CA1111157A

CA1111157A - Treatment of arsenical effluents

Info

Publication number: CA1111157A
Application number: CA324,552A
Authority: CA
Inventors: Alan R. Babcock; Wouterus J.M. Kuit
Original assignee: Teck Metals Ltd
Current assignee: Teck Metals Ltd
Priority date: 1979-03-30
Filing date: 1979-03-30
Publication date: 1981-10-20

Abstract

ABSTRACT OF THE DISCLOSURE

A method is disclosed for treating aqueous effluents containing at least one of the elements arsenic and antimony in which said element is oxidized to the pentavalent stage, lime is then added to the effluent in excess, preferably 10 to 50% in excess, of the amount sufficient to precipitate a calcium compound of the element and maintain the pH of the effluent at a value of at least 12, and the precipitated compound and excess lime permitted to harden. The resulting slurry mixture preferably is partially dewatered after precipitation of the calcium compound to a 10 to 40% by weight solids content prior to hardening. A compound chosen from pozzolanic slag, pozzolanic fly ash, Portland cement, soluble silicates and lime or mixtures thereof in an amount in the range of 10 to 50% by weight of the partially dewatered slurry mixture may be added after dewatering.

Description

This invention relates to the removal of arsenic and antimony from waste water and, more particularly, to the treatment of arsenic and antimony-containing effluents and sludges, and the stabilization of such sludges.
Due to increasingly stringent government regulations for the disposal of noxious materials, and to avoid detrimental consequences to the environment, not only must waste waters and effluents be cleaned up, but sludges resulting from the treatment of waste waters and effluents must be made physically stable and chemical]y inert if these sludges are to be safely impounded or deposited as landfill.
Of particular concern are waste waters and effluents that contain arsenic. According to most methods proposed for the treatment of arsenical effluents, treatment is effected with lime or slaked lime under highly alkaline conditions in one or more stages, whereby calcium arsenite or calcium arsenate precipi-tates. Accordinc~ to other methods, arsenical effluents are treated with ferrous or ferric salts in the presence of lime. Rowever, precipitated compounds in the resulting sludges are not sufficiently insoluble to provide the desired degree of chemical inertness, i-e-do not have a sufficierltly low leachability and physical stability to allow safe disposal. In an effort to overcome these disadvantages, it has heen proposed to add such compounds as lime, cement, gypsum and silicates ' to the effluent precipitates to improve the physical stability and chemical inert~ess. It has also been proposed to subject sludges and precipitates to calcination or to encapsulation in molten slag or sulfur. However, the addition of lime, cement, gypsum and silicates still does not provide solids that are both sufficiently physically stable and chemically inert to allow disposal as landfill, while methods involving calcination or encapsulation are economically unattractive.
We have now found that both arsenic and antimony contained in aqueous effluents can be readily and economi-- cally converted into a physically stable and chemically inert solid which can be safely disposed of as landfill.
We have found that, when arsenic and antimony present in effluents in the trlvalent state are converted to the penta-valent state and the resulting pentavalent arsenic and antimony are treated with excess lime, solids with a low leachability and a high compressive strength are obtained.
Accordingly, there is provided a method for the treatment of aqueous effluent containing at least one element chosen from arsenic and antimony which comprises the steps of oxidizing said element in said effluent to the pentavalent state, adding an amount of lime in excess of the amount necessary to precipitate a calcium compound of said at least one element and permitting precipitated compound and excess lime to harden.

- 2 -.lSi7 According to a preferred embodiment, there is provided a method for the continuous treatment of aqueous effluent containing at least one element chosen from arsenic and antimony which comprises the steps of oxidizing said element in said effluent to the pentavalent state, adding an amount of lime in excess of the amount required to maintain tlle pH
of the mixture of effluent and lime at a value of at least about 12, whereby said element is precipitated, removing at least a portion of the water from said mixture and permitting the at least partly dewatered mixture to harden.
The invention will now be described in detail~ -Effluents that can be treated according to the method of the invention are waste waters and effluents from the mining and metallurgical industry that contain -arsenic and/or antimony. Such waste waters and efflu-ents include mine-drainage waters, solutions obtained in the concentration of mineral values in ore~, the treatment of concentrates for the recovery of metal values, the electrodeposition of metals and the treat-ments of slimes and flue dusts, and include sludges such as obtained from scrubbing of gaseous effluents.
~rsenic and antimony are usually present in these waste waters and effluents in the trivalent state.
Besides arsenic and antimony, such waste waters, effluents and sludges usually contain one or more other elements such as, for example, lead, zinc, copper, iron, chloride and fluoride. In the treat-ment of waste waters, effluents and sludges, arsenicand antimony behave similarly and the following description is therefore given with reference to a~ueous arsenical effluent and it is understood that this term includes waste waters, effluents and sludges that comprise arsenic and/or antimony without the exclusion of other elements.
As stated hereinabove, arsenic is usually present in aqueous arsenical effluent in the trivalent state. r~Jhen lime is added in sufficient amount, calcium arsenite precipitates which has an undesirably hi~h solubility, so that mere treatment o~ effluent with lime results in the formation of solids which have a high rate of leachability upon disposal. If oxidation of trivalent arsenic to the pentavalent state is performed then the solubility of the precipi-tate is substantially lower. If, furthermore, an excess of lime is added, we have found that a further reduction in the solubility of the solids is obtained.
This could, for example, be due to the formation of a complex compound of calcium arsenate and lime which may be represented by the formula 3Ca3(~sO~)2.Ca(O~I)2.
The oxidation of trivalent arsenic to the pentavalent state can be readily accomplished by any one of a large number of suitable oxidizing agents such as oxygen, ozone, peroxides, chlorine, hypochlor-ite, ferric chloride or manganese dioxide. We prefer to accomplish the oxidation with chlorine by sparging chlorine into the effluent at ambient temperatures until trivalent arsenic has been oxidized. The effluent is preferably near neutral pH during the oxidation with chlorine, for example, at a value in the range of about 7 to 8. The p~ is maintained in this range by adding a s~ l suitable alkaline material, preferably lime. At values of the pH below about 7, chlorine ~as becomes a ha~ard. At values of the pH above ahout 8, the oxidation will be impaired due to precipitation of calcium arsenite when lime is used as a neutralizing agent.
Lime is added to the effluent to precipitate the arsenic Lime is preferably added as a slurry of ~round lime and ~later. The amount of lime should be at least sufficient to raise the pH of the efflu-ent to a value of at least about 12. At values of tl-e pH below about 12 the precipitation of calcium arsenate is incomplete, Preferably, the amount of lime is added in an excess in the range of about 10 to 50~ of the amount necessary to maintain the p~l of the reaction mixture of effluent and lime at a value of at least about 12, to precipitate a calcium arsenate compound and to form a mixture of precipi-tated arsenic compound and excess of lime. The addition of an excess o~ lime of at least about 10~
is believed to be sufficient to precipitate arsenic as a calcium arsenate-lime complex. The leacha}~ility of the solids in the reaction mixture ~ecreases with increasing exce~ss of lime. The lime may be added continuously or intermittently.
After the addition of the desired amount of lime! the resultant mixture may be impounded or, alternatively, further treated. If desired, a floccu-lating agent may be added. Preferably, the resultant _ 5 _ .

mixture is treated for the removal of at least a portion of the water and the resultant solids are impounded or stored Removal of water may be accomplished by standard methods such as thickening~ centrifugation, filtration, or a combination of these methods. After dewatering to obtain an at least partly dewatered mixture preferably containing at least 40%
solids by weight, the resultant solids are permitted to harden over a period of time and thereby to attain a very low leachability. The excess of lime in the solids makes it possible for carbonation to take place which results in formation of a hard carbonate matrix and ensures that the alkalinity of the solids is maintained to provide the desired degree of lasting chemical inertness. The physical stability of the hardened solids, i.e. the bearing and compressive strengths~ is sufficient to allow disposal of tl~e solids as landfill. The advantageous properties of the hardened solids are likely due to the formation of solid-solid solutions of arsenates and lime, as well as carbonate bonding, i.e., by the reaction of lime with carbon dioxide, because o the significant amount of lime.
The physical stability o the solids can be further improved by mixing the at least partly dewatered reaction mixture with an amount of a compound chosen from pozzolanic slag and fly ash, Portland cement, soluble silicates and lime. Pozzolanic slag and pozzolanic fly ash are most effective when an excess of lime is present.
Before mixing the at least partly dewatered reaction mixture with an amount of such compound, the dewatering should be carried out to obtain an at least partly dewatered reaction mixture that contains solids in the range of about 10 to 40~ by weight.

15'7 Mixing of such compounds with such at least partly dewatered reaction mixture in a suitable mixing device, such as for example a pusmill, or cement mixer, in an amount in the range of about 10 to 50% by weight of dewatered reaction mixture, increases the bearing and compressive strengths, the amount of such compound being inversely proportional to the degree of dewatering.
The invention will now be illustrated by means of the following non-limitative examples. In the examples 1 through 4~ a synthetic effluent containing,

3.63 g/L As3+ ~s203 3.47 g/L F as ~F
0.51 g/L Pb2+ as PbO
0.43 g/L Sb3+ as Sb2O3 and having a pH of 2.5 was treated. In all of the following examples, 200 mL/min of aqueous arsenical effluent was continuously fed to a reactor with agitation and lime was added in the form o~ a slurry of lime in water contairling 100 g/L lime as calcium hydroxide. The treated effluent was discharged into a clarifier for a 2 hour retention. The clarifier underflow was discharged and the solids recovered by filtration. After a cure period of one to three months, the recovered solids were subjected to a per-colation leach, wherein 150 g recovered solids (in broken form) were placed in a 5 cm diameter column which was filled with water to just submerge the -solids. Water was percolated through the open system from the top of the column at a hydraulic loading of ~0 :~ .

l.~ S~

1.1 m3/day/m2. The volume of water in the column was kept constant at 215 mL to give a solids-leachate retention of 2.4 hr. Leachate samples were collected at specific intervals and assayed for arsenic.
Example 1 ~
Using the effluent and method as described above, lime was added to effluent until the pH had in-creased to 12~0, The total amount of lime was 12.8 g lime/L effluent, which amount is the stoichiometric amount necessary to precipitate the above mentioned elements in the effluent. The clarifier overflow contained 10 mg/L As. The percolation leach samples collected after 1,3 and 14 days assayed 47, 53 and 34 mg/L As~ respectively.
Example 2 The test of Example 1 was repeated hut 26 g lime/L effluent was added, i,e., a 100% excess of the amount necessary to maintain a pH of 12Ø Leachate samples collected after 1,3,7 and 14 days of the per-2b colation test assayed 1.4, 2.2, 3.6, and S.9 mg~L As, respectively. The arsenic content of the clarifier overflow was 6 mg/L.
Example 3 The test of Example 1 was repeated but a portion of the arsenic and antimony in the effluent, i.e. 70%, was oxidized by sparging chlorine gas through the effluent while adding the stoichiometric amount of lime.
Leachate samples collected after 1,3 and 14 days assayed 0,72, 0.54 and 0.51 mg/L As, respectively. The arsenic ~0 content of the clarifier overflow was 4.1 mg/L.

- , - -L5'7 _ ample 4 The test of example 3 was repeated but all arsenic and antimony was oxidized and 26 g lime/L effluent added.
Leachate samples collected after 1, 3, 7 and 14 days assayed 0.01, 0.02, 8.08 and 0.01 mg/L As, respectively.
The arsenic content in the clarifier overflow was 0.3 to 0.7 mg/L.
Example 5 A number of tests were conducted wherein a scrubber effluent from a metallurgical operation containing 490 mg/L
trivalent As, 80 mg/L trivalent Sb, 3420 mg/L F and 270 mg/L
Pb was oxidized by sparging gaseous chlorine into the effluent until arsenic and antimony were present in the pentavalent state. A sufficient amount of lime was added to maintain the pH at a value of 12Ø The resulting slurry was thickened and subsequently filtered. The arsenic content of the thickener overflow was 0.14 mg/L. The filtercake containing 40% solids by weight was mixed with various amounts of either poz~olanic tail slag, or pozzolanic fly ash, lime, pozzolanic tail slag and lime, or sodium silicate and cement, the mixtures permitted to harden over a period of one month and the hardened mixtures subjected to the percolation leach test, as described above, and a confined compressive strength test The tail slag contained 27% SiO2, 11% CaO and 36% iron. The fly ash contained higher amounts of SiO2 and CaO and contained substantially less iron than the tail slag. The results of the tests are given in the following table.

g _ .. l~ .... .. l. ... - .o-F~ N ~ ~ ~ ~I r~ O N ~1 O ~ ~1 ~i O N ~I r-l,O

;1 ~t N ~1 00 N N 11~ ~1 ~1 ~I CO ~ ~ ~1 ~ O r-l ~
R~ ~~ O O 0000 N~OO
.. I. .-- .. I. .... ....

~ V ~ ~I V V ~ ~

~ ~1 _1 t~ O ~I CO Ln ~1 N ~ t~ t~ ~ O 1` N ~ i Q ~ O ~ .-1 ~1 ~1 0 ~1 0 -1 0 0 _-1 0 I-J O ~1 -~

V V

U~ ~ ~ N ~`J ~) N ~ r) ~ ~) Irl N ~ u) a~
u~,~oo oooo u~oo~1 oo~o oooO ~u~oo Z O O O O O O O ~;Z O O O O o o o o o o o o z o o a) ,1u~ o o Ln ~n In U~ O ~ Ul O U~ O O
Ç~ ~ O U~ U~ ~ 00 ~ O CO N ~I CO ~D ~r 0~ G~ N ~D
k Q. . .. .... . .. .... . ~ .... . .
~ I~ ,~ ~1 ~1 ~1 ,~ ~ I ~ ~1 ~1 ~ ~1 o ,~ ~ ~1 o ~ I ~J o ,1 ~
,1 ~
O ~ ~ _i ~ ~ ~1 ~ ~1 ~ E~
a~

~,~ ~ rd tq ~ ~ ~ P~ h P~
lq ~ .Y ,Y .~: ~,Y 0.Y ,Y
~ tC ~:
o o o o o o a~ ~ o ~ o ~ o ~ o ~ o ~ o -r~
V~ N 5~1~ ~ ) 1 N ~ 1 L ) S~ ~r O
O O ~ O Y ~ ~ ~ S

U~
u, a~ ~
U~ U
a) X a~
O O ~ .C
~rl ~ ~U X
Q -JZ O -I E; dP ,1 d oP k d~ ,1 ~ 1 Ei ~ u~
~1 o ~ ~ o ~1 o ~5 ~ o ~ oo ~ o z --10-- :

.

;i7 Results in the above Examples show that solids with a low leachability are obtained when aqueous arsenical effluent is oxidized to convert trivalent arsenic into pentavalent arsenic, an excess of lime is added, the resultant mixture is at least partly dewatered and the dewatered mixture is permitted to harden. The results also show that the physical stability of the solids can be further improved without impairing the chemical inertness wnen the oxidized effluent is treated with an amount of lime to maintain a pH of 12, the effluent is dewatered to obtain a solids mixture containing 40% solids by weight, and the partly ~ewatered solids are mixed with an amount of pozzolanic slag, lime, soluble silicate or pozzolanic fly ash in an amount in the range of 10 to 50~ hy weight.
It will be understood of course that modifications can be made in the embodiment of the invention illustrated and described herein without departing from the scope and purview of the invention as defined by the appended claims.

. _ , . ~, . .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for the treatment of aqueous effluent containing at least one element chosen from arsenic and antimony which comprises the steps of oxidizing said element in said effluent to the pentavalent state, adding an amount of lime in excess of the amount necessary to precipitate a calcium compound of said at least one element and permitting precipitated compound and excess lime to harden.

2. A method for the continuous treatment of aqueous effluent containing at least one element chosen from arsenic and antimony which comprises the steps of oxidiz-ing said element in said effluent to the pentavalent state, adding an amount of lime in excess of the amount required to maintain the pH of the mixture of effluent and lime at a value of at least about 12, whereby said element is precipitated,removing at least a portion of the water from said mixture and permitting the at least partly dewatered mixture to harden.

3. A method as claimed in claim 1,or 2, wherein said oxidizing is accomplished by sparging chlorine into the effluent.

4. A method as claimed in claim 1,or 2, wherein the excess of lime is sufficient to precipitate said element as a complex of said precipitated element and lime.

5. A method as claimed in claim 1,or 2, wherein lime is added in an excess in the range of about 10 to 50% of the amount necessary to maintain the pH of the reaction mixture of effluent and lime at a value of at least about 12, to precipitate said element and to form a mixture of said precipitated element and excess lime.

6. A method as claimed in claim 2, wherein said at least partly dewatered mixture contains at least 40% solids by weight.

7. A method as claimed in claim 2, or 6, wherein the at least partly dewatered mixture is mixed with at least one compound chosen from slag, fly ash, cement, a silicate and lime in an amount in the range of 10 to 50% by weight of said at least partly de-watered mixture.

8. A method for the treatment of aqueous effluent containing at least one element chosen from arsenic and antimony which comprises the steps of oxidizing said element to the pentavalent state, adding an amount of lime sufficient to maintain the pH of the mixture of effluent and lime at a value of at least about 12 whereby said element is precipitated, dewatering the mixture to obtain at least partly dewatered mixture containing solids in the range of about 10 to 40% by weight and mixing said at least partly dewatered mixture with a compound chosen from pozzolanic slag, pozzolanic fly ash, Portland cement, soluble silicates and lime or mixtures thereof in an amount in the range of 10 to 50%
by weight of said dewatered mixture, the amount of said compound being inversely proportional to the degree of dewatering.

9. A method as claimed in Claim 1, 2 or 8, wherein said effluent also contains fluoride.

10. A method as claimed in Claim 1, 2 or 8, wherein said oxidizing is accomplished by sparging chlorine into the effluent while maintaining the pH in the range of about 7 to 8.

11. A method as claimed in Claim 8, wherein the amount of lime is sufficient to precipitate said element as a complex of said precipitated element and lime.

12. A method as claimed in Claim 8, wherein said lime is added in an excess in the range of about 10 to 50% of the amount necessary to maintain the pH of the mixture of effluent and lime at a value of at least about 12, to precipitate a calcium compound of said element and to form a mixture of said precipitated calcium compound and excess lime.

13. A method as claimed in Claim 12, wherein said at least partly dewatered mixture is mixed with a compound chosen from pozzolanic slag and pozzolanic fly ash.

14. A method as claimed in Claim 1, 2 or 8, wherein said effluent also contains lead.