┴───────┴───────┴───────┴───────┴──────┴─────
TABLE VIII.—ANALYSES OF BULLION, REFINED LEAD AND SLIMES
────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────
│ PB. │ CU. │ BI. │ AG. │ SB. │ AS.
│PER CENT.│PER CENT.│PER CENT.│PER CENT.│PER CENT.│PER CENT.
────────┼─────────┼─────────┼─────────┼─────────┼─────────┼─────────
Bullion │ 87.14 │ 1.40 │ 0.14 │ 0.64 │ 4.0 │ 7.4
Lead │ │ 0.0010 │ 0.0022 │ │ 0.0017 │ trace
Slimes │ 10.3 │ 9.3 │ 0.52 │ 4.7 │ 25.32 │ 44.58
────────┴─────────┴─────────┴─────────┴─────────┴─────────┴─────────