Method of processing vanadium-containing residues

To process vanadium-containing residues, the residues are leached in an aqueous slurry with introduction of SO.sub.2, the undissolved solids are removed. To effect a processing which is simple, economical and ecologically satisfactory, vanadium content is precipitated as a tetravalent vanadium compound from the sulfate solution at a pH from 7 to 9 by an addition of alkali metal hydroxide and the precipitate is removed from the alkali metal sulfate solution.

SPECIFICATION 
FIELD OF THE INVENTION 
The present invention relates to a method of processing or working up 
vanadium-containing residues in which the residues are leached in an 
aqueous slurry with introduction of SO.sub.2, the undissolved solids are 
removed, and the vanadium contained in the solution is precipitated. 
BACKGROUND OF THE INVENTION 
Vandaium-containing residues become available mainly in the form of used 
catalysts, e.g. from processes for the catalytic conversion of SO.sub.2 to 
SO.sub.3. The residues consist of the carrier material, generally silica, 
and the catalytically active substances. 
In addition to vanadium compounds the residues may contain compounds of 
other metals, such as iron, arsenic, aluminum, calcium, cesium, and other 
alkali metals. 
For ecological reasons such residues cannot be dumped or may be dumped only 
as hazardous waste because they contain water-soluble toxic substances. 
For this reason and to recover valuable constituents of the residues, the 
residues are increasingly processed or worked up. 
From Czechoslovakian Patent Specification 178,626 it is known to leach 
spent vanadium-containing catalysts in water with introduction of SO.sub.2 
so that the vanadium compounds are dissolved as VOSO.sub.4. After the 
carrier material has been removed the VOSO.sub.4 in solution is oxidized 
to V.sup.5+ by the addition of MnO.sub.2 or NaClO.sub.3 and adjustment to 
a pH from 0.5 to 5.0. V.sup.5+ is extracted by an amine having a high 
molecular weight in a water-insoluble solvent and is then reextracted by 
an alkaline solution. Thereafter, V.sub.2 O.sub.5 is precipitated by a 
neutralization with H.sub.2 SO.sub.4. That process requires a large number 
of process steps and a consumption of relatively expensive chemicals and a 
solution which contains alkali metal sulfate is formed in large 
quantities. That solution poses problems in disposal or recovery. 
U.S. Pat. No. 4,913,885 discloses for the processing of vanadium-containing 
raw materials and particularly of spent catalysts a process in which the 
material is leached in an aqueous suspension in the presence of mixed 
gases comprising SO.sub.2 and oxygen. It has been stated that conventional 
exhaust gases of industrial combustion processes which contain 5 to 20% 
SO.sub.2, 10 to 30% O.sub.2, balance inert gases, such as N.sub.2, are 
particularly desirable. As a result, vanadium as VOSO.sub.4 and other 
components as sulfates enter into solution. When the carrier material has 
been removed, the VOSO.sub.4 in the solution is oxidized by an addition of 
oxidizers, such as MnO.sub.2 or NaClO.sub.3 to V.sup.5+, which is then 
precipitated as NH.sub.4 VO.sub.3 by an addition of NH.sub.3, the 
precipitate is removed and the precipitate is then calcined to form 
V.sub.2 O.sub.5. The oxidation of VOSO.sub.4 to V.sup.5+ requires the 
addition of oxidizers and causes additional metals to enter into solution. 
The solution contains a number of dissolved metals and also surplus 
ammonium sulfates. This renders use of the solution, processing or 
disposal more difficult. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide a process for 
processing vanadium-containing residues which is simple, economical and 
ecologically satisfactory. 
SUMMARY OF THE INVENTION 
This object is accomplished in accordance with the invention in that the 
vanadium content is precipitated as a tetravalent vanadium compound from 
the sulfate solution at a pH from 7 to 9 by an addition of alkali metal 
hydroxide and the precipitate is removed from the alkali metal sulfate 
solution. 
More particularly, the method of processing the vanadium containing residue 
according to the invention comprises the steps of: 
(a) introducing SO.sub.2 into an aqueous slurry containing the 
vanadium-containing residue and leaching vanadium from the residue into a 
sulfate solution forming an aqueous phase of the slurry; 
(b) precipitating vanadium as a tetravalent vanadium compound from the 
sulfate solution by bringing a pH of the sulfate solution to 7 to 9 by 
addition of an alkali metal hydroxide to the sulfate solution; 
(c) removing a tetravalent vanadium precipitate from the resulting sulfate 
solution. 
The residues are pretreated before the leaching in dependence on their 
composition. For instance, in the processing or spent catalysts it is 
possible to separate scale by magnetic separation and to subject packing 
elements to classification. The purified residue is then reduced to the 
particle size required for leaching. The vanadium is usually present in 
the waste materials in pentavalent form. The pentavalent compounds have 
low solubility and for this reason are converted in accordance with 
V.sub.2 O.sub.5 +H.sub.2 +SO.sub.2 =V.sub.2 O.sub.4 +SO.sub.4. 
For technological reasons, the V.sub.2 O.sub.4 is dissolved as a sulfate so 
that H.sub.2 SO.sub.4 is required. The sulfuric acid formed the reduction 
of V.sup.5+ to V.sup.4+ with SO.sub.2 provides only one-half of the 
quantity which is stoichiometrically required to dissolve the V.sub.2 
O.sub.4 In accordance with V.sub.2 O.sub.4 +2H.sub.2 SO.sub.4 =2VOSO.sub.4 
+2H.sub.2 O. 
The sulfuric acid which is additionally required can be provided in various 
waye, namely: 
by the simultaneous use of an oxygen-containing gas with or without inert 
gas in the leaching with SO.sub.2 to generate the additional H.sub.2 
SO.sub.4 required in accordance with SO.sub.2 +1/2 O.sub.2 +H.sub.2 
O=H.sub.2 SO.sub.4 ; 
by the addition of oxygen-releasing chemicals during the leaching with 
introduction of SO.sub.2 so that the released oxygen reacts in accordance 
with the previous relationship; 
by the addition of H.sub.2 SO.sub.4 during the leaching with introduction 
of SO.sub.z ; or 
by a combination of the above methods. 
The undissolved solids left after the leaching are removed from the 
solution. In the processing of spent catalysts, the solids usually are 
constituted by the SiO.sub.2 -containing carrier. 
The solids are washed and may be re-used or dumped. The water used for 
washing can be recycled to the leaching step. 
The alkali metal hydroxide which is added to the sulfate solution to 
precipitate its vanadium content will be selected with a view to the 
further use of the alkali metal sulfate solution. NaOH, KOH or CsOH are 
generally used separately or in a mixture. 
At the pH adjusted in accordance with the invention all metals, with the 
exception of the alkali metal, such as Fe, Al, Ca, etc., are at least 
predominantly precipitated together with the vanadium content and are then 
removed from the alkali metal sulfate solution together with the vanadium 
content. Because the precipitation takes place near the neutral point, it 
is not necessary to use alkali hydroxide in a surplus. Rather, the alkali 
hydroxide is approximately required in an amount which stoichiometrically 
corresponds to the SO.sub.4 =content of the sulfate solution. The pure 
alkali metal sulfate solution may be used as follows: 
in the form in which it has been formed; 
as a solid after crystallization; 
as a solid and/or liquid after fractional crystallization. 
The pure alkali metal sulfate solution can be strengthened and then be 
reused to make catalyst. After crystallization it may be used as a 
fertilizer or in the production of catalysts. By fractional 
crystallization, different alkali metal sulfates can be separated and it 
is possible to recover, e.g., pure or enriched Cs sulfate, which can then 
be used, e.g., to produce catalyst. 
The precipitated vanadium compounds are washed in a countercurrent in a 
plurality of stages. The hydroxide which has been filtered off can be used 
as a valuable raw material for the recovery of vanadium. 
The advantages afforded by the invention reside in that the precipitation 
as V.sup.4+ does not require an oxidation step to a higher oxidation 
number and the use of an oxidizer is not necessary and that the 
precipitation of all metals, with the exception of the alkali metals, 
results in the formation of a pure alkali metal sulfate solution. 
Only one specified pH value is required for formation of the pure alkali 
metal sulfate solution whereas for a precipitation as V.sup.5+ several pHs 
for different metals would be required. The only chemical which is added 
is alkali metal hydroxide, which is required in relatively small amounts. 
Liquids or solids which would have to be dumped are not formed. 
According to a preferred feature, sulfuric acid is added during the 
leaching with SO.sub.2. This accelerates the reaction because it is not 
necessary first to form the sulfuric acid, the consumption of SO.sub.2 is 
correspondingly lower, and it is possible to operate the process in a 
closed cycle without the formation of exhaust gas if pure SO.sub.2 is 
used. The case of an addition of sulfuric acid, an O.sub.2 -free or 
low-O.sub.2 SO.sub.2 gas is preferably used in order to prevent formation 
of surplus sulfuric acid in accordance with SO.sub.2 +1/2 O.sub.2 +H.sub.2 
O=H.sub.2 SO.sub.4. The surplus sulfuric acid would give rise to a 
correspondingly higher consumption of neutralizing agent. 
According to a preferred feature, about 1 mole of sulfuric acid is added 
per mole of V.sub.2 O.sub.5 during the leaching. This results in a 
complete reaction without the need for a larger amount of neutralizing 
agent in the subsequent proceeding. 
According to a preferred feature, gas which is evolved during the leaching 
and contains surplus SO.sub.2 is recycled to the leaching step. This 
approach is particularly desirable when pure or highly concentrated 
SO.sub.2 gas is employed, because no exhaust gas or only a small amount of 
exhaust gag is formed. 
According to a preferred feature, arsenic is precipitated from the sulfate 
solution by the addition of sulfides before the vanadium compounds are 
precipitated and is removed. In this case any arsenic contained in the 
residue is precipitated before the vanudium precipitation with hydroxide. 
Precipitation of As with the vanadium would render the processing of the 
vanadium content more difficult. The sulfide employed is an alkali metal 
sulfide, which is selected with a view to the desired alkali metal content 
of the alkali metal sulfate solution.

SPECIFIC DESCRIPTION 
As can be seen from the drawing, the vanadium containing residue at 10 can 
be subjected to a pretreatment in stage 11 which can include sieving, 
magnetic separation to remove scale, crushing, etc. to provide the solid 
phase 12 which is slurried at the leaching stage 13 in an aqueous phase in 
which sulfate is formed by the introduction of SO.sub.2 at 16, recycled 
leachant at 15 or aqueous sulfuric acid and, if desired, sulfuric acid 
from some other source. 
Following the leaching stage in the presence of SO.sub.2, the slurry is 
delivered at 45 to a suction filter 20 from which the filtrate is removed 
at 22 as the aqueous phase. The solid phase is sujected to a first wash 
with acidified hot water fed at the resulting wash water can contribute to 
the recovered filtrate which contains tho vanadium component. 
This filtrate may also contain arsenic and tho arsenic can be removed in a 
stage 30 in which sulfides are added at 46. The precipitated arsenic 
compounds are separated out at 31. 
The aqueous sulfate phase, containing the vanadium constituent is fed at 32 
to the neutralization stage 33 which is operated at a pH between 7 and 9, 
e.g. about 8, and to which an alkali metal hydroxide, such as KOH added at 
34. 
The vanadium precipitates out as tetravalent vanadium and the resulting 
slurry can be fed at 36 to a multistage filtration and washing and 
represented at 37. 
The solids are removed at 38 as a valuable vanadium product and the 
filtrate is an alkali metal sulfate solution 39 which can be subjected to 
evaporative crystallization at 40. 
From the evaporative crystallization at 40, a condensate 41 is recovered 
which can be recycled partly as a washing liquid for the filtration and 
washing stage 37 and partly as a washing liquid for the filtration and 
washing stage 24 (as wash water 26). 
The evaporative crystallization can yield a mother liquor which can be 
returned at 44 to the neutralization stage 33. 
The balance is a moist solid in crystalline form, i.e. the alkali metal 
sulfate which can be recovered for use at 43. 
From the suction filter 20, the solid phase 23 can be subjected to a second 
wash with wash water 26 as represented by the filter stage 24. 
The solid product recovered from the filter is the SiO.sub.2 carrier of the 
catalyst forming the residue at 10. The aqueous phase forming the filtrate 
at 27 contains sulfate and vanadium and may be stored at 28 as a leachant 
which ultimately is fed at 29 to the leachant by 15 to the leaching stage 
13. 
A flocculent can be added as represented at 35 to the neutralization stage 
33. 
From the leaching stage 13, evolved gases, including surplus SO.sub.2, can 
be collected at 14 and recycled by a blower 17 to the SO.sub.2 distributor 
of the leaching stage 13. 
SPECIFIC EXAMPLE 
A spent catalyst is pretreated by the following preliminary steps: 
sieving with 20 mm mesh sieve to remove ceramic particles; 
magnetic separation to remove scale; and 
crushing to below 2 mm to provide a fine-grained starting material. 
1 kg of the thus prepared fine.-grained material (composed as stated in 
column 1 of the Table), were stirred at 80.degree. in a closed container 
together with 2.26 kg of the second filter cake wash obtained in the 
filtration of a preceding charge. By means of a precalculated amount of 
sulfuric acid the suspension was acidified to provide a molar ratio of 1:1 
of V.sub.2 O.sub.5 to H.sub.2 SO.sub.4. 
Gas was sucked from the gas space of the container by a fan and was forced 
through a submerged gas distributor into the suspension. The consumption 
of gas was compensated so as to maintain a controlled pressure in that 
pure SO.sub.2 was introduced at such a rate that a very slight 
superatmospheric pressure was maintained. 
The leaching was interrupted by a termination of the introduction of 
SO.sub.2 after two hours. The resulting suspension was filtered in a hot 
state through a suction filter and the filter cake was washed in two 
stages with slightly acidified hot water (first stage: 1 kg washing water; 
2nd stage: 2 kg washing water). The first filter cake wash was combined 
with the primary filtrate to provide 2.7 kg sulfate solution. 
The second filter cake wash was stored as a leaching liquor for the next 
charge. The SiO.sub.2 product which had been filtered off (1.36 kg in a 
moist or 0.6 kg in a dry state) constitutes a product which can be used 
for numerous purposes and has the composition indicated in column 2 of the 
Table. 
The sulfate solution was combined with 0.05 kg mother liquor from the 
crystallization of alkali metal sulfate and the resulting mixture was 
neutralized by an addition of potassium hydroxide solution at 60.degree. 
C. with stirring and was adjusted to pH 8.0. After a flocculating agent 
had been added, the resulting hydroxide suspension was filtered and then 
washed in a counter-current in three stages 
(1st stage: 3.3 kg second filter cake wash from the preceding charge, 
2nd stage: 3.3 kg third filter cake wash from the preceding charge, 
3rd stage: 2.83 kg water). 
The primary filtrate and the first filter cake wash were combined to 
provide 5.35 kg alkali metal sulfate solution. The second and third filter 
cake washes were stored as washing liquors for the next charge. The 
vanadium product which had been filtered off (0.8 kg in a moist state or 
0.11 kg in a dry state) is a valuable vanadium raw material having the 
composition which is apparent from column 3 of the Table. The alkali metal 
sulfate solution was subjected to an evaporating crystallization, which 
provided: 
4.77 kg condensate (for use as washing water during the two filtering 
stages of the next charge); 
0.05 kg mother liquor (to remove impurities by the supply into the 
neutralizing stage for the next charge); 
alkali metal sulfate (0.52 kg in a moist state or 0.48 kg in a dry state) 
having the composition stated in column 4 of the Table. 
The ceramic bodies removed by sieving were washed with water. The small 
amount of laden water thus obtained was also used in the leaching step. 
The washed bodies may be used for a desirable use, e.g. reused as a 
covering material in catalyst trays or as a raw material in the ceramic 
industry. The scale removed by magnetic separation was also washed with 
water and the small amount of laden water thus obtained was also supplied 
to the leaching step. Scale may be used as a raw material to make iron. 
Waste materials which cannot be utilized (exhaust gas, waste water, 
solids) were not obtained. 
Where preliminary analysis showed the catalyst to contain arsenic, an 
additional purification step is performed. In that case the arsenic which 
might, during the leaching, have entered the sulfate solution, is 
precipitated out of that solution as a sulfide, e.g., by means of Na.sub.2 
S. In one case, in which the arsenic content of the spent catalyst caused 
the sulfate solution to contain 295 ppm As.sub.2 O.sub.3, that content was 
decreased below 2 ppm (detection limit) by the addition of Na.sub.2 S in 
five times the stoichiometric amount. The resulting arsenic residue 
contained 11.0% arsenic and amounted to about 28 g in a moist state or 7 g 
in a dry state. 
TABLE 
______________________________________ 
Product Compositions in % by weight 
Alkali 
Spent SiO.sub.2 
Vanadium 
metal As 
Catalyst* 
Product Product sulfate 
residue 
Column 1 2 3 4 5 
______________________________________ 
V.sub.2 O.sub.5 
6.4 0.57 54.5 0.0015 0.38 (V) 
K.sub.2 O 
9.1 0.45 2.1 48.3 0.71 
Na.sub.2 O 
2.20 0.06 0.06 4.3 0.05 
SiO.sub.2 
57 95 0.77 0.015 n.d. 
SO.sub.3 
19.1 0.84 1.64 47.0 80 (S) 
Fe.sub.2 O.sub.3 
4.45 2.02 28.1 0.003 1.04 
Al.sub.2 O.sub.3 
0.96 0.60 4.2 &lt;0.002 n.d. 
CaO 0.36 0.12 0.54 0.32 n.d. 
As.sub.2 O.sub.3 
0.004 0.003 0.02 &lt;0.0001 
11 (As) 
H.sub.2 O 
n.d. n.d. 7.85 n.d. n.d. 
______________________________________ 
*Fine-grained starting material after sieving, magnetic separation, and 
crushing n.d. not determined