Note: Descriptions are shown in the official language in which they were submitted.
3~
F I ELD OF I NYENT I ON
This invention relates to a process for recovering
selected metal values from waste waters using sewaye type
bacteria to imbibe and capture and concentrate the metal values
in a sludge, followed by de-watering and burning off the sludge
to elimina~e organic matter and to further concentrate the
metal values, and then treating by pyrometallurgical or hydro-
metallurgical steps to recover the desired metals.
BACKGROUND-AND~PRIOR A~T
_.
; 10 ~t has:been common knowledge ~or many years that -traces
of gold, silver, platinum ~d other valuable metals are found in
natural waters such as sea water and fresh water lakes and
streams, and that these traces are present together with con-
siderably larger quantities of other non-precious metals such as
silicon, iron, magnesium and copper. In U.S. Patent 2,086,384
to Lady the chemical recovery of these metals which are found
in natural waters is discussed. U.S. Patent 3,819,363 to
wanzen~erg suggests a process for t~e recover~ of precious
e'cals from the sediment in sea water, the precious metal tak-
ing the form of organometallic precious metal values which
are contained in organic underwater deposits on the surface of
inorganïc meterials such as s-and, peb~les or shells, hereinafter
referred to as debris, which is dredged and then treated accord-
ing to the Wanzenberg process. The steps in that process in-
` clude detaching the organic phase from the inorganic debris by
treating with benzëne, and then recovering the organic materials
bY floating them as a froth to separate them from the inorganic
ebris which sinks~ The recovered organic froth is then ahemic-
. ally oxidized to spontaneously combust organic components leav-
ing the metallic values, which are subsequently treated by
metal-winning steps of a conventional nature.
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The present disclosure cli~fer5 from the Wanzenbexg
process because the present disc~osure teaches steps which
do not appear in the Wanzenberg process, and because the
Wanzenberg process relies upon steps that would b~ inoper~
ative in the presently disclosed process. In the present
disclosure active bacteria is initially introduced to imbibe,
concentrate, and immobilize, metal values to remove them
from the waste waters wherein such metal values appear in
very low concentrations, thereby capturing ~hem in an
organic phase.
Conversely, Wanzenberg is not concerned with those
precious metal values which are carried in the waters,
and therefore his process does not teach a method for im
mobilizing these precious metal ~alues and extracting them
' ' : :-
from the waters themselves. Instead, Wanzenberg teaches a
- process for separation of a precious metal-bearing ~rganic
deposit from associated,inorganic debris using benzene
,:~-
' which serves the dual purpose of accomplishing detachment
~, , of t~e organic deposit or coating, and al50 of forming a `,'~
buoyant froth in which the organic matter containing the
metallic values is floated free from the debris and made '~
easily recoverable from the water. Treatment of the ,'~
Wanzenberg type with benzene applied to the mixed organic/
-~' inorganic sewage sludge which occurs at one stage in the ';
steps of the present process was tried, but in the present
process this Wanzenberg step was found to achieve no
` useful separation.
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It is known that certain rather speciali~ed bacteria
can be used for the recovery of various metals including
precious metals from mixtures containing them, U.S. Patent
2,829,964 to Zimmerley appearing to be the first to suggest
the use of such a hiological process employing ferrooxidans
for dissolving sulfide minerals and for changing ferrous sulfate
tO ferric sulfate. Certa~n other patents~ teach the use of
ferrooxidans as oxidizing bacteria'useful in the recovery o~
certaln metals such as copper from solutions containing them.
~hese patents include the following U.S. patents: 3,252,756
t Goren; 3,266,88~ to Duncan et al; 3,305,353 to Duncan et al;
3,347,661 to Mayline; 3,6U7,235 to Duncan et al; and 3,679,397
to O'Connor et al.
Another strain of bacteria referred to as denitrificans
is used in the recovery of other metals from salts as taught in
U.S. Patents 3,105,014 to Harrison and 3,272,621 to Zajle.
Still another strain of bacteria known as thiooxidan is dis-
cussed in U.S. Patents 3,433,629 to Imai et al and in 3,455,6~9
t Mayling. In each of these patents the process is essentially
One of leaching to obtain selected metals in the form of other
comPounds which can be further treated.
However, none of these patents suggests the use of
bacterial imbibing to capture metal values followed by burning
Off of the organic phase, for instance by incineration per se,
Or incident to smelting, to concentrate the metal bearing
components.
There are other patents using biological treatment steps
which are of interest in connection with the present subject
matter since they involve the use of an activated sludge in the
recovery of metals. U.S. Patent 3,218,252 to Glover et al
teaches the use of an activated sludge process for
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1~9~36C~
the bacteriological oxidation of ferrous salts in an acid
Solution in which the bacteria is a ferrooxidan. The object
in the Glover process is not to recover the iron per se, bùt to
remove from mine waters metal compounds which are considered
to be pollutants. The separation of the oxidized metal is
accomplished by precipitation, after which part of the acti~
vated sludge is returned to the input stage of the process
for recirculation with new mine waters being introduced there-
into. Thus, the metallic component can be separated from the
oxganic component by precipitation because it can be differ-
entiated by physical properties, the organic matter passing off
with the treated mine waters.
Still another technique involves the recovery of silver
from a solution containing silver halide, where the silver is
in a gelatin, by using the step of fermentation of the gelatin
- in contact with an aerobic bacteria. An example of this pro-
cess is U.S. Patent 3,501,378 to Shinkai.
U.S. Patent 3,537,986 to Watanabe et al is a patent
teaching the use of activated sludge to reduce the amount of -~
coagulant needed to recover silver halide from a solution in-
cluding gelatin by oxidizing and decomposing the gelatin, `~
whereupon the silver halide is adsorbed on the sludge which
is then precipitated, leaving supernatent liquid also con-
taining unprecipitated silver halide. This patent contains `-
no suggestion that bacterial action in waste waters would
capture and immobilize precious metal values appearing in the
waters in such dilute concentrations as to be unrecoverable
by the use of coagulants. It contains no teaching of incin-
- eration in order to concentrate the metallic component, and it
fails to teach the inorganic metal-winning steps which are
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required to achieve recovery of the precious metal values in
the presence of much greater concentrations of the metallic
values appearing as contaminants.
U.S. Patent 3,755,530 to A~ila et al is not a biological
process, but seeks to recover metals from plating solutions
by a freeze-drying method which involves sublimation of the
frozen solution followed by roasting o~ the dried agglomerates,
the roasting serving the purpose of decomposing metallic salts,
rather than serving the purpose of concentrating such salts by
removal of organic flocculents with which the metal-bearing
values are mixed as is the case in the present process.
Canadian Patent 9~3,722, dated Feb. 17, 1976, Matyas
- Korosi and issued to Eastman Kodak Company, teaches the re-
cOvery of silver from the waste waters of emulsion manufacture
containing gelatin by treating the waters with a certain en-
zyme which reacts with the gelatin to form soluble peptides;
then acid treating to effect precipitation of silver metal or
compounds; then separating a concentrate of silver metal. The ;specification mentions incineration as a part of the silver -
recOvery process.
With regard to treatment of the metal bearing ash to
recover the desired metals therefrom, there are a great many
~; prior art teachings including various different approaches,
comprising pyrometallurgical and hydrometallurgical techni~ues
which are typical of the metal-winning steps in the present
process. These steps are of course non-biological and gener-
ally involve inorganic chemical recovery of the metals.
THE INVENTION ~ ~;
According to the invention there is provided a process
` 30 for the recovery of selected metal values from water in which
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the selected metal values each appear in a concentration
- of 0.1 - 5~0 ppB together with other metal values which each
appear in a greater concentration~ and in which said water has
mixed therewith a bacteria containing organic material, said
process aomprising the steps of: c:apturing metal values from
said water by bacterial imbibing of said values to immobilize
and concentrate the values into said organic material; co-
agulating and de-watering said oryanic matter, thereby separ-
ating said organic matter as a sludge from said water; burning
Off the de-watered sludge to eliminate the organic matter and
orm an ash including said selected metal values and said other
metal values; and recovering said selected metal values from
said ash.
The water in which the selected metal values appear may -
be a mixture of natural surface water and industrial waste
water. ;~
The selected metal values are preferably recovered
from said ash by dissolving them and removing them from the ash ~;~
and purifying the dissolved metal values to isolate and recover
2 said selected metal values. `~
It is of advantage if the organic material is a sludge
activated with aerobic bacteria and the mixture of said water
and the sludge is aerated to sustain growth of the bacteria. ~`
Alternatively the organic material may contain anaerobic ~ -
bacteria. ~ ;
The selected metal values can be recovered ~rom the
ash if it is smelted with the ore of a base metal to obtain an
impure mass of the base metal in which said selected metal
values are dissolved, and said impure mass is refined to re-
3~ cover therefrom the selected metal values. Copper sulphide
3~;(3
Ore or lead oxide ore may be used. This process is of partic-
ular advantage if the selected metal valu~s are pre~ious
metals. Thus the mass is, for example, refined and formed into
an anode; the anode is electrolytically separated into a pure
metal cathode and a mud containinq precious metals, and the mud
is further refined to recover the precious metals. Alternatively,
where lead oxide ore is used, the impure mass is preferably
refined by the addition of zinc to form a ~inc crust containing
the selected metal valu~s, and the crust is further refined to
recover the precious metals therefrom.
The selected metal values may be metals which form
soluble cyanides in which case the ash may be treated to con-
vert said selected metal values into dissolved cyanides and
the selected metal values are recovered from their cyanide
solution by precipitation. The precipitation agent may be,
for example, the dust of a non-precious metal if precious metals
are to be recovered.
The step of bacterial imbibing of said metal values is
achieved by bacterial digestion, preferably prior to the
coagulating and separating step.
In a bacterial system in which an activated sludge is
separated from the water and then incinerated to burn off
Organic waste materials, the concentration factor of the trace
etal values entering with the sewage and the water from the
city water supply is up to about 40,000:1 when comparing the
concentration of the metal values in influent waste water with
the concentration in the ash of incinerated sludge. The
efficiency of the process according to the present invention
is such as to permit the economic recovery of precious metal
values from waste waters which contain also a very wide variety
~;23~
of physically undifferentiated organic and inorganic matter,
the latter including undesired metal values appearing as con-
taminants in the present process in much higher concentrations
than the selected trace metal values, see Example #l below.
Although the present discussion is mainly concerned
with gold and silver recovery, there are also significant
quantities of copper, zinc, palladium, chromium, cadmium,
nickel, tin and lead which appear with various other metallic
xides, such as calcium, aluminum, iron and magnesium. In
some areas there is also a recoverable quantity of platinum
and gallium. The concentration of gold found in the inorganic
matter recovered by the presently disclosed process can be as
much as five times the concentration of gold in most of the ;~
commercial gold ore mined in the United States at the present
time. The efficiency of concen~ration and recovery of the
eavy metals from the influent is demonstrated further ~y the ~ -
fact that there are no commercially significant quantities of
these metals to be found in the effluent water leaving the
present system.
2 Most prior art precious metal-winning processes fall ~ ~;
into one of two general categories, i.e. the treatment of
mineral ores, or the treatment of an industrial waste to
recover a specific metal value such as the recovery of silver
from waste waters from a photographic film manufacturing
plant. In both of these cases, the previous metal value being
recovered generally appears in aoncentrations of the order of,
for example, 5-5000 parts per
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million, and usually it is no~ heavily contaminated by the
presence of much higher concentrations of other metal values
which will interfrere with the recovery of the desired prec-
ious metal value.
These factors emphasize a major difference between
such prior art teachings and the present invention, wherein
the purpose of the present proce~s is to recover precious
metal ~alues in useful amounts from the industrial and sani-
tary waste from a city where the precious metal values appear
1~ in mere trace concentrations of the order of 0.1 - 500 PPB,
but wherein there are very many other metal values appearing
in greater concentrations in the same waste waters, these
other metal values tending to contaminate, and thereby impede
efforts to recover the traces of preciou~ metals. Both the
amount and concentration of precious metal values vary from
city to city, and therefore the economics of their recovery ;
vary from one geographic location to another.
The main problems which the present invention solves -~
; are (1) how to concentrate mere trace quantities of precious
metals appearing in very great quantities of waste waters,
and (2) how to recover such precious metals when they appear
in much higher concentratlons of other base metals, and (3)
how to achieve these objectives economically. This invention ;
combines certain steps to achieve these purposes. The
present process achieves a great total ConcentratiGn enhance- -
ment while using ordinary bacteria of the type used for the ~ -
bacterial treatment of sewage wherein the bacteria imbibes
the sewage to concentrate the metal values into a separable
sludge. These metal values in the sludge appear in the ~ `
presence of vast amounts of organic material which are
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separated by incineration. The metal-winning steps set forth
hereinafter recover almost all of the precious metal values
in the waste waters despite heavy concentrations of base
metals in wide variety. It is believed that when a concen-
tration attributable to the bacterial step of about 10,000:1
to 40,000:1 is combined with the concentration attributable ~-
to the subsequent metal-winning steps, the total concentrat~on
is such as to make it possible to recover precious metals
appearing in the waste waters as only a few parts per billion,
this result being achieved virtually without loss of precious
metal values in the original waste waters. The economics of ~
this overall process are greatly improved by the fact that ~ ;
the sewage treatment portion of the process is being carried
out for sanitation purposes already~in many cities, whereby
either dried sludge or incinerated ash is available as a by~
product.
THE DRAWINGS
Fig. 1 shows a block diagram illustrating apparatus
. :
~ suitable for carrying out the steps of a process according to
i; 20 the present invention; and
Fig. 2 is a flow diagram illustrating the steps o~
the present process.
Referring now to Fig. 1 of the drawings, the waste
waters contain metal values, the recovery of which is the
purpose of the present process. These metal values appear
in waste waters which are represented by the block 1, these
waters comprising, for instance, industrial waste water and ;~
natural surface water containinq verv dilute ~uantities of -~
the various metals appearing as dissolved compoun~s. These
waters enter the system having entrained therein organic ~
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waste of a type and quantity suitable to support the growthof bacteria. The source is generally a city sewage system.
In a typical illustrative system, the influe~t including the
organic waste, undergoes bacterial action represented by
the block 2 before passing through a preliminary mixer and
separator 3 designed to remove solids which can be made to
precipitate immediately, and these solids are then delivered
directly to a sludge de-watering device 4 which comprises any
one of the several types which are well known in the sewage
tr ~tment art. In a more advanced type of sewage system the
main stream of the influent passes from the preliminary separ~
ator 3 into a bacterial digestion chamber 6. These bacterial
steps can be either aerobic or anaerobic. In the digestion ;~ -
chamber 6, in the case of aerobic bacterial treatment the
mixture is aerated by a source of air 5. The activated
sludge is circulated in the bacterial digestion chamber 6
for a protracted period of time, for instance, as set forth
in EXAMPLE #l below. The mixture is then separated in a
separator 7 which delivers clarified water and concentrated ; ~ -
sludge through separate outlets. This sludge which may
typically be about 5% solids is also introduced into a ~ ;
sludge de-watering device 4 where it is mixed with the
sludge from the preliminary separator 3. When de-watered, -
the sludge is about 16~ solids by weight.
The diagram of Fig. 1 is a composite showing two
flow routes which can be followed concurrent}y by the sewage,
or alternatively. One route involves ~he mere mechanical ;
separation of the sludge via the flow route 3a, and the other
flow route involves the inclusion of bacterial digestion in
the digester 6, either aerobic or anaerobic, followed by
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delivery to the de-watering device 4 by way of the route 7a.
After the solid waste material has been de-watered in the
sludge de-watering device 4, it can be delivered to an
incinerator 8 where it is incinerated to an ash which as
; discharged is essentially devoid of organic material. The
ash comprises compounds of the various metals, mostly oxides.
This ash is represented ~y the block 9 on the diagram shown
in Fig. 1, the ash comprising about 15% to 50~ by weight of
the sludge entering the incinerator i~ this sludge were
1~ ~ried. The ash is then subjected to selected inorganic metal~
winning process steps represented by ~e block 10, illustrative
examples of which are given hereinafter, which steps recover ;
selected metals comprising the output of the process as repre-
sented by the block 11.
As an alternative process, the de-watered ~ludge
from the block 4 can be mixed as indicated by the dashed
line 4a with a base metal ore and smelted in the metal- ;~
winning step 10. In this case the incineration step 3 is
omitted, and the organic matter in the sludge is burned off
incident to smelting of the ore. For purposes of the
present disclosure the word "ore" is defined to include not
only a substantially untreated raw ore, but includes suitable
intermediates resulting from prior treatment steps of the
raw ore.
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Fig. 2 shows the process of recovering selected
~ ~ ,
metal values from waste watPrs which include natural surface
water 20, industrial waste water 21, and organic waste 22, ;~
. ~
these waste waters being mixed together at 23 and being subject
to bacterial action attributable to the presence of organic
waste in the mixture, this bacterial action being indicated
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by the block 24.
In order to capture me~al values which appear in
these waste waters either in solution or in colloidal suspension,
a bacterial digestion step 25 is used in which bacteria imbibe
such metal values, thereby immobilizing them and concentrating
them within the organic matter. The bacterial action which
occurs in steps 24 and 25 can be aerobic and/or anaerobic.
After a sufficient residence time, during which the bacterial
imbibing of the metal values takes place, the mixture moves on
to step 26 in which the sludge is coagulated and separated from
the water, the water being drained off as shown in 26a, and the
coagulated sludge being introduced into a de-watering step 27 ~ ',
where the percentage by weight of organic material is greatly
increased. -
The organic matter is then burned off in step 28,
wherein the de-watered sludge is introduced for the purpose of
eliminating organic matter and forming an ash comprising the ~,
metal values and ~ny residual contaminants. This burning off ~'
step 28 may comprise either incineration of the sludge as a
separate step, or alternatively it may occur incident to a smelt-
ing step wherein the sludge is mixed with an ore of a certain
metal, selected metal values being thereby dissolved in step 29
in a melt of that certain metal. The undissolved metal values
and contaminants ar,e separated therefrom as shown in the block 31.
,' In the case where the burning off step 28 is accom- ;
plished by incineration to produce an ash, the ash is then sub-
- jected to subsequent inorganic metal-winning steps of the type
illustrated in the specific EXAMPLES given below in this
specificat~n. The ash then passes to step 29 in which the metal-
30 winning inorgani,c process steps are used to dissolve certain ~
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metal values which also include the selected metal values to be
recovered. After certain metal values have been dissolved, the
undissolved metal values and contaminants remain and are
separated therefrom as shown in the block 31.
The dissolved metal values then pass to step 30 in which
the mixture of dissolved values is purified in order to recover -
~from all the dissolved metal values those which have been
selected, i.e., by separating the selected values from the re-
maining dissolved metal values which are non-selected for pur-
10 poses of the present process. The selected metal values are ;~
shown in the block 33 in Fig. 2, and the remaining metal values `~
from which they were separated and recovered in step 30 are shown
in a separate block 32.
EXAMPLE # l -- OFcGA~IC TREATMENT
The following discussion represents an illustrativeexample of organic process steps comprising the first portion of - ~ --
the present process, which organic steps are followed by inor~
ganic metal-winning steps, examples of which are set forth
below as EXAMPLES #2-6. ;~
In this illustrative example, the influent mixed waste
waters comprising natural surface water, industrial waste water,
and organic waste, for instance sanitary waste, enter at a daily
~low ra~e of about 28,000,000 gallons. The influent temper-
ature is about 70F with a pH of about 7. The influent contains
a total solid content of about 500-1500 PPM, the precious metal
content being, for silver about 20-100 PPB, for gold about 1-5
PPB, and for platinum about 0.03-0.16 PPB. These precious metal
values appear in the presence of very much higher relative
quantities of baser metals, which for purposes of the subsequent
metal-winning steps appear as contaminants.
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In the bacterial treatment steps of the process, there
is a residence time of from 3 to 24 hours during which the
bacteria feed on the organic matter and imbibe the metal values,
thereby immobilizing and concentrating them in a sludge, the
temperature remaining at about 70F and the pH remaining at
about 7.
The sludge is then coagulated and separated to recover
clarified water and wet sludge in which the total solid content
has been increased to about 5% by weight. This wet sludge i5 :~
then de-watered so that its total solid content is further in~
creased to about 16% before incineration. However, where the
sludge i5 to be added to an ore in a smelting step, as set forth
for instance in Examples #3 and #4 below, the sludge should be
further de-watered or dried. In either event, the sludge is -~
burned to drive off the organic matter. Where incineration is
used to recover an ash, this ash comprises inorganic contamin-
ants and metal values appearing about as follows: 20~ P205;
5% NO3; 7% Si02; 3.4% Ca; 0.5% ~g; 48800 PPM Fe; 31600 PPM Al;
11400 PPM Zn; 10900 PPM Cu; 2180 PPM Cr; 1900 PPM Nii 1450 PPM
Pb; 472 PPM Mn; 3pO PPM Ga; 190 PPM Cd; 600 PPM Ag; 30 PPM Au; ~ -
4 PPM Pd; 1 PPM Pt; 10 PPM Tl.
From the point of view of physical characteristics the -
ash is a finely divided reddish powder with a density of about
40 pounds/cubic foot, wherein the above metallic values appear -~
mostly as oxides which are physically dispersed and undifferent-
iated from each other in the mass. The ash is produced in
quantities of about 8000 pounds per day, so that its concentration
when compared with total influent is about 30,000:1. The
effluent from the organic steps of the process comprises about
27,000,000 gallons of clarified water per day. The incineration
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is carried out with excess air during ~ residence time of about
one-half hour and the ash leaves the incinerator at about 650F.
When using one of the inorganic smelting processes set
forth in the examples below the following resulks are obtainable:
Percentage of the gold and silver recovered - 90%.
Gold Production per day - 0.36 pounds ~ 5.25 Troy Ounces.
Silver Production per day - 3.6 pounds - 52.5 Troy Ounces.
The content of the incinera~ed ash will of course vary
with the geographic area from which the waste waters are taken.
The mineral content of the surface waters of that area consti-
tute one variable, and the type of industry contributing to the
industrial waste waters is another important variable.
There are various-different metal-winning processes
which can comprise the block 10 of Fig. 1 representing the in-
organic steps required to recover one or more particular metals
from the ash shown in the block 9. Among the commonest of these
are ~arious pyrometallurgical processas including smelting and/or
sintering, and vanous hydrometallurgy processes, which represent
two diferent approaches which can be used alternatively. The
following examples are given to illustrate various inorganic
metal-winning steps which are appropriate to use with the above
, .,
organic sludge concentrating steps.
` EXAMPLE #2 - PYROMETALLURGY ~ ~;
As an example of the pyrometallurgical approach, the
incinerated ash from the block 9 in Fig. 1 of the drawing is
mixed with copper ore and processed as follows: The mixture is
smelted and separated into a valueless slag, and into a matte
which is concentrated and contains the copper and other metal
values including the precious metals. This matte is then
further reined by preliminary fire refinement thereof and cast
into copper bearing anodes.
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The anodeis are then subjected to an electrolytic refin-
ing step so that the anode is dissolved by the passage of
current ana is deposited in the fo,-m of a pure copper cathode
within the electrolytic cell. ~he precious metals remain in the
cell as a mud or slime which is recovered from ~e bottom of the
cell and initially treated for the removal of remaining copper.
The mud is dried and fed into a furnace where it is
further refined by oxidation of impurities into a slag. The
precious metal remains in the form of a melt, often reerred to
as a dore, which is then cast into anodes for further electro-
lytic processing in a manner well known per se. If the platinum
group of metals is also present, they will go along with the
gold and can be separated from it by subse~uent chemical or
electrolytic means. ~ -
EXAMPLE #3 - COPPER SMELTING
Copper ore taken from a mine is first treated by well
,
known flotation methods to concentrate the copper sulfide, and
this concentrate is fed into a furnace together with inciner~
ated ash, fluxed with various chemical reagents such as lime and
silica. The furnace is maintained at a temperature of 1200-1300C
for about two hours to bring about a two-phase separation com- ;
prising a low density siliceous slag and a high density matte
containing copper sulfide and the precious metals from the ash
dissolved therein. The slag has high affinity for iron and other
contamina~ing base metals, while the matte is composea mainly of
the copper sulfide and the dissolved precious metals. In the case ~;~
of the sulfide ore, the furnace contains an SO2 gas atmosphere.
The two-phase mass is easily separated into a black por-
ous slag and a dense shiny metallic matte. The following data
provides a breakdown for the chemical composition of the copper
~~7
~ 32~t~
ore and the sanitary ash together with the flux:
Ca Fe Cu Aq Au
Copper Ore 0.4% 34~ 15% 18 PPM
Sanitary Ash 7.8~ 2.9% 0.2~ 146 PPM 41 PPM
The mixture contains about 81.8% Copper Ore; 8.2~ Ash; 7.4~
Silica and 2.5~ Lime, and the resulting two-phase mass is about
71% matte and 29% slag by weight~ Repeated re~inement reduces
the mass of the matte by further removal of remaining contamin-
ants, principally iron.
10- The matte is then passed into another furnace which is
maintained at 1200-1300C, and air is injected in~o it to supply
oxygen. The sulfur is removed a~ SO2, and after two hours a
blister copper mass of about 95~ purity is removed and cast into
anodes. The anodes are then subjected to electrolysis to re~
cover pure copper. The precious metal residue remains in the
bottom of the plating tanX as a slime or mud, and this mud is '~
subsequently removed and converted in a small' furnace by known
process steps into pure precious metals. It is estimated (by '~
analysis of the ash content) that about 81~ of the silver and
~0 about 95~ of the gold values are recovered by this process. ~;
- Considering further the case in which copper suIfid2
ore is used, dried sludge cannot be advantageously added to the y
substant'ially untreated~raw ore, because usually there is an
absence of'a suitable oxidizing agent to burn off the organic
matter in the initial smalting step. However, the dried sludge
can be advantageously added to an intermediate resulting from
such initial smelting and taking the form of a copper matte, for
instance the matte described above. In this way the separate ~
incineration step is eliminated, and additional furnace process ~ -
30 heat is obtained from the burning off of the organic matter in
the sludge'coincident with the reaction of the sulfur in the
copper sulfide with oxygen in air which is usually added in the
,
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furnace. The precious metals dissolve in the blister copper
produced, and are recovered as discussed above.
However, if the ore is of the copper carbonate type then
either ash or dried sludge can be mixed and smelted with the
substantially untreated raw ore.
EXAMPLE #4 - ~EAD SMELTING
This example is similar to Example #2, except that the
sludge or ash fr~m the incinerator i5 mixed with a lead ore.
In this case, the gold, silver and copper which have been dis-
10 solved in the lead are extracted therefrom by the addition of ~.
zinc to the molten bath ~PARKES' PROCESS). A zinc crust will
form and will contain silver, yold and copper from the melt.
Th;s crust is recovered and further refined to separate the zinc
by distillation. The zinc-free xesidue can then be separated for
the recovery of copper, silver, and gold by chemical or electro- `~
lytic means.
The ability of lead to extract gold, silver and other -~
precious metals from an ore is used in a procedure in which lead
oxide is mixed with either the incinerated sewage ash, or with `;~
dried sludge containing the metal values in the ratio of about
3 to 5 parts of lea~ oxide to one part sludge or ash. Other
compounds are added to the mixture, such as Si02, Na2CO3 and
Na2B407 to form a free flowing, low-viscosity low-density slag
through which the lead will easily settle out. If incinerated `
ash is used, it will be necessary to add a reducing agent to the
mixture such as coke, or flour in laboratory work, to reduce the
lead to metallic form. However, it i5 guite efficient to use
dried sludge in the mixture instead of ash~ the carbonaceous
matter in the sludge replacing the reducing agent in the mixture.
30 The dried sludge typically is in finely divided form and has a ~`
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specific gravity of about 0.91 gm/cc, or 1500 lbs/cubic yard.
Its moisture content typically is in the vicinity of 6%, and it
contains about 50% combustible matter.
The mixture is then fused at 1000-iOsOC in a suitabls
container. The lead metal and slag phases separate J are cooled,
and the slag is broken from the lead mass containing the precious
metal values dissolved in it.
For lead smelting of ash having modera~e amounts of
base metal values, the following mix~ure and flux composition is
satisfactory:
20 parts ash (gold content about 40 PPM, silver about 140 PPM
in the ash).
- 60 parts PbO
5 parts Na2B407
3 parts Flour.
For lead smelting of an ash having high amounts of base
metal values, the mixture includIng the flux composition is
changed to:
20 parts ash (gold CGntent about 40 PPMj silver about 140 PPM
; 20 in the ash).
90 parts PbO
60 parts Na2C0
30 parts Na~B40
8 parts CaF
15 parts SiO
7 parts Flour
For lead smelting sf a dried sludge:
20 parts dried sludge (gold content about 20 PPM, silver about
70 PPM in the sludge). ~;
10-20 parts SiO
100-120 parts PbO
P 2 3
20 parts Na2B407
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3~
These proportions are adjusted to optimize the slag depending
upon the behavior of the ash, or sludge plus the lead ore.
The gold and silver and any other precious metal values
contained in the lead mass are then recovered by standard
techniques, such as by the Parkes' Process.
EXAMPLE #5 - HYDROMETALLURGY
In a typical method accord:ing to the hydrometallurgical
approach, the incinerator ash can be treated by cyanidation.
According to this approach, the ash is trea~ed with a cyanide
solution for dissolving the precious metals. The undissolved
portion of the ash is then removed from ~he solution by filtra-
tion. The solid material is washed to recover any remaining ~-
noble metal-bearing solution, and this wash is added to the
separated cyanide solution. The cyanide solution is then precipi-
tated, for instance, using zinc or aluminum dust in the precip~
itation step, the precipitate then being refined to recover gold `~
and silver in metallic form
EXAMPLE #6 -- CYANIDE EXTR~CTION `
As mentioned above, the incinerated ash containing the ~ `
2Q precious metal values also contains large percentages of base
metals which are undesirable in cyanide extraction because they
consume large amounts of the cyanide. Moreover, tbe cyanide
forms complexes with the base metals as well as with the precious
metals and these complexes tend also to be removed along with the `~
selected precio~ls metal values from the cyanide bath, whereby the
desired isolation of precious metal values is defeated. There~
fore, acid leaching of the base metal values using concentrated
~!
H2SO4 is employed as a pretreatment step to leach out active base
metal values before cyanidation of the remaining ash. As a
practical process, three or more acid baths are ~repared in
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, .. .. . . . .
different vessels. The pH of the baths is maintained below
2.5, and the ash is moved from bath to bath at about 24-hour
intervals in such a way that the weakest acid bath receives
the untreated ash, and then the ash and/or the acid is moved
to the next vessel for another 24 hours, so that with each
move, the increasingly leached ash encounters a more concen-
trated acid. At the end of three days, this pretreatment is
concluded and the remaining ash is ready for cyanidation, the
base metzl values having been removed in solution with the
spent acid, and the precious metal values remaining in the
ash since they are not soluble to a significant extent in the
sulfuric acid.
The acid bath can advantageously contain about 400 lbs
of concentrated H2SO4 per ton of ash. About two tons of water
are used to dilute each ton of concentrated H2SO4, the propor-
tion being non-critical so long as there is sufficientwater
present to facilitate pumping of the bath and filtering out
of the ash.
The leached ash, still containing the precious metal ~,
. . .
values, is washed. Its residual acid content is neutralized
by the addition of 2% to 4~ of the weight of the ash of hy-
drated lime to raise the pH of the bath and prevent the forma-
tion of hydrogen cyanide when the ash is subsequently moved
to a~cyanidation bath which is continuously stirred and aerated
by bubbling-air therethrough. The bath contains from 1~ to
6~ of the weight of the ash of Sodium Cyanide, together with `
the ash from the pretreatment step, and washed coconut charcoal
to the extent of about one ton per thousand ounces of precious
metal. For a cyanide concentration of about one lb. NaCN per
100 lbs of ash, over a period of 24 to 72 hours the silver
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3~
recovery from the ash by adsorption onto the charcoal is
between 85% and 90~, and the goldl recovery is about 95~.
The recovery is not much increased by further increasing
the cyanide concentration.
The charcoal with the precious metal complexes adsorbed
thereon is recovered from the bath and the metal values are
separated, for instance by burning off the charcoal. The
precious metals can then be purified by well known steps,
for instance by electrolytic refining.
The present invention is not to be limited to the
exact process shown in the drawing, for obviously changes
can be made therein within the scope of the following claims:
.
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