4,244,927 Process for recovering arsenic compounds by sodium hydroxide leaching

Patent Number/Link:

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

Unreacted sulfides of

Cu, Zn and Fe; sulfur,

gold, tin. gypsum, and

ul1Ireacted ferric oxides

Hot H2S04

Hot chloride

Hot NaOH

(As fixation)

Types

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.

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

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

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

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

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

4,244,927

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:

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

<0.4

<0.3

SOLUBILITY

ppm/As

4.0

3.6

3.8

4.0

3.6

3.7

3.8

4.0

3.5

4.0

3.4

3.5

TABLE 5

ARSENIC FIXATION

We claim:

BRINE RESIDUE (neutralized)

Days:°I

BRINE RESIDUE (not neutralized)

Days:°I

CAUSTIC RESIDUE (neutralized)

Days:°3

CAUSTIC RESIDUE (not neutralized)

Days:°3

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

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;

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);

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

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

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);

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.