Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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
'
S7
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.
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.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.
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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 :~ .
-- 7 --
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.
8 --
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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. .... ....
00 0 0000 00 0 0000 0000
~ 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 -~
00 0 0000 00 0 0 00 0000
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.
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