United States Patent [19]
Reynolds et a1.
[11]
[45]
4,244,927
Jan. 13, 1981
[56] References Cited
U.S. PATENT DOCUMENTS
[54] PROCESS FOR RECOVERING ARSENIC
COMPOUNDS BY SODIUM HYDROXIDE
LEACHING
[75] Inventors: James E. Reynolds; Eazo L.
Coltrinari, both of Golden, Colo.
(73] Assignee: Hazen Research, Inc., Denver, Colo.
[21] Appl. No.: 61,412
[22] Filed: Jul. 27, 1979
[51] Int. 0.3 COIB 27/02
[52] U.S. O : ,. 423/87; 423/150;
. 423/602
[58] Field of Search 423/87, 602, 150, 38,
423/41,98, 109, 1; 75/101 R, 72, 104, 109, 117,
118 R, 120, 121
Assistant Examiner-Wayne A. Langel
Attorney, Agent, or Firm-Sheridan, Ross, Fields &
McIntosh
ABSTRACT
During the processing of high arsenic materials such as
smelter flue dust, extremely insoluble ferric-arsenic
compounds are generated to immobilize the arsenic
during leaching of the metals. The arsenic may be recovered
in saleable form from the arsenic-containing
residues by leaching with sodium hydroxide and crystallizing
the arsenic salts from the leach residue.
[57]
An arsenic-recovery process primarily for use in conjunction
with the hydrometallurgical processing of
arsenic-containing materials for metal recovery. Arsenic
is recovered from ferric arsenate by reaction with
sodium hydroxide in accordance with the following
general reaction:
Sill , 423/87
Nadkarni et a1. .. 423/87
Sandesara 423/87
2,951,741 9/1960
3,911,078 10/1975
4,118,243 10/1978
Primary E~aminer-,-O. R. Vertiz 19 Claims, No Drawings
Values Leached
Cu, Zn, Cd, Ge,
Pb, Ag, Bi, Au, Sb
As
Unreacted sulfides of
Cu, Zn and Fe; sulfur,
gold, tin. gypsum, and
ul1Ireacted ferric oxides
Hot H2S04
Hot chloride
Hot NaOH
(As fixation)
Types
I
2
3
Tails
BEST MODE FOR CARRYING OUT THE
INVENTION
Leach Stage
The process is effective with a wide range of arsenic
bearing materials including flue dusts containing in
excess of 10% arsenic and less than 6% iron.
In the first leach of the preferred embodiment, copper,
zinc, cadmium, and germanium are solubilized by a
sulfuric acid leach, which also solubilizes the arsenic.
To avoid carrying the arsenic over into the further
processing of the liquor to redeem the metal values, the
arsenic is oxidized under oxygen pressure and precipitated
out as an extremely insoluble ferric-arsenic compound.
The preferred method for supplying the extra ferric
ions when necessary for the stoichiometry of completely
insolubilizing all the arsenic present is the addition
of ferrous sulfate, preferably in excess of the
amounts necessary for combination with all the arsenic.
The leach system is maintained in an oxidizing mode.
The excess ferric ion generated insures precipitation of
virtually all of the arsenic.
Although ferric iron may be present in the system in
the form of hematite, the solubility of hematite is not
high enough to efficiently produce the required excess
of ferric ions.
2
sure leach of the ore wherein some arsenic is insolubilized
and about O.S to 2.0 grams per liter arsenic remain
in solution. Inthis process, no attempt is made to insolubilize
essentially all the arsenic value as is done in the
5 present process,
U.S, Pat. No. 2,686,114 to McGauley, et aI., discloses
the insolubilization of arsenic values in a high pressure,
high temperature ore oxidation leach using air. Arsenic
is precipitated with iron in the ore and with alkaline
10 earth metals as arsenates of these metals. The advantages
of total insolubilization of arsenic with ferric ions
apparently are not known to these inventors.
In accordance with the objectilve of recovering the
most valuable metal products firs,t, and processing the
arsenic with the least possibility of contamination of
recovered products and danger to the environment and
to workers handling the materials, copper smelter flue
dust is first leached under oxygen pressure with acid,
preferably hot sulfuric acid, containing ferric ions to
recover one or more of the following: copper, zinc,
25 cadmium, and germanium and to precipitate arsenic as
an extremely insoluble ferric-arsenic compound. Next,
the residue is leached with hot chloride solution to
extract anyone or more of the metal values lead, silver,
gold, bismuth, and antimony, without solubilizing the
arsenic. Finally, the residue is leached with a hot sodium
hydroxide solution to recover arsenic as a saleable
product, and the. tails, after final arsenic fixation, are
safely disposed to the environment. The sequential
leaching process is summarized as follows:
4,244,927
1
BACKGROUND ART
PROCESS FOR RECOVERING.ARSENIC
COMPOUNDS BY SODIUM HYDROXIDE
LEACHING
TECHNICAL FIELD
The invention lies in the field of hydrometallurgical
recovery of metals, specifically arsenic.
DISCLOSURE OF THE INVENTION
A paper, entitled "Hydrometallurgical Recovery or
Removal of Arsenic From Copper Smelter By-products"
by K. Togawa, Y. Umetsu and T. Nishimura,
presented at the 107th A.I.M.E. annual meeting at Den- 30
ver, Colorado on Feb. 27-Mar. 2, 1978, discusses the
problems involved in recovering valuable metals from
copper ore refining flue dust, as well as the removal of
arsenic as insoluble arsenates and as arsenic sulfide from
aqueous solutions, and the hydrometallurgical recovery 35
of arsenic trioxide from arsenic sulfide. The paper does
not disclose an integrated process for the successful
recovery from the flue dust of metals uncontaminated
with arsenic, or the recovery of arsenic as a saleable
product, nor does it disclose the other advantages or 40
objectives of the present process.
An earlier article entitled "Recovery of Metals from
the Dusts of Flash Smelting Furnace" by Eikichi Mohri
and Minoru Yamada presented at the WorId Mining and
Metals Technology Proceedings of the Denver Joint 45
MMIJ-AIME meeting in 1976 discloses a hydrometallurgical
process for treating copper smelter dusts by
leaching, precipitating some (but not all) copper with
hydrogen sulfide, neutralizing to pH2 with calcium
carbonate to prevent precipitation of iron arsenate dur- 50
ing subsequent copper cementation with iron powder,
and finally oxidizing the solution with blowing air to
form a stable iron arsenate. This process does not overcome
the necessity for handling arsenic in its dangerous
soluble forms during metal recovery steps, as does the 55
present invention, nor does it immobilize arsenic in an
insoluble state early enough so that substantially complete
copper recovery is possible (H2S precipitation
must be halted at 2-3 grams of copper per liter of solution
to avoid precipitating arsenic with the copper in 60
this process). Neither does it provide for the recovery
of arsenic in saleable form as sodium arsenate. The
present process overcomes these problems and allows
for substantially complete recovery of copper, zinc,
cadmium, lead, and other metals, as well as sodium 65
arsenate.
U.S. Pat. No. 4,149,880 to Prater, et aI., discloses a
copper cementation process following an oxygen pres-
A process for recovering arsenic is provided which
may be used in conjunction with metal recovery processes
from slags, flue-dust and the like containing arsenic,
wherein the arsenic has been immobilized as an
extremely insoluble ferric-arsenic compound and car- IS
ried through various leach states.
The material containing the insoluble ferric-arsenic
compound is leached with a hot sodium hydroxidesolution
in excess of stoichiometric amounts, the liquor
evaporated to a high sodium hydroxide concentration, 20
and a soluble arsenic salt crystallized therefrom.
The residue is then treated with ferric ions to fix any
remaining arsenic as an insoluble ferric-arsenic compound
suitable for disposal.
FeAs04+3NaOH-Na3As04+Fe(OHh·
Excess sodium hydroxide is preferred. In addition to
the stoichiometric amount required for the above reaction,
an excess improves the performance of the circuit,
and provides for better crystallization of the arsenate.
The sodium hydroxide leach is conducted for onehalf
to two hours, and preferably about one hour to
insure maximum arsenate solubilization.
4,244,927
3 4
The metal values to be leached are solubilized within Calcium chloride extracts lead as its chloride, which
a very short time, but because the arsenic present in the is highly soluble in the chloride solution, from its insoluflue
dust is also extremely soluble, being 50% soluble in ble sulfate, precipitating gypsum. The chloride solution
water alone, the materials in the system must be allowed will contain calcium chloride if extraction of lead is
to remain in contact for over an hour: depending on 5 desired, and may also contain sodium chloride, hydrotemperature
and pressure as well as economic require- gen chloride, magnesium chloride, and barium chloride
ments, from I to 8 hours, and more preferably from 2 to as the source of the chloride ions necessary to solubilize
3 hours. This extended period of time allows for the the desired metal values.
formation of the insoluble ferric arsenic compound. The oxidizing agent may be sodium chlorate, manga-
An oxidizing mode is maintained at approximately 10 nese dioxide, ozone, chlorine, hydrogen peroxide or
-400 to approximately -500 mv (saturated calomel/- others, and preferably is sodium chlorate or manganese
platinum electrodes) with oxygen at a pressure of ap- dioxide.
proximately 25 to approximately 75 psi, and more pref- The oxidizing agent (added in amounts required to
erably, about 45 to about 55 psi, in order to oxidize the adjust the emf to at least about -700 mv) solubilizes the
ferrous ions to their ferric state, the arsenic to its penta- 15 gold while preventing re-leaching ofthe arsenic. If gold
valent state, and the sulfides to their more soluble suI" recovery is not desired, only slight oxidizing conditions
fates. need be maintained.
The reaction is conducted at a temperature of be- If there is insufficient lead in the oxidizing acid leach
tween about 90° C. and about 130° C., and more prefera- residue to merit recovery or lead recovery is otherwise
bly between about 105° C. and about 115° C. 20 not required, the chloride leach may be adjusted by
The pH is maintained from about 0.1 to about 1.5, reduction of the calcium chloride and the temperature
preferably with sulfuric acid, to solubilize as much of in order that the lead sulfate will not be solubilized
the metal values as possible without dissolving the fer- while silver, gold and bismuth values are leached. The
ric-arsenic compound. chloride level must be sufficient so as to maintain the
As an optional step, in order to increase the filtration 25 silver in solution.
rate of the leach slurry, gypsum may be generated as a The temperature of between about 80° C. and about
filter aid in situ by partially neutralizing the slurry (from 105° C., and preferably approximately 95° C.-100° c.,
100 to 50 gil sulfuric acid) with calcium carbonate. This and the pH, adjusted at between about 0.1 to 1.0 and
partial neutralization was shown to increase the filter preferably between about 0.4 and 0.6 with sulfuric acid,
rate in gpm/ft2 by a factor of approximately 10. This 30 allow for maximum solubilization of the metal values
step should be omitted if later arsenic recovery from the with minimum arsenic extraction.
residue is desired, as the excesss sulfate in the residue The leach materials are allowed to remain in contact
lowers arsenic recovery. for between about one-half and two hours and prefera-
A liquid-solid separation is performed and the leach bly about one hour to insure complete dissolution of the
filtrate is then neutralized to pH approximately 2 to 4 by 35 lead, in the presence of ferric ions in a concentration of
the gradual addition, with agitation, of calcium carbon- about 3 gil, added if necessary as FeCI3. These addiate.
During neutralization, the arsenic is precipitated to tional ferric ions, in the oxidizing conditions of the
less than 100 parts per million, and preferably less than leach, insure that the ferric to ferrous ratio will be high
10 parts per million. This arsenic appears in the gypsum enough to prevent formation of gold and prevent recake
formed during neutralization. The ferric-arsenic 40 leaching of the ferric-arsenic compound.
compound solubility product is nearly constant at this A liquid-solid separation is performed, and the lead
pH, and thus arsenic precipitation increases with in- crystallized as high purity lead chloride, after which it
creased ferric ion concentration in solution. may be reduced to elemental lead by pelletization with
During neutralization, the temperature is permitted a carbonaceous material such as coke and an alkaline
to reduce to between about 50° C. and about 60° C., to 45 material such as limestone or calcium oxide at between
decrease arsenic solubility, and the materials are al- 'about 800° C. and about 1000° C. producing a CaCh
lowed to remain in contact for about ~ hour to allow flux which may be recycled to regenerate the chloride
time to aid the precipitation. leach and return chloride to the system. Silver, gold,
Copper, zinc, cadmium, and germanium are recov- bismuth, and antimony may be cemented out of the
ered from the filtrate by conventional methods, includ- 50 solution with elemental lead.
ing electrowinning of the copper, with sulfuric acid Alternatively, the arsenic-free filtrate may be used to
produced in the copper cells being recycled to the sul- enrich feed material for processing of lead ores.
fate pressure leach. Cadmium and germanium may be Arsenic is recovered in saleable form from the brine
recovered by cementation with zinc, and zinc sulfate leach residue or from other materials bearing insoluble
recovered by evaporation. Because the arsenic has been 55 arsenates in which the arsenic is present in its pentavaremoved,
the filtrate may also be recycled without lent form by leaching in a sodium hydroxide leach of
metal recovery therefrom, to enrich the feed of conven- heated, strong, basic solution to extract arsenic as sotional
metallurgical recovery processes for copper and dium arsenate, according to the generalized reactionzinc.
Arsenic may be extracted from the residue by means 60
of the sodium hydroxide leach process of this invention
described below.
Alternatively, filtered and washed residue from the
oxidizing acid leach is advanced to a chloride leach to
solubilize lead, silver, gold, bismuth and antimony. A 65
hot chloride solution with an oxidizing agent in the
presence of ferric ions extracts the metal values while
leaving the arsenic compound undissolved.
4,244,927
5
It is noteworthy that where the oxidizing acid leach
slurry was partially neutralized with calcium carbonate
prior to filtration, arsenic recovery during the caustic
leach is somewhat reduced, and thus, when arsenic
recovery is to be performed on the residue, calcium 5
carbonate neutralization should be performed on the
original leach liquor after it has been separated from the
arsenic-containing residue so as not to add large quantities
of gypsum" to" the residue.
The sodium hydroxide leach is performed at at least 10
about 40° C. in order to solubilize the arsenate. Upper
temperature limits are determined by the needs of the
crystallization step.
After a liquid-solid" Separation, a soluble arsenate of
high purity is crystallized from the liquid by vacuum 15
6
ing arsenic and allow for safe disposal to the environment.
EXAMPLES
The following examples are descriptive, but not limiting
of the invention.
\. Sulfate pressure leach:
Five-hundred (500) gram samples of copper smelter
flue dust were leached in a 2-Iiter Parr autoclave having
an impellor speed of 1550 rpm at 100° C. under an oxygen
pressure of 50 psig with an oxygen bleed of 300 cc
per minute (except for Test 5 where no oxygen bleed
was used). The dust contained 13.9% copper, 2.05%
zinc, 9.51% arsenic and 5.60% iron. Test results are
summarized in Table I.
TABLE I
SULFATE PRESSURE LEACH
TEST NO. 2 3 4 5 6
LEACH TIME 2 2 2 123 123 2
(hr.)
emf (mv) 4.10 430450460 435460 460
pH 0.35 0.1 0.0
LEACH
SOLUTION
gil H2SO4 100 114 50 98 100 100
gil iron 5Fe+3 16.4FE +3 15.7Fe+3 19.8Fe+2 20Fe+2 34.3Fe203Reagent
RESIDUE
ASSAY (%)
Cu 1.30 1.79 1.36 1.30 1.48 1.24 1.33
Zn 0.25 12.80.230.20 0.27 0.24 0.23
As 11.2 12.8 12.9 13.8 13.4 13.2 12.8
LIQUOR
ASSAY (gil)
Cu 51.8 59.2 56.6 56.6 56.8 52.5 52,0 54.7
Zn 7.40 8.65 8.088.088.16 7.807.767.68
As,o, 13.3 5.58 4.57 5.46 3.65 2.98 5.37 3.47 2.40 18.6
Fe 1.0 3.10 0.56 4.55 3.84 3.80 4.65 3.81 3.92 0.70
% EXTRACTED
Cu 94.7 92.5 93.8 94.1 93.8 94,,494.6
Zn 93.1 92.6 92.9 94.4 92.4 92,,8 92.8
As 34 14 11 13 8.9 7.3 14 9.56.7 45
evaporation and cooling to approximately 25° C., while
stirring. The sodium hydroxide is preferably concentrated
to from about 58-79 gil NaOH to about 154-160
gil NaOH. The crystals are then filtered, and dried.
Excess sodium hydroxide in the liquid insures that 45
substantially all the arsenic will be crystallized when
eOficentrated to about 154-160 gil NaOH at room tem"'
fillite. Further concentration is unnecessary and
mi.ht f@§Uj~ in contamination of the product with crystilliHd
sodiUm hydroxide. 50
Cut to the high solubility of an arsenate like sodium
IrHftlte in wlitet; the crystals are not washed, but are '
dried It approximately 80° C.
The .xca buic filtrate is recycled to the sodium
hydroxidellich. 'fbe leach residue may betreated with 55
Additionil fetric jons to fix the small amount of remain-
2. Neutralization:
The autoclave slurry of Example I is filtered and the
liquor neutralized with calcium carbonate. In Test 1,
prior to filtration, partial neutralization (from 100 gil
H2S04 to 50 gil H2S04) is effected with calcium carbonate
and in Test 2, this partial neutralization is omitted.
The partial neutralization of 'fest 1 precipitates gypsum
which increases the filtration rate for the leach
slurry from 0.013 gpm/ft2 to 0.17 gpm/ft2• After filtration
of the slurry, the liquor is cooled to 50' to 60' C.
and vigorously agitated while calcium carbonate is
added in order to neutralize the solution to a pH of
about 3. After one-half hour, the material is again filtered.
The results of these tests are summarized in Table
2.
TABLE 2
NEUTRALIZATION WITH CALCIUM CARBONATE
, DISTRIBUTION %
TEST I TEST 2
PRODUCT Cu Zn Cd Ge As Cu Zn Cd Ge As
LIQUOR 94.0 92.7 72.4 82 6.2 94.0 93.2 76.9 82 6.7
RESIDUE 6.0 7.3 27.6 18 93.8 6.0 6.8 23.1 18 93.3
NEUTRALIZED
L1QUOD 93.2 92.7 71.0 53 <0.03 92.9 93.1 72.S 42 <0.03
CaS04CAKE 0.8 <0.1 1.4 29 6.2 1.1 0.1 4.4 40 6.7
CaC03
CONSUMPTION
7
4,244,927
8
TABLE 2-continued
NEUTRALIZATION WITH CALCIUM CARBONATE
DISTRIBUTION %
TEST I TEST 2
PRODUCT Cu Zn Cd Ge As eu Zn Cd Ge As
Ib/ton DUST 282 514
CAUSTIC LEACH AND SODIUM ARSENATE
CRYSTALLIZATION DISTRIBUTION AS A PERCENT
OF THE FLUE DUST FEED MATERIAL
TEST I TEST 2
As Bi Sn Ge As Bi Sn
5. Arsenic fixation:
Ge
1.8 82.1 2.9
TABLE 4-continued
72.5
15 PRODUCT
LIQUOR
CRYSTALS
3. Brine Leach:
250 grams of autoclave residue was leached with 250 10
gil NaCl, 25 gil CaCh and 3 gil ferric ion as FeCb.6H20,
at a temperature of 95° C. to 100° C. and a pH of
0.5, adjusted with HCl in an oxidizing mode using NaCl03
to achieve an emf of -690 to -700 mv. Test 1
residues were those in which the autoclave leach slurry
was partially neutralized before filtration with CaC03,
as described in Example 2, and Test 2 residues were not
previously treated with CaC03. Test results are summarized
in Table 3.
TABLE 3
BRINE LEACH
(DISTRIBUTION AS A PERCENTAGE OF
TOTAL FLUE DUST FEED MATERIAL)
TEST I TEST 2
PRODUCT As Pb Ag Au Bi As Pb Ag Au Bi
LIQUOR <0.5 99.5 83.6 89 77.7 <0.5 99.2 85.8 89 77.8
RESIDUE 93.8 0.4 16.0 11 21.0 93.3 0.8 13.8 11 21.8
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.3
<0.4
<0.3
<0.3
<0.3
<0.3
<0.3
SOLUBILITY
ppm/As
4.0
3.6
3.8
3.8
4.0
3.6
3.7
3.8
4.0
3.5
3.5
4.0
3.4
3.4
3.5
pH
TABLE 5
ARSENIC FIXATION
We claim:
BRINE RESIDUE (neutralized)
Days:°I
4
5
BRINE RESIDUE (not neutralized)
Days:°I
4
CAUSTIC RESIDUE (neutralized)
Days:°3
4
5
CAUSTIC RESIDUE (not neutralized)
Days:°3
4
5
60
30 Arsenic solubility in the brine and caustic leach residues,
for residues initially partially neutralized before
filtration of the sulfate pressure leach as described in
Example 2 and for non-neutralized residues, was determined
after fixing the arsenic by the addition of 2 ml 100
gil Fe+3in the form of Fe2(S04)3 to 10 grams of residue
slurried with 75 ml H20, the emf being adjusted to
-500 mv, and the pH being adjusted to approximately
2.0 with sulfuric acid. Calcium carbonate was then
added to adjust the pH to 4.0 and the mixture stirred for
one hour at 25° C. The solubility of the fixed arsenic
was then tested by contacting with demineralized water
for up to 5 days. Test results are summarized in Table 5.
These results show extremely low arsenic solubility,
even at fairly low pH, thus confirming that the brine
and caustic leach residues are environmentally safe for
disposal.
CAUSTIC LEACH AND SODIUM ARSENATE
CRYSTALLIZATION DISTRIBUTION AS A PERCENT
OF THE FLUE DUST FEED MATERIAL
4. Caustic leach and sodium arsenate crystallization:
The residue from the brine leach described in Example
3 was leached in sodium hydroxide at 40° C. to
extract arsenic as sodium arsenate. 3 grams of sodium
hydroxide were used for each gram of arsenic dissolved
plus an excess of70 gil. Test 1 residue was the result of 35
the partially neutralized autoclave slurry described in
Example 2. Test 2 was as described in Example 2. The
leach slurry was then filtered and the liquid evaporated
under vacuum to alter the sodium hydroxide concentration
from 58-79 to 154-160 gil NaOH, and cooled to 40
25° C. while stirring to crystallize sodium arsenate from
the liquid. The crystals were then dried on a Buchner
funnel without washing, and further dried at 80° C. Test
2 residue was derived from non-neutralized autoclave
slurry residue. Test results are summarized in Table 4. 45
The values for "Liquor" and "Residue" resulted from
the caustic leach. The values for "Mother Liquor" and
"Crystals" resulted from the sodium arsenate crystallization.
A further test was performed on non-neutralized 50
residue using a leach time of 4~ hours and an excess
sodium hydroxide of 80 gil, and resulting in 88.3%
arsenic extraction.
S?diu~ arsenate c.rystals from all tests were of good
punty, With only mmor amounts of entrained sodium 55
sulfate and co-crystallized tin as impurities. Bismuth and
germanium, which tended to report in varying degrees
to all prior leach liquors and residues, were substantially
not present in the sodium arsenate crystals.
TABLE 4
TEST I TEST 2
:-P:-R_O_D_U:-C_T_-..:A;.::"S_.:B;:.i_.:S:.:;n_...:G::;e=-_A:::::..s_.::.B:.:.i_.::.S::,:n_....::G.:.e_ 65
LIQUOR 80.9 0.1 1.9 <5 87.0 0.1 3.0 <5
RESIDUE 12.9 20.9 94.6 8-18 6.3 21.7 92.6 8-18
MOTHER 8.4 0.1 4.9 0.1
10
13. The process of claim 10 wherein the solids of step
(b) are treated with ferric ions to fix the arsenic as a
ferric arsenic compound suitably insoluble for disposal.
14. The process of claim 13 wherein the ferric ions
added are ferric sulfate in the amount of 20 grams per
kilogram of residue, and the mixture is neutralized to
about pH 3.5 to about 4.5 with calcium carbonate to aid
precipitation of ferric-arsenic material.
15. The process of claim 10 wherein in step (c) the
10 sodium arsenate is crystallized by vacuum evaporation
of the liquid from a concentration of about 50-80 grams
sodium hydroxide per liter of about 150 to 160 grams
per liter, and allowing to cool to about 25° C.
16. The process of claim 10 wherein following step
15 (c) the crystals are dried without washing.
17. A process for recovering arsenic from arseniccontaining
materials comprising:
(a) oxidizing the arsenic to its pentavalent state;
(b) precipitating the arsenic with ferric ions as ferric
arsenate;
(c) leaching the ferric arsenate with a sodium hydroxide
solution at a temperature of at least about 40°
c.;
(d) performing a liquid-solid separation on the leach
materials; and
(e) crystallizing an arsenic salt from the liquid of step
(d) by evaporation.
18. The process of claim 17 wherein the solid residue
of step (d) is treated with additional ferric ions and an
30 alkaline earth metal carbonate to adjust the pH to about
3.0 to 4.5 to fix all the arsenic present as a highly insoluble
ferric arsenate suitable for disposal.
19. A process for recovering arsenic as sodium arsenate
from a leach residue which contains a ferric-arsenic
compound wherein the arsenic is present in its pentavalent
state which comprises:
(a) leaching the residue with sodium hydroxide in
excess of stoichiometric amounts to solubilize the
arsenic at a temperature of at least about 40° C.;
(b) performing a liquid-solid separation on the material
of step (a);
(c) performing a vacuum evaporation on the liquid of
step (b) to increase the sodium hydroxide concentration
from about 55 to 80 grams sodium hydroxide
per liter to about 150 to 160 grams per liter and
allowing to cool to crystallize sodium arsenate;
(d) drying the arsenate crystals without washing; and
(e) fixing any arsenic remaining in the solids of step
(b) by adding 20 grams ferric sulfate per kilogram
of residue and neutralizing to between about pH 3.5
to 4.5 with calcium carbonate to form ferricarsenic
compounds suitably insoluble for disposal
to the environment.
* * * * *
4,244,927
9
1. A process for recovering arsenic from materials
containing ferric-arsenic compounds in which the arsenic
is present in its pentavalent state, comprising:
(a) leaching the material with a stoichiometric excess
of a sodium hydroxide solution; 5
(b) performing a liquid-solid separation on the leach
materials; and
(c) crystallizing the soluble arsenic salt from the liquid
of step (b).
2. The process of claim 1 wherein ferric ions are
added to fix the arsenic in the leach residue.
3. The process of claim 2 wherein the ferric-arsenic
mixture is neutralized to a pH between about 3.5 and
about 4.5 with calcium carbonate.
4. The process of claim 1 wherein the leach is performed
at a temperature of at least 40° C.
5. The process of claim 1 wherein the leach is performed
for about one-half to about two hours. 0
6. The process of claim 1 wherein the leach is per- 2
formed for about one hour.
7. The process of claim 1 wherein following separation
of the leach liquid, vacuum evaporation is performed
to concentrate the sodium hydroxide from 25
about 50-80 grams per liter to about 150 to 160 grams
per liter, in order to crystallize the sodium arsenate
product.
8. The process of claim 1 wherein during crystallization
the liquid is allowed to cool to about 25° C.
9. The process of claim 1 wherein following crystallization
the crystals are dried without washing at about
80° C.
10. A hydrometallurgical process for recovering ar- 35
senic as sodium arsenate from residues containing arsenic
resulting from the step of serially leaching various
metal values while carrying ferric-arsenic compounds
in which the arsenic is present in its pentavalent state
undissolved in the residues, which comprises 40
(a) leaching the residues with sodium hydroxide in
excess of stoichiometric amounts to permit a reaction
wherein sodium arsenate is formed in solution
and the ferric ions are precipitated by means of the
hydroxide; 45
(b) performing a liquid-solid separation on the material
of step (a);
(c) crystallizing sodium arsenate from the liquid of
step (b). 50
11. The process of claim 10 wherein the leach is performed
at a temperature of at least about 40° C.
12. The process of claim 11 wherein the leach is performed
for about one hour.
55
60
65