CA1089382A

CA1089382A - Oxidation of ferrous salt solutions

Info

Publication number: CA1089382A
Application number: CA287,871A
Authority: CA
Inventors: Eric Livesey-Goldblatt
Original assignee: General Mining and Finance Corp Ltd
Current assignee: General Mining and Finance Corp Ltd
Priority date: 1976-10-18
Filing date: 1977-09-30
Publication date: 1980-11-11
Also published as: BR7706765A; PH13475A; ZM6177A1; PL201584A1; OA05784A; MX4518E; ES462571A1; GB1542600A; AU2786977A; AU512731B2; FR2367704A1; SE443557B; SU925245A3; FR2367704B1; JPS5350089A; DE2742199A1; SE7710810L

Abstract

ABSTRACT OF THE DISCLOSURE
The invention concerns a process for the bacterial oxidation of ferrous salt solutions in the presence of oxygen and at an acid pH, She oxidation is carried out with the bacterial oxidising agent coated on the passage surfaces of a structure which is honeycombed with passages. The process conveni-ently is carried out continuously. The ferric salt obtained may be used for leaching a mineral, eg an ore or coal.

Description

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THIS INVENTION relates to a process for the oxidation of ferrous salt solutions and to the use of the oxidised , ~; solutions in the leaching of minerals.

~1 We have recently discovered that ferrous sulphate solution can be oxidixed continuously to ferric sulphate , - solution, in a vessel containing smooth spaced-apart plates or tubes therein with the bacteria coated on the plates or tubes, - if the solution is passed continuously between the surfaces of ¦ the plates or tubes.
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` 10 We have now found that the process is improved in a surprising manner (which makes it suitable for commercial . -i , . -. . .-.: .. - . : ~ . . - ..
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exploitation) i. ferrous sulphate solution is oxidised bacterially in the vessel containing acidified sulphuric acid and also containing a structure which is honeycomed witn passa~es capable of supporting a film of bacteria thereon.

The present invention provides a process for the oxidation of a ferrous salt solution, which comprises passing the ferrous salt solution in the presence of oxygen and at an acid pH through a vessel containing a bacterial supporting structure which is honeycombed with passages, the surfaces of the passages having a bacterial oxidising agent present as a film thereon, and the passages being of a size to permit the movement of the solution therethrough, said bacterial oxidising agent be~n~ capable of oxidising ferrous ions to ferric ions.
' The term 'bacterial supporting structure which is honeycombed with passages' means any structure containing a large plurality ~ie more than 10) passages therethrough and on ., 'r the walls of which a film of bacteria can be supported.
. Conveniently, there may be considerably more, eg 50 or more ` preferably a hundred or more, passages. The structure may be a .1, . . . .
~' 20 preformed artificial structure and may contain tortuous ,. passages.

The structure may be a unitary structure comprising a plurality of plates haviny relief formations projecting outwardly from one or both sides thereof. The relief t 25 formations may be permanently attached to a further adjacent '~ plate. The relief formations may be corrugations projecting .

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out~lardly from one or both sides of the plates. The corru-gations may be substantially waveform in plan view so that the passages are tortuous. The corrugations may be of substanti-ally triangular or hemispherical cross-section. In one embodiment, the str~cture may resemble a honeycomb in appear-ance. The ferrous salt solution may be ferrous sulphate, eg a solution which is used in the mining industry or in steel plant pickling liquors. The oxidation process can be carried out continuously by passing a moving stream of the solution over a submerged structure which is honeycombed with passages. In this way a fast rate of oxidation can be obtained. The product -~obtained, when ferrous sulphate is oxidised may be used for leaching minerals eg for extractive metallurgy or for removing sulphur from coal.

The invention al50 provides a process for treating mineral materials which comprises leaching the mineral with a ferric salt solution obtained by the oxidation process of the ' invention.
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Any suitable bacteria that will oxidise ferrous ions -~ 20 to ferric ions may be used. For example, bacteria of the genus thiobacillus ferrooxidans, thiobacillus thiooxidans, ferro--, bacillus ferrooxidans or ferrobacillus sulphooxidans may be 5 ~ . used to oxidise ferrous sulphate. The reaction proceeds '~ according to the following equation:-.~ . , . ' ' .
,~ 25 4FeSO4 + 2H2S04 + 2 Bacteria-~ 2Fe2~504)3 + 2H20.
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: . - - ~: , .: -1~893~32 The equation shows that sulphuric acid is required for c~nversion of ferrous sulphate to ferric sulphate, and that the amount of ferrous sulphate governs the acid re~uirement. The acid may be sulphuric acid on its own or may, for eY~ample, be sulphuric acid mixed with another suitable acid, eg hydrochloric acid in amounts up to about 10~ by weight. To achieve oxidation of high concentrations of fer~ous sulphate, equivalent high concentrations of sulphuric acid optionally containing other acids, should be used. The bacteria is sensitive to acidity and the pH conveniently is in the range 0,8 to 2,5 more preferably 1,3 to 2. A pH of about 1,5 is particularly preferred for oxidising ferrous sulphate to ferric sulphate.
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We have found it convenient continuously to pass a small volume of ~cldified ferrous sulphate solution into a large volume of an iron sulphate solution, mainly comprising acidified ferric sulphate but which may also contain other ! substances (for example ferrous and ferric chloride and hydrochloric acid) and the bacteria, and in which the structure . .
which is honeycombed with passages is submerged. The walls of '~ the structure are coated with the film of bacteria. By having a large volume comprising mainly acidified ferric sulphate, dilution of the bacteria, and the pH of the solution, are not unduly affected. The bacteria film is a surface film and is continuously being destroyed and reformed over the large surface area of the honeycomb or passages of the structure.
Thus the process can be carried out continuously.

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1~938;2 The structure may be of any suitable material having passages therethrough and along which the solution may pass.
The distance apart of opposite sides of the walls of the passages in the structure is sufficiently wide to enable the .
solution containing entrained o~ygen contin~ously to pass therethrough without being blocked by bacterial oxidising agent. Experimentation has shownlthat the bacterial layer generally is about 1 mm thick, so that a the passage walls should be greater than 2 mm apart and conveniently between - 10 about 3,5 and 10 mm apart. It is convenient to have the maximum possible surface area available for the bacteria to grow on. The surfaces of the structure may be of a plastics material which is inert to the iron salts and to the acid. A
very suitable substantially rigid structure is the material known by the Trade Mark 'Flocor' and sold by Imperial Chemical Industries Ltd. 'Flocor' is a high void space and llght weight ` structure of polyvinyl chloride. It appears to be resistant to ; degredation by bacteria and acids under the reaction conditions used. 'Floror' comprises a plurality of parallel planar plates separated by and joined to a plurality of corrugated plates, the corrugations being of waveform appearance in plan view and ` having transverse ribs. With this invention, there are not base structures such as sand or rocks which could cement -~ together, and hence the passages are not blocked. The process ~- 25 can therefore be carried out continuously. The plastics structure may be totally submerged in the reaction vessel.

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The solution of the ferric salt can contain trace elements or other materials to assist bacterial growth. Trace elements or other materials to assist bacterial growth may be traces of one or more of ammonium, nitrate, potassium, calcium, magnesium, phosphae and chloride ions. ~ mixture of iron sulphates, sulphuric acid, trace elements and bacteria can be saturated with oxygen and passed over the film of bacteria grown on the passage walls of the structure. The bacterial film may also be present on the surfaces of the vessel con-taining the structure. Once grown, the film can be self re-generating to provide the oxidising surfaces for an apparently indefinite period. The vessel in which the bacterial oxidation takes place may be enclosed or jacketed to maintain it at an optimum temperature in the range - 10 to 45C, preferably 30 to 35C. Oxygen saturation of the solution can be achieved by lS introducinq air or oxygen into the solution using any suitable type of submerged sparging, eg through a pipe, using a mechanical aerator, or through a fritted disc. It appears that ; the rate of oxidation is directly proportional to the available i surface area of the honeycomb. The air in contact with the , ... ..
solution may be enriched with either or both pure oxygen and carbon dioxide.

¦ The vessel containing the structure may be an i elongated vessel, which conveniently is vertical. The ferrous . t salt to be oxidised may be introduced at the top, while air is introduced from the bottom.
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Ferric sulphate solution obtained by the process is suitable as a leach liquor for use as a lixivant of minerals.
Thus, it can be used to treat coal to remove sulphur therefrom, or can be used to obtain metals from their ores or from dumps or the like. For extractive metaliurgy, where bacterial action is used and the leach solution containing bacteria is recircu-lated for re-use, high leach temperatures destroyed the bacteria. With the present invention, it does not matter unduly if the bacteria are destroyed in a high temperature leach as, after the metal has been removed from the solution and cooled, the present invention ls carried out under conditions which favour reinoculation of the returned iron sulphate solution with bacteria.

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-: I The ferric sulphate-containing solution can be used lS to extract a number of metals from their ores. Examples are ` - uranium, copper, gold, lead, zinc, nickel, cobalt, molybdenum, . j.
arsenic, antimony, cadmium, mercury, and other base metals.

~,, - , The ore can be leached with the ferric salt solution, ^ under the action of heating and/or stirring and/or agitation . 20 and/or applying superatmospheric pressure, if necessary. The ~ ,. - .
desired metal can be separated from the leach liquor, and i residual liquor may be recycled to the oxidation process of the invention for re-oxidation. Ores which may be treated can be ~! oxides, silicates, and particularly sulphides of the metal.
`- 25 Low grade, as wel~ as high grade ores can be treated. The ores ..... .
to be treated may be mine dump waste, copper-containing .

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silicate gangue, copper sulphides such as copper pyrites, covellite, chalcocite, chalcopyrite, bornite (Cu5FeS5), cubanite (CuFe2S3), enargite (Cu3AsS~), tetrahedrite (Cul2Sb4S13) and tennantite (Cul2As4S13~; nickel sulphides and nickel sllicate minerals SUC}l as garnierite (Ni6Si4Olo(OH)g), nickel mattes (Ni3S2) pentlandite (Fe,Ni)gS8), millerite (NiS); cobalt ~ sulphides, arsenides or sulphur-arsenides, such as cobaltite ; (CoAsS), tetrahydrite, pyrrhotite mixtures with tetrahedrite and/or chalcopyrite, sphalerite; copper seleniaes and tellurides, carrollite (Co2CuS4); zinc and lead minerals such as sphalerite ~ZnS), galena (PbS), galena-sphalerite concentrates, Pb-Cu-Fe mattes, boulangerite, (Pb5Sb4Sll), zinkenite ~Pb5Sb4S27), jamesonite, (Pb5Fesb6Sll) and geocronite ~Pb55b2S8), iron minerals such as pyrrhotite and pyrite (FeS2), molybdenum minerals such as molybdenite (MoS2); arsenic and antimony mlnerals such as orpiment (AS2S3), arsenopyrite ~FeAsS), I ~tibnite, (Sb2S3); cadmium and mercury minerals; and uranium ,- minerals, such as uraninite or pitchblende (UO2 + x)' -brannerite (UTi2O6), carnotite (K2(~O2)2 (UO4)23H2O), autunite . 20 (Ca(UO2)2 (PO4) ; lOH2O) and gold slimes-~. I , . .
-~ The ore may be milled in a first step, graded, and ~ the fine ore passed to a tank where it is mixed with water. ~he :: ~
l aqueous mixture can be milled further and then passed to a ¦ -plurality of leach vessels. The leach vessels may be heated, eg with steam and agitated, eg by stirring. Concentrated acid and the ferric solution is added continuously to one or more of the leach vessels. After passing through the leach vessels, ~ I .
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the leach liquor containing metal values is filtered and the filtrate split into two parts. One part passes to an ion-exchange column (from which the metal can subsequently be s, eluted), and the barren solution is discarded. The other part passes to a tank containing the honeycomb or other structure . coated with a film of bacterial oxidising aqent. Oxygen or air can be passed upwardly through the tank, and the ferric solution obtained is recycled to the leach vessels.

~he accompanying drawings illustrate.a method of carrying out the invention, and apparatus for use therein, in . which drawings:
Figure 1 is a flow sheet showing a process for ~ 8eparating metals from their ores according to the lnvention;
;' ' Flgure 2 is a schematic section through a tank for oxidising a ferrous salt; and Figure 3 is a three-dimensional view of part of a honeycomb for use in the tank.
, ', Referring to Figure l,.milled ore from mill lO passes to vessel 12 where it is mixed with water from pipe 13 and then . 20 passed through a second mill 14. From here the slurry of ore and water passes into a leaching vessel 16 which is in series with further leachlng vessels 16.1 and 16.2. Each leaching --, vessel is heated by steam from pipe 18 and contains a stirrer ~ . . 18, 18.1, 18.2.

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Concentrated acid and ferric salt solution are passed to thè first leaching vessel through piplines 20 and 22 respectively. The leached material is filtered at 24 and the filtrate passed through pipeline 26 to point 28 where it is split into approximately equal parts. The residue from the filtér is sent for disposal along line 30.

, One part of the leach solution is passed along pipeline 32 to an ion exchanger 34 from which barren solution s discarded along pipeline 36. The other part of the leach solution is passed along pipeline 38 to a tank 40 containing a submerged supporting structure 42 which is hone~combed with passages. This tank and supporting structure are illustrated ln greater detail in Figures 2 and 3. ~he walls 44 are of waveform appearance and are triangular in cross-section.

' 15 The leach li~uor passing along pipellne 38 is an - acidic solution comprising a ferrous salt possibly containing : ~ .
, ` some ferric salt. The solution passes through passages 43 in '~I the support, the walls 44 of which are coated on their surfaces . ~ .
45 with a bacterial oxidising agent. Air is passed upwardly zO through aerator 46, by means of pump 48. The solution is oxidised by the bacteria in the presence of the air, and the : ferric salt solution formed is recycled along the pipeline 22.

~, The metal is eluted from time to time from the exchanger 34 by . ..
i passing eluant from pipe 50 through the exchanger and recover-ing the metal-containing solution from plpeline 52.

~ The invention is illustrated by reference to the l following non-limiting Examples.

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., . EXAM~LE l:
A cylindrical tank 3 metre high and 1,2 metre in diameter was filled with ferrous sulphate solution. The volume of solution was 3400 litre. A block of plastics honeycomb-like 'Flocor' packing, 1000 mm by 1800 mm by 600 mm, of the shape illustrated . in Figure 3, was submerged in the tank liquor. The specific surface area was 93 M2/M3. Aeration was provided at the base of the tank by a commercially available aerator giving 50 M /hour nominal air induction. After some weeks during which the bacterial activity (using ferrobacillus ferrooxidans) increased and reached a maximum the following data was recorded:-Temperature 32C
Feed flow rate 72 lltre/hour Peed Ferrous 9,8 g/l ferric 1,3 g/l Sulphuric acid 9,0 g/l . . Product Ferrous 0,25 g/l Ferric l0,5 g/l ~:? 20 Sulphuric acid 0,88 g/l H 1,5 . . Oxidation Rate 688 gram/hour ;' Surface area 100 M?
:~ . 2 ;~ . Specific Rate . 6,88 gram/M /hour : ' .
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The distance apart of the pas~age walls in the honeycomb was about 4 mm.

l'he ferric iron/bacteria solution obtained was added to an ore containing a useful mineral and pyxites using the flow scheme shown in Figure 1. The ore ~tas acidified with sulphuric acid ~o destroy any residual alka-linity. ~-;:
EXAMPLE 2: (Comparison Example) Various other ways of oxidising ferrous sulphate were carried out for comparison purposes, as explained below. The apparatus used were:
1. A plate and spraY unit ! , . In this apparatus, ferrous sulphate solution was I sprayed over the top of a plurality of plates coated with ~t lS bacteria, and the ferric sulphate was collected at the ;l bottom.

i 2. A bio-disc unit :
; In this apparatus, a spindle, having a plurality of bacteria-coated discs attached thereto, was rotated with the discs ln the iron sulphate solution, fresh ferrous ~; sulphate solution being fed in from one end.
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l~ 3. A filter-pack unit In this apparatus, ferrous sulphate solution was ~i; passed downwardly through a tower packed with rings and `'¦ 25 which were coated with bacteria. --. . . . .
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4. A submerged honevcombed unit In this apparatus (see the accompanying drawing) ferrous sulphate was passed into an iron sulphate solution containing an insert as illustrated in Figure 3 of the drawings and coated with bacteria. Air was passed upwardly. This treatment is referred to as 'Bacfox' treatment.

In each case, thiobacil]us ferrooxidans was used.
~he ferric sulphate was used to leach milled ore. In each case, the following were used.
Solution treated 66560 tons/month Ferrous iron concentration 5,0 g/litre -~ Total ferric iron produced 460 kg/hr.

The following ~s a summary of the rate of oxidatlon I and the capital and operating costs.

~ Fe Oxidation rateCapital Operating Cost .
Un~t Gram/M2/hr. Gram/M3/hr. R R/ton -Cent/kg ` Solution Ferric . Treated Produced.
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Biodisc 3,78 -204 558 000 0,145 2,91 Filter ~`l pack 1,125 212 548 000 0,145 2,91 .s~ Bacfox 25treatment 7,46 693 - 113 800 0,031 0,63 , , . . . . . _ . .

, The economic advantages of the invention are - ~ clear.
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EX~PLE 3.
Using the leacning part of the scheme shcwn-in Figure 1 of the dxa~ings, 500 g. of milled ore containing the equivalent of O,36 kg. U3O8/ton were slurried with water. Thereafter the slurry was milled and then passed to the leaching vessel 16 ~here 350 g. of ferric sulphate solution from the tank 40 (prepared according to Example 1) were added. This solution contained 8 g. of ferric iron. Concentrated sulphuric acid equivalent to 15 kg./ton of ore were added, and the slurry was leached for 2 hours at 55C. Thereafter MnO2 equivalent to l,O
~g./ton was added to oxidise any ferrous ions present. The leaching was continued for a further 16 hours.

The slurry was filtered, washed, dr~ed, and assayed.
~he uranium content of the leached residue showed that 9O~ of the uranium had been dissolved from the ore.

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A control test using the same ore but not using ferric sulphate produced according to the invention, required 25 kg./ton of sulphuric acid and.3 to 4 kg./ton of MnO2 for oxidation.
:i . ' ' " : ' EXAMPLE 4.
(a) The procedure illustrated in the flow sheet can be carried out using covellite as the ore and acidic t .- . ferric sulphate at abcut 35C, or an acidic ferric ' chloride and ferric sulphate mixture at about 98c for .
.l 25 the leaching.

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(b~ When repeating the procedure for chalcocite, temperatures in the range of about 23 to 95~C may be used.

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(c) When repeating the procedure for chalcopyrite, tempera-tures in the range of about 27 to 100C may be used.

~d) ~en repeating the pxocedure for bornite, temperatures in the range of about 23 to 98C may be used.

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~e) When repeating the procedure for cubanite, and using ferric sulphate, the temperature advisably should be in the range of about 45 to 90C.

~f) When repeating the procedure for enargite, and using ' erric sulphate, the temperature advisably should be in the ran~e of about 60 to 95C.

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~g) When repeating the procedure for tetrahedrite, and using ferric sulphate, the temperature advisably should be about 35C.
; ., '''i , ~h) When repeating the procedure for pentlandite, and using ferrlc sulphate, temperatures may be in the range of about , 25 to 60C.
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) When repeating the procedure for pyrrhotite, and using `r 20 ferric sulphate, temperatures may be in the range of about 32 to 50C.

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(j) When repeating the procedl~re for pyrite, and using ferric sulphate, the temperature may be about 33 C.

EXAMPLE 5.
The procedure of Example 1 ~as repeated but using Thiobacillus ferrooxidans to oxidise a ferrous sulphate and ferrous chloride mixture containing 6,8 g/l of ferrous chloride and 13,6 g/l of ferrous sulphate. A mixture comprising ferric chloride and ferric sulphate was obtained. This is a stronger oxidising agent than ferric sulphate alone and so can be used advan-~o tageously for leachlng minerals.

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Claims

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

1. A process for the oxidation of a ferrous salt solution, which comprises passing a ferrous salt solution at an acid pH
in the range of 0.8 to 2.5 through a vessel containing a bacterial supporting structure fully submerged in an aqueous iron sulphate solution, said bacterial supporting structure comprising a plurality of plates of plastics material which is inert to the action of iron salts and acid, said plates having relief formations projecting outwardly from one or both sides thereof in the form of a honeycomb-like structure containing tortuous passages through which the solution will pass, the surfaces of the passages having a bacterial oxidizing agent present as a film thereon, and the passages being of a size to permit the movement of the solution therethrough, said bacterial oxidizing agent being capable of oxidizing ferrous ions to ferric ions, and causing oxygen to be bubbled upwardly through the passages.

2. In a process for treating mineral materials to effect leaching thereon which comprises the steps of (i) passing a ferrous salt solution in the presence of oxygen at an acid pH of 0.8 to 2.5 through a vessel containing a bacterial supporting structure to form a ferric salt solution, (ii) passing the ferric salt solution formed to at least one leaching tank provided with a stirrer, (iii) passing a slurry of milled mineral to said tank, (iv) removing from said tank a suspension comprising solid matter and a solution containing a ferrous salt and material leached from said mineral, (v) separating the solid matter from the suspension to form a ferrous salt solution containing the material leached from the mineral, (vi) and removing the leached material in known manner from the ferrous salt solution obtained in step (v), with the improvement comprising:
(a) passing the ferrous salt solution in step (i) at an acid pH of 0.8 to 2.5 through a vessel containing a bacterial supporting structure fully submerged in aqueous acidic iron sulphate solution, said bacterial supporting structure comprising a plurality of plates of plastics material which is inert to the action of iron salts and acid, said plates having relief formation projecting outwardly from one or both sides thereof in the form of corrugations defining a honeycomb-like structure containing tortuous passages therein through which the ferrous salt solution will pass, the surface of the passages having a bacterial oxidizing agent present as a film thereon, and the passages being of a size to permit the movement of the solution there-through, said bacterial oxidizing agent being capable of oxidizing ferrous ions to ferric ions and causing oxygen to be bubbled upwardly through the passages; and (b) recirculating the ferrous salt-containing solution obtained after treatment to step (i) where ferrous salts are oxidized again to ferric salts.

3. A process for treating mineral materials to effect leaching thereof which comprises the steps of (i) passing a ferrous salt solution in the presence of oxygen at an acid pH, through a vessel containing a bacterial supporting structure which is honeycombed with passages, the surfaces of the passages having a bacterial oxidizing agent present as a film therein, and the passages being of a size to permit the move-ment of the solution therethrough, said bacterial oxidizing agent being capable of oxidizing ferrous ions to ferric ions, whereby a ferric salt solution is formed, (ii) passing the ferric salt solution formed to at least one leaching tank provided with a stirrer, (iii) passing a slurry of milled mineral to said tank, (iv) removing from said tank a suspension comprising solid matter and a solution containing a ferrous salt and material leaching from said mineral, (v) separating the solid matter from the solution, (vi) subjecting said solution containing a ferrous salt and the material leached from the mineral to ion-exchange to remove the leached material from the iron salt solution, (vii) and recirculating the ferrous salt containing solution, after ion-exchange treatment, to step (i) where ferrous salts are oxidized again to ferric salts, wherein the supporting structure is submerged in an iron salt solution present in the vessel, and an ore containing at least one element selected from the group consisting of uranium, copper, gold, lead, zinc, nickel, co-balt, copper molybdenum, arsenic, antimony, cadmium or mercury is leached to separate the element from the ore.

4. A process for treating mineral materails to effect leaching thereof which comprises the steps of (i) passing a ferrous salt solution in the presence of oxygen at an acid pH, through a vessel containing a bacterial supporting structure which is honeycombed with passages, the surfaces of the passages having a bacterial oxidizing agent present as a film therein, and the passages being of a size to permit the move-ment of the solution therethrough, said bacterial oxidizing agent being capable of oxidizing ferrous ions to ferric ions, where a ferric salt solution is formed, (ii) passing the ferric salt solution formed to at least one leaching tank provided with a stirrer, (iii) passing a slurry of milled mineral to said tank, (iv) removing from said tank a suspension comprising solid matter and a solution containing a ferrous salt and material leaching from said mineral, (v) separating the solid matter from the solution, (vi) subjecting said solution containing a ferrous salt and the material leached from the mineral to ion-exchange to remove the leached material from the iron salt solution, (vii) and recirculating the ferrous salt containing solution, after ion-exchange treatment, to step (i) where ferrous salts are oxidized again to ferric salts, wherein the supporting structure is submerged in an iron salt solution present in the vessel, and wherein a mine dump mineral is leached.

5. An apparatus for use in the production of a ferric salt comprising a vessel containing a bacterial supporting structure, said bacterial supporting structure comprising a plurality of plates of plastics material which are inert to the action of iron salts and acid, said plates having relief formations projecting outwardly from one or both sides thereof in the form of a honeycomb-like structure containing tortuous passages through which the solution will pass, the surfaces of the passages having a bacterial oxidizing agent present as a film thereon, and the passages being of a size to permit the movement an acidified ferrous sulphate solution there-through, said bacterial oxidizing agent being capable of oxidizing ferrous ions to ferric ions, means for introducing oxygen into said vessel, inlet means for introducing such acidified ferrous salt solution and outlet means for such acidified ferrous salt solution, wherein said bacterial support structure is submerged in such acidified ferrous salt solution when the vessel is in use and said means for introducing oxygen is positioned such that oxygen is bubbled between said plates of said bacterial supporting structure.