5,762,891
Jun. 9, 1998
United States Patent [19]
Downey et al.
111111111111111111111111111111111111111111111111111111111111111111111111111
US005762891A
[11] Patent Number:
[45] Date of Patent:
FOREIGN PATENT DOCUMENTS
[73] Assignee: Hazen Research, Inc.. Golden, Colo.
[54] PROCESS FOR STABILIZATION OF
ARSENIC
[75] Inventors: Jerome P. Downey, Parker; Harry
Mudgett. Lakewood. both of Colo.
[57] ABSTRACT
(List continued on next page.)
RG. Robins and J.c.Y. Huang, "The Adsorption Of Arsenate
Ion By Ferric Hydroxide", Department of Mineral Processing
and Extractive Metallurgy. School of Mines. University
of New South Wales. Kensington, N.S.W. 2033
Australia, no date.
E. Krause and V.A. Ettel. "Solubilities And Stabilities of
Ferric Arsenates", J. Roy Gordon Research Laboratory,
INCO Limited. Mississauga. Ontario, Canada. Oct. 1987.
RG. Robins. 'The Aqueous Chemistry Of Arsenic In Relation
To Hydrometal1urgica1 Procsses". The University of
New South Wales, Kensington, N.S.Woo Australia. Aug..
1985.
EJ. Arriagada and K. Osseo-Asare, "Gold Extraction From
Refractory Ores: Roasting Behavior Of Pyrite And Arsenopyrite".
Department of Materials Science and Engineering,
The Pennsylvania State University, University Park. PA
16802, no date.
Mahesh C. Jha and Marcy J. Kramer, "Recovery Of Gold
From Arsenical Ores", AMAX Extractive Research &
Development. Inc., 5950 McIntyre Street. Golden. CO
80403, no date.
The present invention is a method to remove arsenic from
arsenic-containing materials. such as an ore or concentrate.
by roasting the arsenic-containing material to convert
arsenic sulfides into arsenic oxides. The arsenic oxides are
contained in the roasted arsenic-containing material. The
roasted arsenic-containing material is contacted with a lixiviant
to solubilize the arsenic in the oxide in a pregnant
leach solution. Ferric arsenate, an environmentally stable
compound. is formed in the lixiviant. The ferric arsenate can
be removed to provide a treated solution complying with
environmental regulations. The method provides a simple
and effective technique for removing arsenic from arseniccontaining
materials.
30 Claims, 2 Drawing Sheets
Primary Examiner-8teven Bos
Attome); Agent, or Firm-Sheridan Ross P.C
4/1993 Australia.
8/1968 France 423/47
9/1984 Japan 423/87
OTHER PUBLICATIONS
4/1910 Dewey 423/47
9/1916 Anderson, Jr 423/47
5/1952 McKay et a1 75/9
3/1965 Vian-Ortuno et a1 75/9
5/1977 Pagel 210/53
12/1980 Koh et aI 423/531
1/1981 Reynolds et a1 75/101 R
1/1986 Allgulin 21on11
1lI1986 Devuyst et a1 210m7
2/1988 Pabmeier et a1 2101709
3/1988 Ramadorai et a1 751118 R
3/1992 Domville 2101713
6/1992 Fernandez et a1 75/423
8/1992 Poncha 2101724
954,263
1,198,095
2,5%,580
3,172,755
4,025,430
4,241,039
4,244,734
4,566,975
4,622,149
4,724,084
4,731,114
5,093,007
5,123,956
5,137,640
WO 93108310
1536266
59-164639
[21] Appl. No.: 607,882
[22] Filed: Feb. 27, 1996
[51] Int. CI.6
•••••••••••••••••••••••••••••• COlG 3/00; COlG 28/00
[52] U.S. CI. 423/87; 423/87
[58] Field of Search 423/47. 87
[56] References Cited
U.S. PATENT DOCUMENTS
G. Ramadorai andRK Mishra. "Roasting ofArsenical Gold
And Silver Bearing Minerals". Metallurgical Department.
Newmont Gold Company, Carlin. Nevada. Refinery Department.
Pease and Curren Company. Warwick. Rhode Island.
no date.
AAml>U)ltl.IJNI~
N-lal.ll. In
~::;;:;:~ ".e-.(;:~
E]
5,762,891
Page 2
OTHER PUBLICATIONS
Piret. Norbert L. and Melin. "An Engineering Approach To
The Arsenic Problem In The Extraction Of Non-Ferrous
Metals". Stolbert Ingenieurberatung GmbH. Consulting
Engineers. D-5190 StolberglRhld. no date.
AS. Block-Bolton et al.. "Separation Of Arsenic From
Nickel". Materials Processing Center. Massachusetts Institute
of Technology. Cambridge. MA 02139. no date.
M. Stefanakis and A Kontopoulas. "Production Of Environmentally
Acceptable Arsenites-Arsenates From Solid
Arsenic Trioxide". MErnA S.A. I Eratosthenous Street.
GR 166 35 Athens. Greece. no date.
Taylor. "Cyclone Roasting Of Refractory Sulfide. Precious
Metal Concentrates". University of Idaho. no date.
u.s. Patent Jun. 9, 1998 Sheet 1 of 2 5,762,891
FIG. lA ARSEN IC-CONTAINING
MATERIAL 10
+
OXYGEN- AND ROAST
SULFUR-CTG GAS 26 14
I I
+
ROASTED ARSENIC- OFFGAS
CONTAINING 18
MATERIAL 22 .. •
( SOLIDS "
L1XIVIANT HSOlUBllIIATION 46) SOLIDS l COLLECTION J
34 \ 30) 58 • I
I I PRODUCT SLURRY OFFGAS
I I 62 38 III
+ • WASH / L1QUID/SOLID " SCRUBBING
,
SCRUB
-- SOLUTION -~ PHASE SEPARATION ) SOLUTION \ 42 )
78 66 ) 50 • I I • I I SCRUBBED I ,
I I t t II I I I OFFGAS 58 I I I LEACH PREG. LEACH .-_1 I
--'- I I
I I RESIDUE 70 SOLUTION 54 -------
(FURTHER TREATMENT ,
, , • + 62
HETAlS RECOVERY) (OXIDATION \ I
I I OXIDANT
I I 82.J 86 J
I 88
i
OXIDIZED
PREG. LEACH
I SOLUTION 90 II
• I
I
" METAL
PRECIPITATION r-- HYDROXIDE
94 )
+ 98 I
I
,
u.s. Patent Jun. 9, 1998 Sheet 2 of 2 5,762,891
IWATER I PPCT-CTG LEACH
II
118 SOLUTION III
102 II
II
II
TREATED STABILIZED LI __ L1QUID/SOLID PHASE
SOLUTION SOLIDS
74 SEPARATION 106 110
FIG.IB
5,762,891
FIELD OF THE INVENTION
SUMMARY OF THE INVENTION
BACKGROUND OF THE INVENTION
1
PROCESS FOR STABILIZATION OF
ARSENIC
The present invention generally relates to a process for
stabilizing arsenic which occurs in arsenic-containing materials
and more specifically to a process for converting
arsenic in sulfide ores and concentrates into ferric arsenate.
Many metals are derived from sulfide deposits. which
deposits contain significant amounts of arsenic compounds
(e.g.. 60 ppm or more). The arsenic compounds occur
mainly as sulfides. arsenides, or oxidation products thereof.
By way of example, the arsenic compounds can be compounds
of arsenic and sulfur alone (e.g., AS2S2• AS2S3 and
As2SS ) or can contain a variety of metals compounded with
the arsenic and sulfur (e.g.. FeAsS, Cu3AsS4 , Cu12As4S13,
CoAsS and Ag3AsS3 ). Arsenic not only adversely impacts
the recovery of non-ferrous basemetals and precious metals
from sulfide ores and the conversion of sulfur-containing
compounds into sulfuric acid but is also a highly toxic
substance that is the subject of strict environmental regulations.
The primary process employed to separate the arsenic in
sulfide ores from the metals contained in the ores or ore
concentrates is roasting. The sulfide ore is roasted to volatilize
the arsenic as arsenic trioxide (i.e., AS20 3 ) in the
roasting offgas and the calcine is then treated to recover the
desired metals. This method has a number of problems. The
roasting offgas can contain significant amounts not only of
arsenic trioxide but also of sulfur dioxide. Both compounds
are subject to strict emissions controls and can be difficult to
remove from the offgas by scrubbing techniques. Even if the
arsenic trioxide were to be efficiently removed in the scrubbing
solution. the supply of arsenic trioxide currently
exceeds demand. This disparity has resulted in the need to
store large amounts of arsenic trioxide at a high cost. Arsenic
trioxide can be environmentally unstable and, therefore, is
SUbject to strict environmental controls.
There is a need for an improved process to remove arsenic
from sulfide ores. There is a related need for a process to
convert the arsenic sulfides into an environmentally stable
form.
2
arsenic in the arsenic oxide being in the pentavalent state. In
the instance where the arsenic-containing material contains
iron, the roasting is also conducted in the presence of a
controlled amount of sulfur dioxide to convert a substantial
5 portion of the iron into a salt The ferric arsenate can be
removed from the pregnant leach solution to form a treated
solution and a recovered product. each of which can be
acceptable under existing environmental regulations.
A substantial portion of the arsenic sulfide in the arsenic-
10 containing material is converted into arsenic oxide in the
roasting step. Preferably, at least about 90% of the arsenic
sulfide in the arsenic-containing material is converted into
arsenic oxide during roasting.
A substantial portion of the arsenic oxide and sulfur in the
arsenic-containing material is retained in the roasted arsenic-
15 containing material and not volatilized into the roasting
offgas. More preferably, at least about 95% of the arsenic in
the arsenic-containing material is in the roasted arseniccontaining
material. The offgas from the roasting step preferably
includes about 10% or less of the arsenic from the
20 arsenic-containing material. Preferably. no more than about
75% of the sulfur in the arsenic-containing material is
contained in the roasted arsenic-containing material after
roasting.
25 The roasting is conducted in an atmosphere containing
controlled amounts of oxygen and sulfur dioxide to provide
sufficient oxygen and sulfur potentials to fully oxidize
arsenic oxides and any iron-containing materials and to
convert the iron-containing materials into a form that is
30 soluble in the lixiviant, preferably a salt. The potentials are
realized by providing partial pressures of oxygen and sulfur
dioxide in the roasting atmosphere each ranging from about
0.01 to about 0.1 atm.
The temperature of the arsenic-containing material during
35 roasting is also controlled to inhtbit arsenic and sulfur
volatilization and sintering of the roasted arsenic-containing
material. Preferably, the temperature of the arseniccontaining
material and roasting atmosphere ranges from
about 500° C. to about 650° C.
40 The roasted arsenic-containing material is contacted with
the lixiviant to form the pregnant leach solution containing
dissolved arsenic. To solubilize the arsenic oxides from the
roasted arsenic-containing material, the lixiviant includes a
leaching agent. The preferred leaching agents are sulfuric
45 acid and/or, (hot) water with (hot) water being most preferred.
Ferric sulfate can be added to the lixiviant or pregnant
leach solution to facilitate subsequent formation of the
ferric arsenate. Preferably, at least about 90% by weight of
The present invention addresses these and other needs by the arsenic from the roasted arsenic-containing material is
providing a process that converts arsenic sulfide into ferric 50 dissolved in the leach solution.
arsenate, which is an environmentally stable compound. As In one embodiment. a leach residue is separated from the
used herein, "arsenic sulfide" refers generally to any com- pregnant leach solution to recover any metals in the residue.
pound containing arsenic and sulfur, whether or not com- To enhance metal recoveries, the leach residue is washed
pounded with other elements and preferably refers to com- with a wash solution to remove arsenic from the residue. A
pounds having no oxygen, such as FeAsS, Cu3AsS4, 55 portion of the wash solution can be combined with the
CU1~S4S13' CoAsS andAg3AsS3•The arsenicin the arsenic pregnant leach solution prior to the formation of ferric
sulfide is in the trivalent or pentavalent state. The process arsenate.
includes the steps: (i) roasting the arsenic-containing mate- In a preferred embodiment, the ferric arsenate is formed
rial in the presence of a controlled amount of oxygen to form by first contacting the pregnant leach solution with an
a roasted arsenic-containing material containing arsenic 60 oxidant and second precipitating the arsenic as ferric arsenoxide;
(ii) contacting the roasted arsenic-containing material ate. The preferred oxidant is hydrogen peroxide, oxygen, or
with a lixiviant to solubilize the arsenic oxide in the leach mixtures thereof. Before precipitation. the pH of the pregsolution;
and (ill) forming ferric arsenate in the pregnant nant leach solution preferably ranges from about pH 0 to
leach solution containing dissolved arsenic. Arsenic oxide about pH 1.0 and the temperature from about 20° C. to about
refers to any compound containing both arsenic and oxygen 65 95° C. During precipitation, the pH preferably ranges from
atoms. In the roasting step, a substantial portion of the about pH 1.5 to about pH 2.5 and the temperature from about
arsenic sulfide is converted into arsenic oxide with the 20° C. to about 90° C.
5,762,891
4
roasted arsenic-containing material. The arsenic and much
of the sulfur are maintained in the solid phase to suppress the
production of environmentally harmful compounds. specifically
arsenic trioxide and sulfur dioxide. as byproducts and
5 thereby avoid the attendant problems associated with their
removal from the roasting offgas and subsequent disposal.
As will be appreciated. the sulfur dioxide in the roasting
atmosphere is maintained at a level sufficient for it to be
consumed in the formation of ferric sulfate. Thus. relative to
existing processes there is a significantly reduced amount of
residual sulfur dioxide from the reaction to be scrubbed from
the offgas.
As will be appreciated. the amount of arsenic and sulfur
retained in the roasted arsenic-containing material depends
15 upon the amount in the arsenic-containing material. the
mineralogy. and the specific roasting conditions. Preferably.
the roasted arsenic-containing material contains a majority
of the arsenic and sulfur in the arsenic-containing material.
More preferably. at least about 95% of the arsenic in the
20 arsenic-containing material and at least about 50% of the
sulfur in the arsenic-containing material is contained in the
roasted arsenic-containing material.
The arsenic in the roasted arsenic-containing material is
preferably in the form of an arsenic oxide and more pref-
25 erably an arsenic oxide in the pentavalent state. At least
about 90% of the arsenic in the arsenic-containing material
is preferably converted into arsenic oxide in the roasting
step. The arsenic oxide can include a variety of oxides. such
as arsenic trioxide and arsenic pentoxide. Preferably. at least
30 about 90% by weight of the arsenic oxides in the roasted
arsenic-containing material are in the form of arsenic pentoxide
(e.g.. As20 S)' It is preferred that at least about 90%
of any trivalent arsenic in the arsenic-containing material be
converted into pentavalent arsenic during roasting.
Due to the retention of the arsenic and sulfur in the roasted
arsenic-containing material. the roasting offgas preferably
includes at most a small proportion of the arsenic and sulfur
in the arsenic-containing material. More preferably. the
offgas includes at most about 10% and most preferably at
40 most about 1.0% of the arsenic. and at most about 35% and
most preferably at least about 25% of any sulfur in the
arsenic-containing material. This yields an offgas containing
preferably at most about 0.01% by volume arsenic and 5%
by volume sulfur.
Valuable metals in the roasted metal-containing material
are preferably in a recoverable form after roasting. By way
of example. the above roasting conditions cause substantially
all of the copper to be in a form. such as copper sulfate.
(CUS04 ). which is amenable to recovery by conventional
50 hydrometallurgical recovery techniques.
Iron in the arsenic-containing material. if present. is
preferably sulfated during the roasting step to yield a salt.
with ferric sulfate (Fe2(S04h) being most preferred.
Preferably. at least about 20% of the iron in the arsenic-
55 containing material is converted into a salt. Sulfation
promoters. such as sodium sulfate (Na2S04). can be
employed to increase the amount of ferric sulfate produoed
during roasting. The amount of sulfation promoters used
preferably ranges from about 0% to about 3% by weight of
6() the arsenic-containing material.
The above-noted results are produced by the appropriate
roasting conditions. To yield proper oxidation conditions.
the oxygen and sulfur potentials in the roasting step are
carefully controlled. It is preferred that the roasting input gas
contain more than the stoichiometric amount of oxygen
required to oxidize the arsenic in the arsenic-containing
material to the pentavalent state. If an insufficient amount of
DETAILED DESCRIPTION
BRIEF DESCRIPTION OF THE DRAWINGS
FlGS. lA-B show a flow schematic of a preferred
embodiment of the subject invention. illustrating the conversion
of arsenic sulfides into ferric arsenate. 10
3
The ferric arsenate can be removed from the pregnant
leach solution to provide a treated solution having no more
than about one milligram per liter of dissolved arsenic.
Metals in the treated solution can be recovered from the
treated solution by suitable techniques.
It is desired that the roasting conditions employed inhibit 65
the volatilization of arsenic and any sulfur associated therewith
and cause the arsenic and sulfur to be retained in a solid
The method of the present invention solubilizes the
arsenic in an arsenic-containing material using a lixiviant
and precipitates the arsenic from a pregnant leach solution as
ferric arsenate (FeAs04). Ferric arsenate is an environmentally
stable form of arsenic oxide that is acceptable for
disposal under most environmental regulations. as determined
by the EPA Toxicity Characteristic Leaching Procedure.
The process is of particular importance in the roasting
of gold ores. the pretreatment of copper sulfide concentrates
for hydrometallurgical processing. and thermal treatment of
arsenic-containing waste materials.
The arsenic-containing material can contain a wide range
of arsenic contents. The arsenic-containing material generally
contains from about 0.01 to about 10% by weight
arsenic. Typically. at least about 90% by weight of the
arsenic is present in the form of a sulfide. The arsenic in the
arsenic-containing material can be in the trivalent and/or
pentavalent state. The arsenic-containing material will generally
contain trivalent arsenic. particularly in the form
As2S3 •
The arsenic-containing material can also contain iron.
typically in the form of a sulfide such as iron pyrite. The 35
arsenic-containing material typically contains an amount of
iron ranging from about 25 to about 45% by weight.
The arsenic-containing material can be an ore. concentrate
or other metal-containing material that includes one or
more metals to be recovered from the arsenic-containing
material. Such metals include gold. copper. nickel. silver.
cobalt. zinc. and mixtures thereof. The amount of the metal
in the arsenic-containing material typically ranges from a
few ounces per ton to about 40% by weight. The metal is
typically in the form of a sulfide. 45
To cause the arsenic in the arsenic sulfide to be in a
soluble form. the arsenic-containing material is first roasted
under conditions sufficient to convert arsenic sulfides in the
arsenic-containing material into arsenic oxides. preferably
pentavalent. and to convert iron sulfides into ferric sulfate
(Fe2(S04)3)' The roasting reactions proceed according to the
following equations:
5,762,891
5
oxygen is present in the roasting input gas. the oxidized
arsenic will primarily be in the trivalent state due to the
limited availability of oxygen during roasting. To verify that
the input gas contains sufficient oxygen for formation of
arsenic pentoxide. it is preferred that the partial pressure of
oxygen in the roasting offgas ranges from about 0.01 to
about 0.1 atm and more preferably from about 0.04 to about
0.08 atm. The roasting input gas can be air or oxygenenriched
air. provided that the gas contains a sufficient
amount of excess oxygen to maintain the oxygen partial
pressure at the levels prescribed. Although the sulfur potential
is also an important roasting condition for sulfation. the
roasting input gas is preferably substantially free of sulfur
dioxide. The degree of sulfation can be determined based
upon the amount of sulfur dioxide in the offgas. The sulfur
dioxide is a byproduct of the roasting reactions. The partial
pressure of sulfur dioxide in the roasting offgas preferably
ranges from about 0.01 to about 0.1 atm and more preferably
from about 0.04 to about 0.06 atm. On a volumetric basis the
amount of oxygen in the roasting offgas preferably ranges
from about 4 to about 8% and the sulfur dioxide in the
roasting offgas from about 4 to about 6% of the roasting
atmosphere.
The arsenic-containing material during the roasting step is
preferably maintained at a temperature of no more than
about 6500 C.. more preferably from about 5000 C. to about
6000 C.. and most preferably from about 5200 C. to about
5500 C. In addition to reducing the volatilization of arsenic,
such temperatures are low enough to avoid sintering of the
arsenic-containing material. especially silicates in the
arsenic-containing material. while being high enough to
optimize the oxidation and sulfation reactions, particularly
the oxidation of the arsenic and sulfur in the arseniccontaining
material and the sulfation of any iron in the
material. Silicate sintering can decrease the porosity of the
roasted arsenic-containing material. hindering the penetration
of the lixiviant into the material and therefore the
solubilization of arsenic and metals. High porosity and low
sintering are thus desirable for the subsequent recovery of
the metals from the roasted arsenic-containing material.
Because the oxidation reaction of the arsenic and/or
sulfur-containing components of the arsenic-containing
material is generally exothermic. the control of the roasting
temperature can be difficult. One method to control the
roasting temperatures is to add suitable amounts of a heat
sink. such as silicates, water. and mixtures thereof.
Although the oxidation reaction of the arsenic- and/or
sulfur-containing components of the arsenic-containing
material is generally exothermic. it may be necessary to
initially raise the temperature of the arsenic-containing
material and the temperature of the oxygen-containing atmosphere
in the roasting reactor to initiate the oxidation reaction.
This can be accomplished by initially adding coal,
propane. or butane. or another low combustion material to
the roasting reactor.
The preferred roasting reactor is a circulating fluidized
bed or an ebulating fluidized bed to facilitate the transfer of
reactants and heat produced by the oxidation reaction and
thereby increase both reaction rate and reaction uniformity.
An additional advantage of a circulating fluidized bed is that
it enables the precise control of the bed temperature.
The residence time of the arsenic-containing material in
the roasting reactor preferably ranges from about 10 to about
120 minutes and more preferably from about 20 to about 40
minutes for the substantial completion of the oxidation and
sulfation reactions.
The roasting offgas can be contacted with a scrubbing
solution to remove the arsenic- and any sulfur-containing
6
compounds. such as arsenic trioxide and sulfur dioxide. and
other contaminants. such as entrained particulate matter. The
scrubbing solution is preferably water and can be added as
a portion of the pregnant leach solution discussed below.
5 The scrubbing solution can be combined with the pregnant
leach solution to enable the arsenic in the scrubbing solution.
which is typically in the trivalent state. to be converted into
ferric arsenate.
The roasted arsenic-containing material is next contacted
10 with a lixiviant to form the pregnant leach solution containing
dissolved arsenic oxides. The lixiviant preferably contains
a leaching agent. which is dilute sulfuric acid and/or
water. to facilitate the solubilization of the arsenic oxides.
More preferably. the lixiviant is an aqueous solution which
15 contains a sufficient amount of sulfuric acid to maintain the
pH below about 0.5. Alternatively. the lixiviant can be hot
water. To facilitate solubilization, the lixiviant can be contacted
with the roasted arsenic-containing material in an
agitated reaction vessel at elevated temperatures. The pre-
20 ferred temperature of the lixiviant ranges from about 200 C.
to about 600 C. The contacting time preferably ranges from
about 15 to about 60 minutes for substantially complete
solubilization.
In a preferred embodiment. the pregnant leach solution
25 includes a significant amount of the arsenic oxides in the
roasted arsenic-containing material. More preferably. at
least about 90% of the arsenic in the roasted arseniccontaining
material is dissolved in the pregnant leach solution.
The pregnant leach solution desirably contains from
30 about 1.0 mg/l to about 50 gil arsenic.
If the roasted arsenic-containing material contains iron.
which is preferably in the form of ferric sulfate and/or ferric
arsenate, a significant amount of the iron is preferably
solubilized in the pregnant leach solution. More preferably.
35 at least about 20% of the iron in the roasted arseniccontaining
material is dissolved in the pregnant leach solution.
After contacting the roasted arsenic-containing material
with the lixiviant. the roasted arsenic-containing material
40 (e.g.. leach residue) preferably contains a significantly
reduced amount of arsenic. More preferably. the leach
residue contains less than about 10% of the total arsenic
content of the roasted arsenic-containing material before the
contacting step.
45 In another embodiment. a product slurry from the solubilization
step is subjected to liquid/solid phase separation to
separate the pregnant leach solution from the roasted
arsenic-containing material (e.g.. leach residue). The leach
residue can contain a significant portion of metals, such as
50 gold and silver. to be recovered from the arsenic-containing
material. It is therefore preferred that the arsenic content of
the leach residue be less than about 0.05% by weight. To
remove the pregnant leach solution from the leach residue
and thereby substantially minimize the arsenic content of the
55 leach residue. the leach residue is washed after the liquid/
solid phase separation. The phase separation can be achieved
using conventional thickeners or filters. As desired. the leach
residue can be disposed of or subjected to further treatment
to recover metals contained therein. For example. to recover
60 gold the leach residue can be neutralized with a base
material. such as lime. and treated by cyanidation techniques.
The wash solution is preferably combined with the
pregnant leach solution before the solubilized arsenic is
converted into ferric arsenate.
65 The pregnant leach solution is treated to convert the
solubilized arsenic into a ferric arsenate precipitate. This
step should be conducted for a sufficient period of time to
5.762.891
7 8
Fe,(SO')3+As,O,+3Ca(OHh+2H20,---+2FeAsO.+3CaSO•.2H20+
3H,O
Fe2(SO.h+As,O,+3Ca(OHh+H,02---+2FeAsO.+3CaSO•. 2H,o+
H20
15 or
from about pH 1.5 to about pH 2.5. The temperature during
the precipitation step preferably ranges from about 20° C. to
about 90° C. Under such conditions. a variety of other
compounds. such as hydrated calcium sulfate
5 (CaS04.2H20) and hydrated iron oxide (e.g.• Fe(OHh and
FeO(OH». will coprecipitate with the ferric arsenate.
While not wishing to be bound by any theory. it is
believed that the net chemical reaction in the preferred
embodiment for the oxidation and precipitation steps is as
10 follows:
As will be appreciated. a variety of other materials can also
be employed in lieu of the reactants in the preceding
equations to yield ferric arsenate.
It is possible that a portion of the arsenate ion may be
removed by adsorption or coprecipitation with amorphous
ferric hydroxide. As the pH increases above about pH 2 (Le.•
becomes more basic) and the arsenic concentration of the
solution decreases due ferric arsenate precipitation. ferric
hydroxide becomes a stable phase that is precipitated. This
is especially true at the pH range from about pH 4 to about
pH 5. Typically. the majority of the arsenic will precipitate
as ferric arsenate.
In the preferred embodiment. the ferric arsenate precipitate
is removed from the precipitate-containing pregnant
leach solution by filtration or gravity separation techniques
35 to form stabilized solids containing the arsenic. As noted
above. the stabilized solids are environmentally acceptable
and can therefore be readily disposed of. Using the present
invention. the treated solution can have an arsenic content of
less than about 1 milligram per liter as determined by the
40 EPA Toxicity Characteristic Leaching Procedure. which is
sufficient to comply with environmental regulations.
The treated solution can be subjected to subsequent
treatment steps to recover dissolved metals. By way of
example. copper. nickel. cobalt or zinc will occur as water
45 soluble sulfates in the treated solution. After ferric arsenate
precipitation and removal. the sulfates can be recovered by
suitable techniques from the treated solution. such as solvent
extraction and electrowinning.
FIG. 1 depicts a preferred embodiment of the present
50 invention applied to the arsenic-containing material. The
preferred embodiment is particularly suited to an arseniccontaining
material that is an ore or concentrate and contains
copper and precious metals.
An arsenic-containing material 10 is roasted 14 to form an
55 offgas 18 and a roasted arsenic-containing material 22. An
oxygen-containing gas 26 is contacted with the arseniccontaining
material during roasting to facilitate oxidation of
the arsenic and sulfur and sulfation of the iron in the
arsenic-containing material. The oxygen reacts with sulfides
60 in the arsenic-containing material to form sulfur dioxide. If
the arsenic-containing material is too low in sulfur. a pyrite
concentrate may be blended with it prior to roasting.
The offgas 18 is subjected to solids collection 30 to form
solids 34 and an offgas 38 for scrubbing 42. The solids 34
65 are combined with the roasted arsenic-containing material
22 before solubilization 46. The offgas 38 is subjected to
scrubbing 42 with a scrubbing solution 50 to form a
precipitate ferric arsenate and reduce the dissolved arsenic
concentration in the pregnant leach solution to desired
levels. For best results. this step is conducted in an agitated
tank.
To enable the treated solution to pass the applicable
environmental regulations. the concentration of the iron in
the pregnant leach solution must be sufficient to convert a
sufficient amount of the solubilized arsenic into the ferric
arsenate precipitate to reduce the dissolved arsenic concentration
to the required levels. The stoichiometric ratio of iron
to arsenic in ferric arsenate is l: 1. It is therefore preferred
that the ratio of iron to arsenic in the pregnant leach solution
be at least about 2.5: 1. The preferred concentration of ferric
sulfate in the pregnant leach solution is at least three times
that of the arsenic in the pregnant leach solution. To provide
such iron levels in the pregnant leach solution. it may be
necessary to add ferric sulfate or another soluble form of
ferric iron. such as a ferric salt. to the pregnant leach
solution.
In a preferred embodiment. the pregnant leach solution is 20
contacted with an oxidant to convert trivalent arsenic in the
pregnant leach solution. if any. to pentavalent arsenic. any
ferrous iron into ferric iron. to prevent the reduction of any
pentavalent arsenic and ferric iron which was oxidized
during roasting. and to form ferric arsenate. The trivalent 25
arsenic can be either incompletely oxidized arsenic from the
roasting step or arsenic removed from the offgas by the
scrubbing solution. The preferred oxidant is hydrogen
peroxide. oxygen. and mixtures thereof. For hydrogen peroxide
as the oxidant. the oxidation of trivalent arsenic into 30
pentavalent arsenic proceeds according to the following
equation:
2H20,+As,O,---+As,O,+2H20
The pregnant leach solution preferably has a concentration
of oxidant ranging from about 0.5 to about 5.0% by volume.
The preferred temperature of the pregnant leach solution
during oxidation ranges from about 20° C. to about 90° C.
The preferred pH of the pregnant leach solution during
oxidation ranges from about pH 2.5 to about pH 3.5.
A metal oxide. hydroxide. or carbonate is contacted with
the pregnant leach solution to precipitate ferric arsenate. The
ferric arsenate precipitate can be removed from the
precipitate-containing pregnant leach solution to form the
treated solution.
The preferred metal oxide or hydroxide is lime. hydrated
lime. and mixtures thereof. with the most preferred base
being hydrated lime. The preferred metal carbonate is calcium
carbonate. sodium carbonate. and mixtures thereof.
with calcium carbonate being most preferred. Hydrated lime
can be easily added to the pH adjustment circuit as a slurry
(milk of lime). lime (CaO) or calcium carbonate (CaC03 )
will both react to form hydrated lime (Ca(OHh) in an
aqueous medium within the pH range of interest. A commercial
operation would be likely to purchase lime. slake it
at their site. and then add the milk of lime slurry to the pH
adjustment step. Calcium carbonate is less expensive than
lime but may react too slowly for this application. Calcium
compounds are preferred over sodium compounds for the
pH adjustment step because their use will provide an outlet
for calcium and sulfate through the precipitation of gypsum
(CaS042H20). If a sodium compound were employed. it
would be necessary to bleed from the system to prevent the
accumulation of sodium sulfate.
A sufficient amount of metal hydroxide is added to the
oxidized pregnant leach solution to provide a pH ranging
5,762.891
9 10
EXAMPLE 2
An experiment was conducted using an aqueous makeup
solution of ferric sulfate and arsenic trioxide to analyze the
ability to remove arsenic in the trivalent state from a
solution. The aqueous solution of ferric sulfate was prepared
(189.14 g in 500 ml H20) and heated to 590 C. Arsenic
trioxide (35.14 g) was added and allowed to dissolve. After
1 hour. the temperature was 81 0 C.. but the AS20 3 did not
appear to be totally dissolved; a small amount of yellow
suspended particles was evident. After another hour and 12
minutes, the pH was below zero and the AS20 3 had completely
dissolved. One hundred five m1 of 30% hydrogen
peroxide were slowly added to the solution. After this
addition. the solution appeared lighter in color and had a
slight green tint. A 25 m1 sample of the solution was taken
("S/,). A total of 70.5 g of calcium hydroxide was added and
the pH rose from less than zero to 2.6. The mixture was
filtered and the filter cake washed. The washed filter cake
("P") was dried and submitted for analysis. The precipitate
in the filter cake was tan with red chunks and had a dried
weight of 284.33 g. The mixture was filtered and analyzed
for arsenic. The final filtrate ("S/') was clear and had a
volume of 560.1 ml.
TABLE 1
25 Copper Arsenic in
Protocol Sample Extraction, % Filtrate. mg/L
1 BOF 50,0 1.05
1 FBOF 34.4 0.59
2 BOF 46,1 70
30 2 FBOF 39,6 50
3 BOF 67.3 0.09
3 FBOF 39,0 1.67
4 BOF 69,0 0.01
4 FBOF 55.3 0.35
leach solution was filtered and the leach residue solids were
thoroughly washed with deionized water with the leach
liquor (filtrate) being kept separate from the wash water. The
filtrate was then added to a clean beaker and its temperature
5 elevated to 600 C. with the pH being adjusted to 1.5 with
sulfuric acid during aerating and agitating of the solution.
Fifteen grams of ferric sulfate and 15 ml of 30% hydrogen
peroxide were added and the filtrate agitated for approximately
30 minutes. The pH was slowly adjusted by adding
10 calcium hydroxide until a pH of 2.6 was achieved. The
resulting pulp was then allowed to agitate for an additional
20 minutes before filtration. This protocol was added to
establish whether the copper and arsenic extraction efficiency
from the leach residue could be improved by leaching
15 samples of the roasting bed overflow (BOF) and final bed
overflow (FBOF) calcine in dilute sulfuric acid and whether
arsenic stabilization efficiency could be improved by adjusting
the pH of the filtrate and spiking filtrate with additional
ferric sulfate and hydrogen peroxide.
20 The analytical results of the experiments are summarized
in Table 1.
Based on the experiments. it appeared that the sulfation
roastlleach approach was effective in stabilizing arsenic. The
sulfuric acid leach solubilized significantly more arsenic
than the hot water leach. The arsenic was readily precipi-
40 tated from the filtrate to enable the filtrate to comply with the
EPA Toxicity Characteristic Leaching Procedure while
maintaining the copper in a recoverable form in the filtrate.
EXAMPLE 1
scrubbed offgas 58. The scrubbing solution 50 is a portion of
a pregnant leach solution 54. The scrubbed offgas 58 is
subjected to further treatment 62 to remove contaminants,
such as sulfur dioxide, from the scrubbed offgas 58 prior to
disposal.
The roasted arsenic-containing material 22 is subjected to
solubilization 46 with a lixiviant 58 to form a product slurry
62. During solubilization 46, arsenic oxides, ferric sulfates.
and ferric arsenates in the roasted arsenic-containing material
22 are dissolved in the liquid phase of the product slurry
62.
The product slurry 62 is subjected to liquid/solid phase
separation 66 to form a leach residue 70 containing metals
and the pregnant leach solution S4 containing the arsenic
oxides, ferric sulfates, and ferric arsenates. A portion of a
treated solution 74 is combined with water 118 to form a
wash solution 78. The wash solution 78 is used during
liquid/solid phase separation 66 to wash the leach residue 70
to remove arsenic and other contaminants from the leach
residue 70. The leach residue 70 is subjected to metals
recovery 82.
The pregnant leach solution S4 is contacted with a portion
of the scrubbing solution 50, which also contains dissolved
arsenic, and subjected to oxidation 86 with an oxidant 88 to
form an oxidized pregnant leach solution 90, Through
oxidation 86, the trivalent arsenic and iron in the pregnant
leach solution 54 are converted into pentavalent arsenic and
iron sulfate. respectively.
The oxidized pregnant leach solution 90 is subjected to
precipitation 94 with a metal hydroxide 98 to provide a
precipitate-containing pregnant leach solution 102. The precipitate
includes ferric arsenate.
The precipitate-containing pregnant leach solution 102 is
subjected to liquid/solid phase separation 106 to form stabilized
solids 110 and the treated solution 74. The stabilized 35
solids 110 contain the precipitated ferric arsenate for disposal
114. The treated solution 74 can be contacted with
water 118 and re-used as the lixiviant S8 or the wash solution
78,
A series of leaching and precipitation experiments were
conducted using roasted samples to analyze the ability to
convert arsenic in the samples into ferric arsenate without
compromising copper recovery from the filtrate and/or leach 45
residue,
Four basic test protocols were followed. In Protocol No.
L 50 g of calcine were added to 150 ml of water at 600 C.
The resulting pulp was agitated for 5 minutes and then
filtered. The leach residue solids were thoroughly washed 50
with deionized water, dried, and submitted for chemical
analysis. The leach liquor and the wash water were combined
to form the filtrate and diluted to a known volume in
preparation for chemical analysis. Protocol No. 2 was the
same as Protocol No.1, except that the leach was terminated 55
after 2 hours instead of after 5 minutes. Protocol No.3 was
similar to Protocol No.2. except that after 2 hours of
leaching, the pH was adjusted to 2.5 by adding a calcium
hydroxide (Ca(OH):0 slurry. The resulting pulp was then
held at temperature and agitated for another hour before 60
filtration. Protocol No.3 was designed to determine whether
the arsenic concentration of the leach liquor could be
effectively reduced without adversely affecting the extraction
of copper from the solution. Protocol No.4 was similar
to Protocol No.2 with the following exceptions. The starting 65
pregnant leach solution contained 30 gIL sulfuric acid at 600
C. The hot slurry from contacting the calcine with pregnant
5,762,891
EXAMPLE 5
12
The calculated balance indicates 104.8% arsenic retention
in the bed solids. Realistically. some volatilization of AS40 6
(g) would undoubtedly take place in a commercial operation.
but this could be easily handled with a well designed offgas
5 treatment system. Approximately 56% of the copper in the
calcine and 43% of the copper in the cyclone discharge are
present as sulfate. The remainder is suspected to exist as the
oxide CuO and. in the cyclone underflow. as unreacted
sulfide. The iron appears to have been predominantly con-
10 verted to Fe20 J • but there was some evidence that a minor
amount of ferric sulfate may also have formed.
Another experiment was conducted to determine the
15 effectiveness of precipitating ferric arsenate from a chlorinecontaining
solution using a sodium hydroxide base. A solution
containing 64.64 g of ferric chloride was dissolved in
500 rnl of deionized water. The solution was heated to 600
C. and 10 g of As20 J was added. Temperature and heating
20 were maintained for 1 hour. The solution was allowed to
cool overnight and was then filtered. The dried residue had
a weight of 2.02 g and laboratory analysis showed that it
contained 45.9% arsenic. Assuming that all other arsenic
remained in solution. it was then calculated that 80% of the
25 arsenic trioxide had dissolved in the ferric chloride solution.
The initial pH of the filtrate was 0.89. After adding 50 ml of
sodium hydroxide solution (5.33 g/100 rnl H20). the pH rose
to 1.28 and 3.94 g (dried) of precipitate formed. The filtrate
was reacted with 15 ml of 30% hydrogen peroxide and the
30 pH dropped from 1.15 to 0.26. Sixty milliliters of sodium
hydroxide solution (40.58 g/100 rnl H20) was added. and
the pH went from 0.25 to greater than 3.5.A thick precipitate
weighing 41.1 g (dry) was filtered from the filtrate. The
filtrate was clear and contained 0.89 mgIL of arsenic.
35
While various embodiments of the present invention have
been described in detail. it is apparent that modifications and
adaptations of those embodiments will occur to those skilled
in the art. However. it is to be expressly understood that such
40 modifications and adaptations are within the scope of the
present invention. as set forth in the following claims.
What is claimed is:
1. A method for stabilizing arsenic from an arseniccontaining
material. the arsenic being in the trivalent state in
45 the form of a sulfide. comprising:
(a) roasting an arsenic sulfide-containing material in the
presence of oxygen to form a roasted arseniccontaining
material wherein (i) the partial pressure of
the oxygen is controlled during the roasting step to
convert a substantial portion of said arsenic sulfide into
arsenic oxide and sulfur dioxide. with the arsenic in the
arsenic oxide being in the pentavalent state. while
retaining at least about 95% of said arsenic oxide in
said roasted arsenic-containing material and (ii) in the
instance where the arsenic-sulfide containing material
contains iron. the partial pressure of sulfur dioxide is
controlled to convert a substantial portion of said iron
into an iron-containing salt;
(b) contacting said roasted arsenic-containing material
with a lixiviant to solubilize said arsenic in said oxide
and said iron s.alt. if any. from said roasted arseniccontaining
material to form a pregnant leach solution;
and
(c) forming ferric arsenate in said pregnant leach solution
containing dissolved arsenic.
2. The method as claimed in claim 1. wherein said
arsenic-containing material contains a metal selected from
55
As, %
Distribution
As, g
Analysis.
EXAMPLE 3
TABLE 2
Volume
Mass or
35.14 g 26.62 100
284.33 g 8.73% 24.82 93.2
0.0251 50.0 gil 1.25 4.7
0.5601 OJXl2 gil 0.001
26.07 97.9
Stream
Feed: As,0,
Total
Total
Products
TABLE 4
Sample As, % Cu,% Fe, % 5,..., SO/-
1100 BOF 3.2 35.2 13.8 10.0 29.6
1100 CYC 1.0 25.7 'n.7 6.8 16.7
Final BOF 3.3 36.7 14.7 8.7 25.3
Final CYC l.l 24.4 28.4 6.9 17.3
P
Si
SF
11
The experimental results are shown in Table 2.
As is evident from Table 2. the precipitate contained
substantially all of the arsenic.
A modified EPA Toxicity Characteristic Leaching Procedure
was conducted on the precipitate. The precipitate (60.1
g) was slurried with 500 ml H20 and then filtered. Five
grams of the filtered solid were mixed with 100 ml ofTCLP
Solution No.1 for 18 hours. The solution was filtered and a
laboratory analysis of the filtrate indicated 0.305 mg/l
arsenic. The regulatory limit for arsenic is 5 mg/I. The water
filtered from the slurry was also analyzed and contained
0.205 mgIl arsenic.
A mass balance was calculated based upon the respective
copper and iron analyses of the 1100 bed overflow (BOF)
and cyclone discharge (CYC) samples. The calculated balance
indicated that approximately 0.888 kilograms (kg) of 65
bed overflow and 0.204 kg of cyclone discharge were
recovered for each kg of concentrate charged to the reactor.
An experiment can be conducted to develop process
parameters for converting the pyrite in ores to ferric sulfate.
The experiment was completed in a 4-inch fluidized bed
reactor.
Chemical analysis revealed that a subsample of the concentrate
contained 2.9% arsenic. 36.5% copper. 17.9% iron.
34.4% sulfur and 3.1% silica. Accordingly. the estimated
mineralogical composition of the subsarnple would include
15.4% arsenopyrite. 36.4% chalcocite and 38.5% pyrite.
The desired operating windows for the roasting tests were
based on phase stability diagrams for the Cu-S-O.
Fe-S-a and As-8-a systems at 6500 C. Based upon
this information and upon previous operating experience. a
reasonable target offgas composition is approximately 4%
oxygen and 8% sulfur dioxide (remainder nitrogen). During
the actual test. adjustments to the fluidizing gas composition
were made as needed to maintain the desired bed temperature
and target offgas composition levels. The bed overflow
(BOF) calcine and cyclone underflow streams were periodically
sampled during the test and selected samples were 50
submitted for analysis. The resultant analytical data are
summarized in Table 4.
5,762,891
13 14
5
45
60
19. The method as claimed in claim 1. wherein at least
about 90% by weight of said arsenic in said roasted arseniccontaining
material is dissolved in said pregnant leach
solution.
20. The method as claimed in claim 1. further comprising:
(i) contacting said pregnant leach solution with a metal
hydroxide at a pH ranging from about pH 1.5 to about
pH 2.5 to precipitate said ferric arsenate.
21. The method as claimed in claim 1. wherein in said
10 contacting step. metals from said roasted arsenic-containing
material are dissolved in said pregnant leach solution and
further comprising:
(j) removing said ferric arsenate from said pregnant leach
solution; and
(k) recovering said metals from said pregnant leach
solution.
22. A method for stabilizing arsenic from a material
containing trivalent arsenic and iron in the fonn of a sulfide.
20 comprising:
(a) converting at least about 90% of said trivalent arsenic
into pentavalent arsenic in the fonn of an oxide and said
iron into a salt. wherein at least about 95% of said
arsenic in said oxide and iron salt are contained in said
material;
(b) contacting said material with a lixiviant at a temperature
and pressure sufficient to solubilize said oxide and
iron salt in a pregnant leach solution;
(c) converting any trivalent arsenic in said oxide into
pentavalent arsenic by contacting said pregnant leach
solution with an oxidant; and
(d) forming at a temperature of at least about 90° C. a
precipitate from said pregnant leach solution containing
said pentavalent arsenic.
23. The method as claimed in claim 22. wherein said
converting step (a) comprises:
roasting said material at a temperature less than about
650° C. in the presence of oxygen and sulfur dioxide.
24. The method as claimed in claim 22. wherein said
40 contacting step (b) is conducted at a temperature ranging
from about 20° C. to about 60° C.
25. The method as claimed in claim 22. wherein said
converting step (c) is conducted at a temperature ranging
from about 20° C. to about 90° C.
U. The method as claimed in claim 22. wherein in said
converting step (c) the pH of said pregnant leach solution
ranges from about pH 2.5 to about pH 3.5.
27. The method as claimed in claim 22. wherein said
converting step (c) comprises:
50 contacting said pregnant leach solution with hydrogen
peroxide; and wherein;
said pregnant leach solution has a concentration of hydrogen
peroxide ranging from about 0.5 to about 5.0% by
volume.
28. The method as claimed in claim 22. wherein said
forming step comprises:
contacting said pregnant leach solution with a metal
hydroxide at a pH ranging from about pH 1.5 to about
pH 2.5.
29. The method as claimed in claim 22. further comprising:
(e) removing said precipitate from said pregnant leach
solution to yield a treated solution having at least about
1 mgIL arsenic.
30. A method for stabilizing arsenic from an arsenic- and
iron-containing material. comprising:
the group consisting of: gold. copper, nickel, silver. cobalt.
zinc. and mixtures thereof.
3. The method as claimed in claim 1. wherein said
roasting step is conducted at a temperature ranging from
about 500° C. to about 650° C.
4. The method as claimed in claim 1. wherein said partial
pressure of oxygen ranges from about 0.1 to about 0.01 atm.
5. The method as claimed in claim 1. wherein said
arsenic-containing material comprises iron and said partial
pressure of sulfur dioxide ranges from about 0.1 to about
0.01 atm.
6. The method as claimed in claim 5. wherein said iron is
in the fonn of a sulfide and at least about 20% of said iron
is converted into said iron-containing salt.
7. The method as claimed in claim 1. wherein at least 15
about 90% of said arsenic sulfide in said arsenic-containing
material is converted into arsenic oxide in said roasting step.
8. The method as claimed in claim 7. wherein at least
about 90% of said arsenic in said arsenic oxide is in the
pentavalent state.
9. The method as claimed in claim 1. wherein said
arsenic-containing material contains sulfur and at least about
50% of said sulfur is in said roasted arsenic-containing
material after said roasting step.
10. The method as claimed in claim 1. wherein said 25
roasting step produces an offgas and said offgas includes at
most about 10% of the arsenic in the arsenic-containing
material.
11. The method as claimed in claim 1. wherein said
arsenic-containing material includes sulfur. said roasting 30
step produces an offgas. and said offgas includes at most
about 35% of the sulfur in the arsenic-containing material.
12. The method as claimed in claim 1. wherein said
arsenic-containing material comprises trivalent arsenic and
in said roasting step at least about 90% of said trivalent 35
arsenic is converted into pentavalent arsenic.
13. The method as claimed in claim 1. wherein said
lixiviant comprises a leaching agent that is selected from the
group consisting of sulfuric acid. water. and mixtures
thereof.
14. The method as claimed in claim 1. wherein said
contacting step forms a leach residue and further comprising:
(d) separating said leach residue from said pregnant leach
solution.
15. The method as claimed in claim 14. wherein said
contacting step forms a leach residue and further comprising:
treating said leach residue to recover a metal in said leach
residue.
16. The method as claimed in claim 14, further comprising:
(e) washing said leach residue with a wash solution after
said separating step; and .
55
(f) contacting a portion of said wash solution with said
pregnant leach solution in said forming step.
17. The method as claimed in claim 1. further comprising:
(g) scrubbing an offgas from said roasting step with a
scrubbing solution; and
(h) contacting a portion of said scrubbing solution with
said pregnant leach solution in said forming step.
18. The method as claimed in claim 1. wherein said
contacting step comprises:
contacting said pregnant leach solution with an oxidant 65
selected from the group consisting of hydrogen
peroxide. oxygen. and mixtures thereof.
5.762.891
15
(a) roasting said arsenic-containing material in the presence
of oxygen and sulfur dioxide at a temperature
ranging from about 500° C. to about 650° C. to form a
roasted arsenic- and iron-containing material containing
arsenic pentoxide and ferric sulfate. wherein said 5
oxygen has a partial pressure ranging from about 0.01
to about 0.1 atm and said sulfur dioxide has a partial
pressure ranging from about 0.01 to about 0.1 atm. and
wherein at least about 95% of said arsenic. after said
roasting step, is contained in said roasted arsenic- and 10
iron-containing material;
16
(b) dissolving at least a portion of said arsenic pentoxide
and ferric sulfate in a lixiviant to form a pregnant leach
solution;
(c) forming ferric arsenate in said pregnant leach solution
containing dissolved arsenic pentoxide and ferric sulfate;
and
(d) precipitating said ferric arsenate from said pregnant
leach solution at a temperature ranging from about 20°
C. to about 90° C. and a pH ranging from about pH 1.5
to about pH 2.5.
* * * * *