CA1250407A - Selenium recovery - Google Patents
Selenium recoveryInfo
- Publication number
- CA1250407A CA1250407A CA000469557A CA469557A CA1250407A CA 1250407 A CA1250407 A CA 1250407A CA 000469557 A CA000469557 A CA 000469557A CA 469557 A CA469557 A CA 469557A CA 1250407 A CA1250407 A CA 1250407A
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- CA
- Canada
- Prior art keywords
- selenite
- calcium
- added
- selenium
- solution
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B19/00—Selenium; Tellurium; Compounds thereof
- C01B19/008—Salts of oxyacids of selenium or tellurium
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Inorganic Chemistry (AREA)
- Removal Of Specific Substances (AREA)
Abstract
SELENIUM RECOVERY
ABSTRACT
Discloses a process for recovering relatively pure selenium IV
compound from acid solution comprising sequential precipitations with lime water with solid-liquid separation between precipitations.
ABSTRACT
Discloses a process for recovering relatively pure selenium IV
compound from acid solution comprising sequential precipitations with lime water with solid-liquid separation between precipitations.
Description
~2504(~7 SELENIUM RECOVERY
The present lnvention is concerned with recovery of selenium from acidlc solutlon particularly with recovery of selenium from roaster scrubber solutions.
BACKGROUND OF THE INVENTION
Selenium is usually recovered from process intermediates (e.g., anode slimes, acid plant sludges etc.) by a roasting operation. Scrubber liquor derived from roasting often contains sulphuric acid and various metal lmpurlties. Selenlum is usually recovered from this liquor as elemental selenium by treatment with S02 gas. Acid is generated by the S2 treatment which must be neutralized adding to the cost and various impurities e.g., Te is and Pb can be co-preclptated wlth the selenlum. A general description of selerium recovery techniquea is found in Encyclopedia of Chemical Technology, Kirk-Othmer Third Edition Vol. 20 15 pages 578-j82. Oxidic compounds of selenium are usually prepared indirectly by oxidlzing elemental selenium and are thus more expensive than they would be if recovered directly. This added cost could limlt certain potential large scale applications of selenium e.g., as a soil 1;~5~ 07
The present lnvention is concerned with recovery of selenium from acidlc solutlon particularly with recovery of selenium from roaster scrubber solutions.
BACKGROUND OF THE INVENTION
Selenium is usually recovered from process intermediates (e.g., anode slimes, acid plant sludges etc.) by a roasting operation. Scrubber liquor derived from roasting often contains sulphuric acid and various metal lmpurlties. Selenlum is usually recovered from this liquor as elemental selenium by treatment with S02 gas. Acid is generated by the S2 treatment which must be neutralized adding to the cost and various impurities e.g., Te is and Pb can be co-preclptated wlth the selenlum. A general description of selerium recovery techniquea is found in Encyclopedia of Chemical Technology, Kirk-Othmer Third Edition Vol. 20 15 pages 578-j82. Oxidic compounds of selenium are usually prepared indirectly by oxidlzing elemental selenium and are thus more expensive than they would be if recovered directly. This added cost could limlt certain potential large scale applications of selenium e.g., as a soil 1;~5~ 07
2 PC-2168 additive. In certain geographical areas selenium deficiency in the soil can cause the white muscle disease in animals and, it is currently believed, certain nutritional deficiencies in humans. Thus provision of less expensive, fertilizer compatible, selenium compounds is a highly desireable goal. A cheaper and non-volatile selenium compound could also be useful for glass, ceramics - and steel - making industries.
SUMMARY AND GENERAL DESCRIPTION OF THE INVENTION
The aforestated ob~ect of the present invention is accomplished by treating an acidic solution containing selenium IV, sulfate ion (or perhflps more accurately bisulfate ion), metallic impurities and optionally selenium VI with a base e.g., a calcium base such as calcium carbonate and/or hydroxide in a first step or steps to raise the pH of the solution to between 2 and 6, separating from the aqueous solution the precipitate formed in the first step or steps and thereafter further neutrallzing the thus treated solution with a base to provide an essentially pure selenite salt. The base used in the second neutralization step is advantageously slaked lime, i,e., calcium hydroxide. Use of calcium hydroxide in the second neutrali%ation results in an easily filterable precipitate of an essentially pure hydrate of calcium selenite which i8 fertilizer compatible.
The use of calcium base (preferably as a slurry in water) for neutralization is highly advantageous in that reagent cost is at fln absolute minimum and, in that the ultimate product, calcium selenite, is of definite character because of its low solubility and is very easily recoverable.
The process of the present invention is based upon the phenomena that by first neutralizing to a pH of between 2 and 6 with Ca(OH)2 in one or more stages practically all of sulfate species in acid solution is precipitated as CaSO4 hydrate along with hydroxides or hydrated oxides of heavy metals. Very little SeO3 ion is precipieated in this first neutralization. Within the pH range of 2 to 6 there is a trade-off in that at pH 3 relatively little heavy metal (e.g., Fe, Cu, Ni) is co-precipitaeed with hydrated CaSO4 but practically no Se IV is lost in - :12S04~7
SUMMARY AND GENERAL DESCRIPTION OF THE INVENTION
The aforestated ob~ect of the present invention is accomplished by treating an acidic solution containing selenium IV, sulfate ion (or perhflps more accurately bisulfate ion), metallic impurities and optionally selenium VI with a base e.g., a calcium base such as calcium carbonate and/or hydroxide in a first step or steps to raise the pH of the solution to between 2 and 6, separating from the aqueous solution the precipitate formed in the first step or steps and thereafter further neutrallzing the thus treated solution with a base to provide an essentially pure selenite salt. The base used in the second neutralization step is advantageously slaked lime, i,e., calcium hydroxide. Use of calcium hydroxide in the second neutrali%ation results in an easily filterable precipitate of an essentially pure hydrate of calcium selenite which i8 fertilizer compatible.
The use of calcium base (preferably as a slurry in water) for neutralization is highly advantageous in that reagent cost is at fln absolute minimum and, in that the ultimate product, calcium selenite, is of definite character because of its low solubility and is very easily recoverable.
The process of the present invention is based upon the phenomena that by first neutralizing to a pH of between 2 and 6 with Ca(OH)2 in one or more stages practically all of sulfate species in acid solution is precipitated as CaSO4 hydrate along with hydroxides or hydrated oxides of heavy metals. Very little SeO3 ion is precipieated in this first neutralization. Within the pH range of 2 to 6 there is a trade-off in that at pH 3 relatively little heavy metal (e.g., Fe, Cu, Ni) is co-precipitaeed with hydrated CaSO4 but practically no Se IV is lost in - :12S04~7
3 PC-2168 the precipitate. At the high end of the pH range of 2 to 6, much more heavy metal i9 coprecipitated with hvdrated CaS04 but, at the fiame time, more SeIV i8 lost to the precipitate. Preferrably for purest end product the first neutralization should be carried to an end point in the pH
range of 4 to 6. For maximum yield of Se IV end product, the first neutralization should be carried to an end point in the pH range of 2 to
range of 4 to 6. For maximum yield of Se IV end product, the first neutralization should be carried to an end point in the pH range of 2 to
4. As an alternative the first step can be accomplished in two or more stages e.g., a first neutralization stage to pH 3, separation of solids followed by a second neutralization stage to pH 5.5 again followed by separation of solids. In this way both yield and purity can be maintained near optimum.
If desired, after solids from the first neutralization have been removed, suitable reagents may be used to precipitate selectlvely certain remaining contaminants in solution. For example, if desired, the sulfate ion content of the solution can be lowered still further by precipitation with barium ion added as an aqueous solution of a water soluble salt, e,g., BaCl2, preferably in an amount not exceeding the stoichiometric equivalent of sulfate ion in solution. Alternatively, a base such as lime could be added to hvdrolyse impurities. Any precipitate from such an intermediate purification step is then removed from the partially neutralized aqueous solution.
In the final neutralization using calcium hydroxide, the pH of the solution containing Se IV is raised to the range of 7 to 9 or higher.
When this is done CaSeO3 as a hydrate precipitates in essentially pure form. The purity of the calcium selenite hydrate depends upon several factors including the pH of initial neutrallzation and the quantity and character of the impurities in the original acidic solution. However in following the present teachings, one can reasonably expect to obtain a product in which the weight ratio of selenium to sulfur is in the neighborhood of 120; the weight ratio selenium to tellurium i9 in the neighborhood of at least 200 and; the weight ratio of selenium to total heavy metal is at least about 40. Higher than the stated minimum weight ratios of selenium to tellurium and to total heavv metals can reasonably be expected if the initial neutralization is carried out to an end point between ph 4 and 6. A higher ratio of selenium to sulfur can be ~ZS0~07 . .
gravimetric analysis (9~ weight loss at 150 - 450C) indicated that the final product was the monohydrate, CaSeO3.H2O (theoretical analysis 21.6 Ca, 42.7 Se).
EXAMPLE II
Table 1 shows the fate of impurity elements in another example of the invention. Only 0.8% of the Se was precipitated, and 94% of the SO4 was removed in the calcium sulfate precipitate. When the fi]trate was neutralized to pH 11 by further lime addition, the precipitate analyzed (~): 39.5 Se, 22.0 Ca, 2.18 S04 0.19 Te, 0.40 Cu, 0.12 Ni, 0.25 Fe, and 0.05 As, Sb, Bi, Sn.
TABLE I
PRECIPITATION OF CALCIUM SELRNITE s INITIAL NEUTRALIZATION TO pH 3 FEED LIQUOR CaSO HYDRATE CALCIUM SELENITE PPT
(g/l)Analysi~q ~PPTD Analysis ~PPT
(%) (%) ~t/Vol 180Q 11.14 Kg 12.22 Kg Se 28.0 0.38 0.8 39.5 95.8 SO4 43.4 65.9 94.0 2.18 3.4 Ca 27.0 22.0 Cu 0.32 0.40 84.9 Ni 0.09 0.12 90.5 Fe 0.03 0.25 100.0 Te 0.20 0.19 64.5 As 0.053 0.0003 0.4 Sb 0.006 Bi 0.0045 Sn 0.01 ~LZ504(~7 achleved, if desired by use of barium salt or other suitable reagent as previously de6cribed between the initial and final neutralizations.
PARTIC~LAR DESCRIPTION OF THE INVENTION
In carrying the present invention into practice it is advantageous to treat aqueous acidic scrubber solutions containing sulfate ionJ selenium IV and heavy metal ion at a temperature of about 20 to about 100C e.g., 60C with 8 slurry of lime (e.g., about 200 g/l) added at a rate to neutralize the acidic solution to a pH of 3 to 5 over a period of about 0.5 to about 3 hours e.g., l to 2 hours. The initial pH of the solution is normally 1.5 or less. When the process is carried out in this manner the precipitated product comprising hydrated calcium sulfate settles rapidly and is readily filterable.
Once the hydrated calcium sulfate precipitate is removed calcium selenite is precipitated by adding the same lime slurry to the filtrate solu~ion maintained at a temperature in the same range as employed in the lnltlal neutrallzation. The lime slurry is added over a period of one hour or two to an end point of pH 7 to 7.5. The resultant neutralized filtrate and newly precipitated solids are digested for another hour or two until the pH increases to 9. After dlgestlon 18 complete, the newly preclpitated and digested solids comprlslng calclum selenite settle at a very rapld rate and are filterable at a rate of the order of 12,000 1 /m2h .
~,.
EXAMPLE I
A scrubber liquor derived from roasting anode slimes contalning H2S04 was treated. The liquor analyzed 28 g/l Se and 43 g/l S04 and had a pH of <l. The solution was neutrallzed to pH 3 with lime slurry and the precipitate obtained analyzed (~): 23.3 Ca, 54.6 S042 , 0.8 Se representlng 94% precipitation of S042 and only 2% Se precipitation~
After solids removal, further neutralizat~on to pH 7.2 and digestlon to pH 9.4 gave a precipitate which analyzed (%) 2l.8 Ca, 41.6 Se and l.O
S042 . 99.7% of the selenium was precipitated. The analysis and thermal - ~25V407 EXAMPLE III
A scrubber llquor similar to that treated in Example II was treated ln the same manner as set forth in Example II except that the initial neutralization was to an end point of pH 5. Table II sets forth the analysis of each of the feed solution, calcium sulfate precipitate, purified solution and calcium selenite and the distribution of elements between the precipi~a~es.
"
TABLE II
PRECIPITATION OF CALCIUM SELENITE
ELEMENT FEED ~ PURIFIED CaSeO~ DISTRIBUTION (%) SOLN PPT SOLN PPT ~ CaSeO3 (g/l) (~) (g/l) (%) Se 28.0 1.0 16.0 38.0 2.6 97.1 S04 44.9 64.3 1.2 0.91 95.8. 1.55 C~ 28.2 25.0 Cu 0.11 0.14 0.0037 0.00894.2 0.9 Ni 0.033 0.007 0.017 0.03815.1 81.8 Te 1.16 1.6 0.0280.060 96.2 3.7 Pb 0.0095 0.092 0.0007 0.00598.3 0.1 Fe 0.004 0.05 0.0005 0,01997.7 2.1 As 0,083 0.0063 0.0002 0.086 93.5 6.3 Sb 0.001 0.005 0.001 0.005 Bi 0.001 0.0045 0.001 0.0045 Sn 0.001 0.005 0.002 0.005 The data in Table II shows that the calcium selenite produced after initial neutralization to a pH of 5 and removal of precipitated solids is 2504~:)7 7 PC~2168 purer ~han the calcium selenite produced in Example I especially in regard to tellurlum and total heavy metals.
EXAMPLE IV
THERMAL STABILITY OF CALCIUM SELENITE
Calcium selenite monhydrate was heated in air to 1000C, using a heatlng rate of 3C/min. After initial los3 of water (9% weight 109s between 150 and 450C, corresponding to 108s of one mole of water), two further small weight losses of 0.8X (425-575C) and 3.2% (600-1000C) were observed. The residue analyzed (~): 43.0Se(total), 40.4Se(lV), and 26.7Ca, indicating that the calcium selenlte is thermally stable and is not oxidized in air.
EXAMPLE V
CONVERSION TO OTHRR SELENITE SALTS
Calcium selenite monohydrate (60.lg) analyzing 38.0Z Se and 23.0%
Ca was slurried in water at 80C. Approxlmately stoichiometric addition of H2S04 was made, and the precipitated gypsum (50g, which analyzed 0.6X
Se) was filtered off. Approximately stoichiometric NaOH (14.6g) was added to the filtrate, when a further precipitate of CaSeO3 (12.lg, analyzing 44.0% Se) came down and was filtered off analyzlng 75g/1 Se, was evaporated to a small volume. The crystallized product (l9.g) analyzed (%): 47.5 Se, 23.5 Na, and 0.02 Ca (theoretical for Na2SeO3 (Z):
45.65 Se and 26.6% Na).
The dlstrlbutlon of Se was:
Fraction Wt/VolDistribution (%) Feed 60.lg 100 Gypsum Ppt 50.1~ 2.4 2nd Ppt 12.lg 23.4 (CaSeO3) Solutlon 225ml 74.4 ~ZS~)~V7 ~ PC-2168 Although the conditions were not optimized, it is evident that CaSeO3 can be converted to Na2SeO3 in reasonable yield. The CaSeO3 which re-preclpitated on neutralization can be recycled. Other bases such as KOH, Zn(OH)2 can also be used to make the appropriate selenite salt.
Calcium selenite product produced in accordance with the present invention can be used as such, for example, in fertilizer compositions, or as an inexpensive and non-volatile source of Se for ceramics, glass and steel industries. It can also be used as a raw material for production of other selenium compounds such as Na, K, Zn, NH4 selenites.
The product selenites could also then be oxidized to form the corresponding selenate compounds.
In cases such as fertilizer applications where co-presence of sulfate and selenite is acceptable, the neutralization of scrubber liquor can be accomplished with a base such as ammonia. Precipitate of heavy metals formed at a pH ln the range of 2 to 6 is removed prior to final neutralization to produce a solution from which sulfate-selenite salts can be recovered.
While in accordance with the provisions of the statute, there is described herein specific embodiments of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.
If desired, after solids from the first neutralization have been removed, suitable reagents may be used to precipitate selectlvely certain remaining contaminants in solution. For example, if desired, the sulfate ion content of the solution can be lowered still further by precipitation with barium ion added as an aqueous solution of a water soluble salt, e,g., BaCl2, preferably in an amount not exceeding the stoichiometric equivalent of sulfate ion in solution. Alternatively, a base such as lime could be added to hvdrolyse impurities. Any precipitate from such an intermediate purification step is then removed from the partially neutralized aqueous solution.
In the final neutralization using calcium hydroxide, the pH of the solution containing Se IV is raised to the range of 7 to 9 or higher.
When this is done CaSeO3 as a hydrate precipitates in essentially pure form. The purity of the calcium selenite hydrate depends upon several factors including the pH of initial neutrallzation and the quantity and character of the impurities in the original acidic solution. However in following the present teachings, one can reasonably expect to obtain a product in which the weight ratio of selenium to sulfur is in the neighborhood of 120; the weight ratio selenium to tellurium i9 in the neighborhood of at least 200 and; the weight ratio of selenium to total heavy metal is at least about 40. Higher than the stated minimum weight ratios of selenium to tellurium and to total heavv metals can reasonably be expected if the initial neutralization is carried out to an end point between ph 4 and 6. A higher ratio of selenium to sulfur can be ~ZS0~07 . .
gravimetric analysis (9~ weight loss at 150 - 450C) indicated that the final product was the monohydrate, CaSeO3.H2O (theoretical analysis 21.6 Ca, 42.7 Se).
EXAMPLE II
Table 1 shows the fate of impurity elements in another example of the invention. Only 0.8% of the Se was precipitated, and 94% of the SO4 was removed in the calcium sulfate precipitate. When the fi]trate was neutralized to pH 11 by further lime addition, the precipitate analyzed (~): 39.5 Se, 22.0 Ca, 2.18 S04 0.19 Te, 0.40 Cu, 0.12 Ni, 0.25 Fe, and 0.05 As, Sb, Bi, Sn.
TABLE I
PRECIPITATION OF CALCIUM SELRNITE s INITIAL NEUTRALIZATION TO pH 3 FEED LIQUOR CaSO HYDRATE CALCIUM SELENITE PPT
(g/l)Analysi~q ~PPTD Analysis ~PPT
(%) (%) ~t/Vol 180Q 11.14 Kg 12.22 Kg Se 28.0 0.38 0.8 39.5 95.8 SO4 43.4 65.9 94.0 2.18 3.4 Ca 27.0 22.0 Cu 0.32 0.40 84.9 Ni 0.09 0.12 90.5 Fe 0.03 0.25 100.0 Te 0.20 0.19 64.5 As 0.053 0.0003 0.4 Sb 0.006 Bi 0.0045 Sn 0.01 ~LZ504(~7 achleved, if desired by use of barium salt or other suitable reagent as previously de6cribed between the initial and final neutralizations.
PARTIC~LAR DESCRIPTION OF THE INVENTION
In carrying the present invention into practice it is advantageous to treat aqueous acidic scrubber solutions containing sulfate ionJ selenium IV and heavy metal ion at a temperature of about 20 to about 100C e.g., 60C with 8 slurry of lime (e.g., about 200 g/l) added at a rate to neutralize the acidic solution to a pH of 3 to 5 over a period of about 0.5 to about 3 hours e.g., l to 2 hours. The initial pH of the solution is normally 1.5 or less. When the process is carried out in this manner the precipitated product comprising hydrated calcium sulfate settles rapidly and is readily filterable.
Once the hydrated calcium sulfate precipitate is removed calcium selenite is precipitated by adding the same lime slurry to the filtrate solu~ion maintained at a temperature in the same range as employed in the lnltlal neutrallzation. The lime slurry is added over a period of one hour or two to an end point of pH 7 to 7.5. The resultant neutralized filtrate and newly precipitated solids are digested for another hour or two until the pH increases to 9. After dlgestlon 18 complete, the newly preclpitated and digested solids comprlslng calclum selenite settle at a very rapld rate and are filterable at a rate of the order of 12,000 1 /m2h .
~,.
EXAMPLE I
A scrubber liquor derived from roasting anode slimes contalning H2S04 was treated. The liquor analyzed 28 g/l Se and 43 g/l S04 and had a pH of <l. The solution was neutrallzed to pH 3 with lime slurry and the precipitate obtained analyzed (~): 23.3 Ca, 54.6 S042 , 0.8 Se representlng 94% precipitation of S042 and only 2% Se precipitation~
After solids removal, further neutralizat~on to pH 7.2 and digestlon to pH 9.4 gave a precipitate which analyzed (%) 2l.8 Ca, 41.6 Se and l.O
S042 . 99.7% of the selenium was precipitated. The analysis and thermal - ~25V407 EXAMPLE III
A scrubber llquor similar to that treated in Example II was treated ln the same manner as set forth in Example II except that the initial neutralization was to an end point of pH 5. Table II sets forth the analysis of each of the feed solution, calcium sulfate precipitate, purified solution and calcium selenite and the distribution of elements between the precipi~a~es.
"
TABLE II
PRECIPITATION OF CALCIUM SELENITE
ELEMENT FEED ~ PURIFIED CaSeO~ DISTRIBUTION (%) SOLN PPT SOLN PPT ~ CaSeO3 (g/l) (~) (g/l) (%) Se 28.0 1.0 16.0 38.0 2.6 97.1 S04 44.9 64.3 1.2 0.91 95.8. 1.55 C~ 28.2 25.0 Cu 0.11 0.14 0.0037 0.00894.2 0.9 Ni 0.033 0.007 0.017 0.03815.1 81.8 Te 1.16 1.6 0.0280.060 96.2 3.7 Pb 0.0095 0.092 0.0007 0.00598.3 0.1 Fe 0.004 0.05 0.0005 0,01997.7 2.1 As 0,083 0.0063 0.0002 0.086 93.5 6.3 Sb 0.001 0.005 0.001 0.005 Bi 0.001 0.0045 0.001 0.0045 Sn 0.001 0.005 0.002 0.005 The data in Table II shows that the calcium selenite produced after initial neutralization to a pH of 5 and removal of precipitated solids is 2504~:)7 7 PC~2168 purer ~han the calcium selenite produced in Example I especially in regard to tellurlum and total heavy metals.
EXAMPLE IV
THERMAL STABILITY OF CALCIUM SELENITE
Calcium selenite monhydrate was heated in air to 1000C, using a heatlng rate of 3C/min. After initial los3 of water (9% weight 109s between 150 and 450C, corresponding to 108s of one mole of water), two further small weight losses of 0.8X (425-575C) and 3.2% (600-1000C) were observed. The residue analyzed (~): 43.0Se(total), 40.4Se(lV), and 26.7Ca, indicating that the calcium selenlte is thermally stable and is not oxidized in air.
EXAMPLE V
CONVERSION TO OTHRR SELENITE SALTS
Calcium selenite monohydrate (60.lg) analyzing 38.0Z Se and 23.0%
Ca was slurried in water at 80C. Approxlmately stoichiometric addition of H2S04 was made, and the precipitated gypsum (50g, which analyzed 0.6X
Se) was filtered off. Approximately stoichiometric NaOH (14.6g) was added to the filtrate, when a further precipitate of CaSeO3 (12.lg, analyzing 44.0% Se) came down and was filtered off analyzlng 75g/1 Se, was evaporated to a small volume. The crystallized product (l9.g) analyzed (%): 47.5 Se, 23.5 Na, and 0.02 Ca (theoretical for Na2SeO3 (Z):
45.65 Se and 26.6% Na).
The dlstrlbutlon of Se was:
Fraction Wt/VolDistribution (%) Feed 60.lg 100 Gypsum Ppt 50.1~ 2.4 2nd Ppt 12.lg 23.4 (CaSeO3) Solutlon 225ml 74.4 ~ZS~)~V7 ~ PC-2168 Although the conditions were not optimized, it is evident that CaSeO3 can be converted to Na2SeO3 in reasonable yield. The CaSeO3 which re-preclpitated on neutralization can be recycled. Other bases such as KOH, Zn(OH)2 can also be used to make the appropriate selenite salt.
Calcium selenite product produced in accordance with the present invention can be used as such, for example, in fertilizer compositions, or as an inexpensive and non-volatile source of Se for ceramics, glass and steel industries. It can also be used as a raw material for production of other selenium compounds such as Na, K, Zn, NH4 selenites.
The product selenites could also then be oxidized to form the corresponding selenate compounds.
In cases such as fertilizer applications where co-presence of sulfate and selenite is acceptable, the neutralization of scrubber liquor can be accomplished with a base such as ammonia. Precipitate of heavy metals formed at a pH ln the range of 2 to 6 is removed prior to final neutralization to produce a solution from which sulfate-selenite salts can be recovered.
While in accordance with the provisions of the statute, there is described herein specific embodiments of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.
Claims (8)
1. A process for isolating selenium in the valence state of IV in usably pure form from an aqueous acid solution containing selenious acid, sulfate ion and heavy metal ion comprising adding a base in one or more stages, to said aqueous acidic solution to provide a pH of 2 to 6, separating solids thus formed at each stage from said aqueous solution, adding additional base to said residual aqueous solution to provide a higher pH and thereafter recovering a product selenite from said residual aqueous solution.
2. A process as in claim 1 wherein said additional base is a calcium base and said product selenite is a substantially pure precipitated calcium selenite.
3. A process as in claim 1 wherein calcium hydroxide is first added to said aqueous acid solution to provide a pH of about 2 to 4.
4. A process as in claim 1 wherein calcium hydroxide is first added to said aqueous acid solution to provide a pH of about 4 to 6.
5. A process as in claim 1 wherein calcium hydroxide is the base added after separation of solids and it is added to an end point of a pH of about 7 to 7.5.
6. A process as in claim 1 carried out at a temperature of 20° to 100°C.
7. A process as in claim 2 in which precipitated calcium selenite is digested in said residual aqueous solution prior to being separated from said residual aqueous solution.
8. A process as in claim 7 in which calcium hydroxide is added to an end point of pH 7 to 7.5 and said residual aqueous solution and calcium selenite precipitate is digested until the pH
rises to about 9.
rises to about 9.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000469557A CA1250407A (en) | 1984-12-07 | 1984-12-07 | Selenium recovery |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000469557A CA1250407A (en) | 1984-12-07 | 1984-12-07 | Selenium recovery |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1250407A true CA1250407A (en) | 1989-02-28 |
Family
ID=4129324
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000469557A Expired CA1250407A (en) | 1984-12-07 | 1984-12-07 | Selenium recovery |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1250407A (en) |
-
1984
- 1984-12-07 CA CA000469557A patent/CA1250407A/en not_active Expired
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