Process for separating arsenic from acid solutions which contain it

A process for separating arsenic from acid aqueous solutions containing arsenic and other metal ions. The process involves bringing the acid aqueous solution into contact with a water-immiscible organic diluent consisting of hydrocarbons, alcohols or their mixtures, in which there is dissolved as solvent at least one polyol. The solvent thereby extracts the arsenic, which is then re-extracted with a counter-solvent.

This invention relates to a process for separating arsenic from acid 
solutions which contain it. 
More particularly, the invention relates to a process for separating 
arsenic from sulphuric solutions of copper and other ions. 
Still more particularly, the invention relates to a process for separating 
pentavalent arsenic from acid solutions, in particular sulphuric solutions 
of copper or other ions. 
Almost all commercial copper is produced by electrolytically refining a raw 
material comprising about 99.5% of Cu and containing As together with many 
other extraneous elements. This raw material (blister copper), after 
suitably remelting and casting into slabs, forms the anode of an 
electrolytic cell, the cathode of which can be a slab of copper or, in 
more modern plants, stainless steel or titanium. 
The electrolyte is a copper sulphate solution containing much free 
sulphuric acid. 
On passage of electric current, copper dissolves from the anode and 
simultaneously deposits on the cathode. Of the impurities contained in the 
anode, some remain undissolved and form the so-called "anode mud", whereas 
others (elem-ents less nobel than copper) dissolve electrochemically. One 
of these is arsenic, which therefore constantly increases in concentration 
and has to be removed in order to prevent product contamination. 
There are also other metallurgical problems in which arsenic control and 
removal are very important. This problem exists in the case of copper 
recovery by sulphuric electrolysis with insoluble anodes from residues 
such as the copper froth originating from purification operations in the 
primary metallurgy of lead. 
The input-output equilibrium of arsenic (and of the other impurities) must 
therefore be controlled, and in a known method a part of the copper 
electrolyte is periodically deviated from the cycle, and this is subjected 
to multi-stage electrolytic separation to eliminate the As in the form of 
a Cu-As alloy. However, when solutions poor in coper but rich in arsenic 
are electrolysed, there is the risk of arsine evolution, an extremely 
toxic gas, and special arrangements and precautions are therefore 
required. 
In addition, the subsequent recycling of the alloy sometimes gives rise to 
problems of a technical and cost nature. A more simple process for 
selectively removing arsenic from the copper electrolyte or in any event 
from strongly acid solutions is therefore of particular interest. 
Methods are already known, consisting of extracting arsenic from the copper 
electrolyte using suitable organic solutions, which have tributylphosphate 
as their main active component. The arsenic can be re-extracted from these 
solutions with water or acid or alkaline aqueous solutions, and separated 
by precipitation either as the sulphide by using hydrogen sulphide or 
alkaline sulphides, or as arsenic trioxide by using sulphur dioxide or 
sulphites. 
For example, German patent application No. 2603874 claims substantially the 
separation of arsenic from copper in acid solutions containing sulphuric 
acid, by carrying out the extraction with hydrocarbon solutions containing 
between 50 and 75% of tributylphosphate together with 5% or less of a 
high-boiling alcohol, the explicitly stated purpose of which is to prevent 
the formation of a third phase thus increasing the arsenic concentration 
in the organic phase. 
German patent application No. 2615638 claims the use of a trialkyl, triaryl 
or triarylalkylphosphate, preferably tributylphosphate, diluted in a 
hydrocarbon medium and containing small quantities of isodecanol as an 
emulsion inhibitor. 
British Pat. No. 1551023 claims the use of a solution in the form of a 
hydrocarbon medium containing between 40 and 75% of tributylphosphate and 
up to 15% of a quaternary ammonium salt, preferably 
tricaprylylmethylammonium chloride (Aliquat 336) or a mixture of methyl 
tri(C.sub.8 -C.sub.10)alkylammonium chlorides (Adogen 464). 
All the three cited cases use substantial quantities of tributylphosphate, 
a considerably toxic substance, and in the third case there is a further 
increase in toxicity of the organic extraction mixture due to the addition 
of a quaternary ammonium salt. Belgian Pat. No. 900107 of the present 
applicant describes a process for separating arsenic from acid solutions 
which contain it, by bringing the arsenic-containing solution into contact 
with a water-immiscible organic solvent containing as active components 
for the arsenic extraction one or more alcohols with at least six carbon 
atoms. 
Although the process of said Belgian patent allows considerable separation 
of trivalent arsenic, it does not allow effective removal of pentavalent 
arsenic other than after reduction of the pentavalent arsenic to trivalent 
arsenic, for example by means of sulphur dioxide. 
It has been surprisingly found possible to separate arsenic, including 
pentavalent arsenic, from copper electrolyte or in general from acid 
solutions containing other metal ions in a simple and economical manner, 
with high efficiency, by simply extracting the arsenic with a 
water-immiscible organic diluent containing polyols, thus avoiding the use 
of the aforesaid toxic substances and the need for reduction ot a valency 
of three. The process according to the present invention comprises 
bringing the copper electrolyte containing arsenic or in general the acid 
aqueous solution containing ions of arsenic and of other metals, into 
countercurrent or cocurrent contact with a water-immiscible organic 
diluent consisting essentially of hydrocarbons, alcohols or their 
mixtures, in which there is dissolved as solvent at least one polyol, 
particularly and preferably a diol, said solvent extracting the arsenic 
which is then re-extracted with a counter-solvent. 
The solvent polyols which can be used according to the process of the 
present invention include diols having 7 or more carbon atoms, and triols 
having 10 or more carbon atoms. Particularly suitable diols include 
octane-1,2-diol, decane-1,2-diol, dodecane-1,2-diol, tetradecane-1,2-diol 
and hexadecane-1,2-diol. Phenylethane-1,2-diol, 2-phenylpropane-1,2-diol, 
2-ethylhexane-1,3-diol, 2-methyl-2-propylpropane-1,3-diolo, 
2,2-dimethylhexane-3,5-diol, octane-1,8-diol, dodecane-1,12-diol and 
4-octylcyclohexane-1,2-diol also deserve mention. 
The polyol concentration in the diluent is between 1 and 60%, and 
preferably between 4 and 40%. 
Aliphatic, aromatic and naphthenic hydrocarbons, monohydric alcohols and 
other compounds can be used, either alone or in mixture, as diluents. 
It is essential that the diluents are good polyol solvents and are 
practically insoluble in the aqueous phase. The ratio of the organic phase 
(diluent plus solvent) to the aqueous phase can vary from 0.1 to 10 and 
preferably from 0.5 to 5. 
The arsenic can be re-extracted from the organic phase without difficulty, 
using a counter-solvent consisting of alkaline solutions such as 1N NaOh, 
neutral solutions, water or acid saline solutions such as 0.1M H.sub.2 
SO.sub.4. 
A neutral or alkaline aqueous solution is preferably used.

The examples given hereinafter illustrate the main aspects of the 
invention, but in no case do they constitute a partial or total limitation 
to the scope of the invention. 
EXAMPLE 1 
20 ml of an aqueous solution containing: 
200 g/l of sulphuric acid 
45 g/l of copper 
6.1 g/l of trivalent arsenic 
are fed into a cylindrical separating funnel. 40 ml of 2-ethylhexanol are 
added, and the mixture shaken energetically for 10 minutes. After 
decanting, the aqueous phase is found to contain 4.3 g/l of As.sup.+3 
(29.5% extraction). 
The extraction is repeated on a further 20 ml sample of the same aqueous 
solution but using a 6% solution of tetradecane-1,2-diol in 2-ethylhexanol 
instead of 2-ethylhexanol alone. After decanting, the aqueous phase is 
found to contain 1.95 g/l of As.sup.+3 (68% extraction). 
EXAMPLE 2 
The procedure of Example 1 is followed, but using a 6% solution of 
tetradecane-1,2-diol in 9:1 decane-isodecanol. After decanting, the 
aqueous phase contains 1.13 g/l of As.sup.+3 (81.5% extraction). 
EXAMPLE 3 
Using the procedure described in the two previous examples, 2.times.20 ml 
portions of an aqueous solution containing: 
200 g/l of sulphuric acid 
45 g/l of copper 
6.8 g/l of pentavalent arsenic 
are extracted. Using 2-ethylhexanol, an aqueous phase is obtained 
containing 6.55 g/l of As.sup.+5 (3.7% extraction). When extracting with 
a 6% solution of tetradecane-1,2-diol in 2-ethylhexanol, the aqueous phase 
contains 4.2 g/l of As.sup.+5 (38% extraction). 
EXAMPLE 4 
The procedure of example 3 is followed, except that 9:1 decane-isodecane is 
used instead of the 2-ethylhexanol as solvent for the 
tetradecane-1,2-diol. An aqueous phase is obtained containing 2.7 g/l of 
As.sup.+5 (60% extraction). 
EXAMPLE 5 
The aqueous solution described in Example 1 (trivalent arsenic) is 
extracted with an equal volume of a 10% solution of octane-1,2-diol in 
Solvesso 100. Solvesso 100 is an Esso commercial mixture of aromatic 
hydrocarbons. 
An aqueous phase is obtained containing 0.90 g/l of trivalent arsenic (85% 
extraction). 
EXAMPLE 6 
The aqueous solution described in Example 3 (pentavalent arsenic) is used 
in the procedure of Example 5. 
An aqueous phase is obtained containing 1.10 kg/l of pentavalent arsenic 
(84% extraction). 
EXAMPLE 7 
The aqueous solution described in Example 1 is extracted with an equal 
volume of a 10% solution of 2-phenylpropane-1,2-diol in 9:1 solvesso 
100-isodecanol. 
An aqueous solution is obtained containing 2.00 g/l of trivalent arsenic 
(67% extraction). 
EXAMPLE 8 
The aqueous solution described in Example 3 is used in the procedure of 
Example 7. 
An aqueous solution is obtained containing 3.00 g/l of pentavalent arsenic 
(56% extraction). 
EXAMPLE 9 
The aqueous solution described in Example 1 is extracted with an equal 
volume of a 15% solution in Solvesso of a mixture consisting of isomer 
dodecanediols with vicinal hydroxyls in different positions, this mixture 
being obtained by hydroxylation of a propene tetramer. 
An aqueous solution is obtained containing 2.5 g/l of trivalent arsenic 
(59% extraction). 
EXAMPLE 10 
The aqueous solution described in Example 3 is used in the procedure of 
Example 9. 
An aqueous solution is obtained containing 4.7 g/l of pentavalent arsenic 
(31% extraction). 
EXAMPLE 11 
The aqueous solution discribed in Example 1 is extracted with an equal 
volume of a 10% solution of 2-methyl-2-propylpropane-1,3-diol in Solvesso. 
An aqueous solution is obtained containing 0.8 g/l of trivalent arsenic 
(87% extraction). 
EXAMPLE 12 
The aqueous solution of Example 3 is used in the procedure of Example 11. 
An aqueous solution is obtained containing 1.1 g/l of pentavalent arsenic 
(84% extraction). 
EXAMPLE 13 
The aqueous solution of Example 1 is extracted with an equal volume of a 
20% solution of 2-ethylhexane-1,3-diol in Solvesso. The aqueous solution 
contains 1.3 g/l of trivalent arsenic (78% extraction). 
EXAMPLE 14 
The aqueous solution of Example 3 is extracted in the same manner as in the 
preceding example. An aqueous solution is obtained containing 5.3 g/l of 
pentavalent arsenic (22% extraction). 
EXAMPLE 15 
The aqueous solution of Example 1 is extracted with an equal volume of a 4% 
solution of dodecane-1,12-diol in 2-ethylhexane-1,2-diol. An aqueous 
solution is obtained containing 4.2 g/l of trivalent arsenic (33% 
extraction). 
EXAMPLE 16 
The aqueous solution of Example 3 is extracted in the same manner as in the 
preceding example. An aqueous solution is obtained containing 5.2 g/l of 
pentavalent arsenic (23% extraction). 
EXAMPLE 17 
A solution originating from copper refining and containing inter alia: 
copper: 58 g/l 
nickel: 10.5 g/l 
trivalent arsenic: 0.5 g/l 
pentavalent arsenic: 10.5 g/l 
sulphuric acid: 168 g/l 
is extracted in a single stage with three times its volume of a 6% solution 
of tetradecane-1,2-diol in a 9:1 mixture of ISO L and isodecanol. 
ISO L is an Esso hydrocarbon mixture of isoparaffins. 
An aqueous solution is obtained containing 1.9 g/l of pentavalent arsenic 
(83% extraction). 
EXAMPLE 18 
The organic solution obtained as described in the preceding example and 
containing 3.0 g/l of arsenic is re-extracted twice with equal volumes of 
distilled water. The two aqueous solutions obtained contain 2.6 g/l and 
0.34 g/l of arsenic respectively. Thus 98% of the extracted arsenic was 
re-extracted. 
EXAMPLE 19 
The organic solution regenerated as described in the preceding example is 
again used for extracting arsenic as described in Example 17 and 
subsequent re-extraction as described in Example 18. This operation is 
repeated a further three times. The organic solution resulting from this 
series of treatments is able to extract 80% of the arsenic contained in 
the same solution when operating with a ratio of organic phase to aqueous 
phase of 3:1.