Patent Publication Number: US-9416023-B2

Title: Method for producing aqueous solutions of cobalt sulphate

Description:
The present invention relates to a process for preparing an aqueous cobalt sulfate solution having a pH of at least 4, wherein
         (a) metallic cobalt is dissolved in aqueous sulfuric acid in an atmosphere of hydrogen, of inert gas or nitrogen/oxygen mixtures which comprise nitrogen and oxygen in a volume ratio in the range from 6:1 to 100:1, and   (b) the resulting acidic cobalt sulfate solution is treated with oxygen, oxygen-comprising gas or with a substance which can release oxygen in an aqueous medium.       

     Cobalt sulfate can be used as starting material for numerous applications, for example for pigments or for producing glazes for clay. Cobalt sulfate has attained particular importance recently as cobalt source for electrode materials of lithium ion batteries. Mixed hydroxides or carbonates of appropriate transition metals are precipitated from aqueous solutions of transition metal sulfates, for example from mixed nickel-cobalt-manganese sulfate solutions and these are then processed thermally together with a lithium compound, for example lithium hydroxide or lithium carbonate, to give an electrode material. 
     A key compound for cobalt here is cobalt sulfate, either in solid form or in aqueous solution. Cobalt sulfate is in many cases prepared by dissolution of metallic cobalt in sulfuric acid. 
     For the further processing, it is of great importance to obtain solutions of cobalt sulfate which ideally have a comparatively high pH, for example at least 4. It is desirable to obtain solutions of cobalt sulfate which comprise elements which interfere in lithium ion batteries and are therefore undesirable only in an extremely small amount. Depending on the use and structure of the lithium ion battery, copper, aluminum, chromium, iron or calcium, for example, can be considered to interfere. Chromium, copper and iron can be separated off with a justifiable outlay at a pH of about 4 or above, but only with extreme difficulty at pH values of 1. 
     Thus, SU 385927 discloses a process in which metallic cobalt is firstly treated simultaneously with 40% sulfuric acid and 30% aqueous H 2 O 2  at from 80 to 90° C. in an enameled apparatus. A hydrogen/oxygen mixture can be formed in the apparatus as a result of such a procedure, which is disadvantageous for safety reasons. The cobalt sulfate solution which can be obtained in this way has a residual sulfuric acid content of from 0.2 to 0.4%, corresponding to a pH in the range from 1.5 to 1.8. Such solutions are difficult to purify. 
     It is conceivable to adjust the pH of cobalt sulfate solutions by means of strong bases such as alkali metal hydroxide. However, it needs to be taken into account that this step requires very accurate titration. In addition, solutions of cobalt sulfate which comprise considerable proportions of sodium salts are obtained by this route. 
     It is also possible to increase the pH of cobalt sulfate solutions by means of other complexing agents, for example by means of ammonia. However, ammonia also forms very stable and highly soluble copper complexes. Removal of copper complexes from a cobaltammine sulfate solution is likewise difficult. 
     It is therefore an object of the invention to provide a process by means of which it is possible to prepare cobalt sulfate solutions which have a pH of at least 4 and which can readily be carried out industrially. In particular, it is an object to provide a process by means of which it is possible to prepare cobalt sulfate solutions which have only extremely small proportions of ions which interfere in lithium ion batteries. 
     We have accordingly found the process defined at the outset, also referred to as process of the invention for short. 
     Carrying out the process of the invention gives aqueous cobalt sulfate solutions having a pH of at least 4, preferably from 4 to 8, particularly preferably from 4.5 to 7. The pH is in each case determined at 23° C., preferably by means of a glass electrode, particularly preferably by means of a combination electrode. 
     The process of the invention has at least two process steps (a) and (b), also referred to as step (a) and step (b) for short. 
     In step (a) of the process of the invention, metallic cobalt is dissolved in aqueous sulfuric acid in an atmosphere of hydrogen, of inert gas or of those nitrogen/oxygen mixtures which comprise nitrogen and oxygen in a volume ratio in the range from 6:1 to 100:1. 
     Metallic cobalt is used as starting material for carrying out the process of the invention. Metallic cobalt can be used, for example, in the form of powder, sheets, plates, lumps, turnings, electrode fragments, optionally cut, in the form of briquettes, with preference being given to lumps, briquettes and electrode fragments, which may be cut. Powders can, for example, have an average particle diameter in the range from 0.1 to 1 mm. Sheets can, for example, have a thickness in the range from 0.1 mm to 5 mm and a length and width which are identical or different and are each in the range from 2 cm to 10 meters, in particular in the range from 2 to 10 cm. Plates can, for example, have a thickness in the range from 5.5 mm to 2 cm and a length and width which are identical or different and are each in the range from 10 cm to 1 m. Lumps for the purposes of the present invention can, for example, have a length, width and height in the range from 5 mm to 10 cm, with the smallest dimension and the largest dimension differing by a factor of not more than three. Turnings, can, for example, have a thickness in the range from 0.1 to 1 mm, a width in the range from 1 mm to 5 mm and a length in the range from 1 cm to 20 cm. Briquettes can have a length in the range from 2 to 3 cm and a diameter in the range from 12 to 14 mm. Electrode fragments can, for example, have a thickness in the range from 0.5 to 7 mm. In many cases, uncut electrode fragments have a thickness in the range from 1 to 3 mm and irregular cross sections, with the diameter at the broadest place not exceeding 40 mm and the average diameter being in the range from 10 to 30 mm. Cut electrodes can have a thickness in the range from 0.5 to 7 mm. For example, it is possible to obtain cut electrode fragments having a thickness of 1 mm and a cross section of 10.10 cm or a thickness in the range from 5 to 7 mm and a cross section of 55·55 mm. 
     In an embodiment of the present invention, metallic cobalt having a high purity, for example less than a total of 100 ppm of foreign metals, is used as starting material. As foreign metals, mention may be made of, for example, Ca, Al, Fe, Cu, Cr and Mg. In another embodiment of the present invention, cobalt which is contaminated with one or more of the abovementioned foreign metals, for example with from 100 ppm up to a total of 1% by weight of Ca, Al, Fe, Cu, Cr and/or Mg, in each case based on cobalt, is used as starting material. 
     For example, metallic cobalt can comprise from 10 to 200 ppm of Ca, based on cobalt. 
     For example, metallic cobalt can comprise from 10 to 500 ppm of Fe, based on cobalt. For example, metallic cobalt can comprise from 2 to 500 ppm of Cu, based on cobalt. 
     For example, metallic cobalt can comprise from 2 to 500 ppm of Cr, based on cobalt. 
     For example, metallic cobalt can comprise from 10 to 1000 ppm of Mg, based on cobalt. 
     For example, metallic cobalt can comprise from 1 to 100 ppm of Al, based on cobalt. 
     For the purposes of the present invention, ppm are always ppm by weight unless expressly indicated otherwise. 
     In one variant, metallic cobalt comprises from 10 to 200 ppm of Ca, from 10 to 500 ppm of Fe, from 2 to 500 ppm of Cr, from 10 to 1000 ppm of Mg, from 1 to 100 ppm of Al and from 2 to 500 ppm of Cu, in each case based on cobalt. 
     In an embodiment of the present invention, metallic cobalt can comprise nickel and/or manganese, for example in each case up to 5% by weight. Metallic cobalt preferably comprises no measurable proportions of manganese and up to 1% by weight of nickel. Such proportions of manganese and/or nickel do not interfere particularly when cobalt sulfate is to be used for applications in lithium ion batteries. 
     In an embodiment of the present invention, cobalt obtained from electrode scrap is used as starting material, or the electrode scrap concerned is comminuted. 
     In the context of step (a), “dissolve” refers to a chemical reaction in which metallic cobalt which is not soluble in an aqueous medium is converted into water-soluble cobalt sulfate. 
     In addition, aqueous sulfuric acid is used for carrying out step (a). 
     For the purposes of the present invention, aqueous sulfuric acid is a sulfuric acid which comprises water which can be unprotonated or protonated. For example, the water content of aqueous sulfuric acid can be in the range from 5 to 95% by weight, based on H 2 SO 4 , preferably from 15 to 30% by weight, in each case at the beginning of step (a). 
     In an embodiment of the present invention, metallic cobalt and aqueous sulfuric acid are brought into contact with one another. For example, a reaction vessel can firstly be charged with cobalt and sulfuric acid can then be added. In another variant, aqueous sulfuric acid can firstly be placed in a reaction vessel and metallic cobalt can then be added. 
     In a specific variant, metallic cobalt is placed in a reaction vessel and aqueous sulfuric acid is then added. The acidic cobalt sulfate solution formed is removed continuously and fresh aqueous sulfuric acid is added, for example by means of a pump, or acidic cobalt sulfate solution is circulated. This procedure makes particularly simple removal of heat possible. 
     In an embodiment of the present invention, step (a) of the process of the invention is carried out at a temperature in the range from 80 to 110° C., preferably from 95 to 99° C. It is possible here to work under superatmospheric pressure, for example from 1.1 bar to 5 bar, when step (a) is to be carried out at temperatures of 100° or more. Step (a) is preferably carried out with evaporative cooling and pressure and temperature are set automatically as a result. 
     In an embodiment of the present invention, step (a) of the process of the invention is carried out at atmospheric pressure. In another embodiment of the process of the invention, step (a) is carried out at the pressure which is automatically established with evaporative cooling, for example from 1 bar to 5 bar. 
     Step (a) is carried out in an atmosphere of hydrogen or of inert gas or of nitrogen-oxygen mixtures which comprise nitrogen and oxygen in a volume ratio in the range from 6:1 to 100:1, i.e. metallic cobalt is dissolved under an atmosphere of hydrogen or of inert gas, or of nitrogen-oxygen mixtures which comprise nitrogen and oxygen in a volume ratio in the range from 6:1 to 100:1. Suitable inert gases are, for example, noble gases such as argon. Noble gases, for example argon, and in particular nitrogen are suitable as inert gases. Nitrogen/oxygen mixtures comprising nitrogen and oxygen in a volume ratio in the range from 6:1 to 100:1, preferably from 23:1 to 80:1, are also referred to in the context of the present invention as low-oxygen air. Hydrogen is formed during the dissolution of cobalt and can accumulate and is inert in respect of the actual chemical reaction of the dissolution of cobalt. 
     Preference is given to working in an atmosphere of hydrogen or inert gas. The preferred inert gas is nitrogen. 
     In order to produce an atmosphere of low-oxygen air, it is possible, for example, to mix air and nitrogen with one another and feed this mixture into the reaction vessel. Thus, for example, it is possible to select a tube reactor which is configured as a column and has a sieve plate or a mesh as bottom through which low-oxygen air, inert gas or hydrogen is passed as reaction vessel. 
     In an embodiment of the present invention, step (a) is carried out continuously, with the aqueous cobalt sulfate solution formed being continuously taken off and aqueous sulfuric acid and optionally metallic cobalt being replenished when required. In another embodiment of the present invention, step (a) is carried out batchwise. 
     In an embodiment of the present invention, step (a) is carried out in a stirred vessel. In another embodiment of the present invention, step (a) is carried out in a tube reactor which is configured as a column and has a sieve plate or a mesh as bottom. 
     Possible materials for the reaction vessel are, for example, epoxy resins, unfilled or filled with glass fibers, also laminated epoxy-glass fiber materials, plastics such as polypropylene, PVC or PVDF, and also plastic tubing introduced into steel outers. Further possible materials are lead and lead alloys. Particular preference is given to polypropylene or glass-fiber-reinforced epoxy. Metal vessels, for example steel vessels, can be lined with the abovementioned material. 
     The process of the invention can be carried out with step (a) being carried out once, for example for a time in the range from 30 seconds to 30 minutes. In another embodiment of the present invention, step (a) is carried out a plurality of times, for example from twice to ten times and in each case for a time, identical or different, in the range from 40 seconds to 5 minutes. 
     In an embodiment of the present invention, step (a) can be carried out until the content of H 2 SO 4  has decreased to from 0.05 to 1% by weight, based on acidic cobalt sulfate solution which can be obtained in step (a), preferably from 0.1 to 0.5% by weight. 
     An acidic cobalt sulfate solution which can, for example, have a pH in the range from 0.7 to 2 is obtained. 
     In step (b) of the process of the invention, the acidic cobalt sulfate solution formed in this way is treated with oxygen, oxygen-comprising gas or with a substance which can release oxygen in an aqueous medium. Here, “treat with oxygen” means that the solution is treated with pure (99% or more) gaseous oxygen. As oxygen-comprising gas, it is possible to choose, for example, air or oxygen-enriched air. Preference is given to air. 
     As substances which can release oxygen in an aqueous medium, particular mention may be made of peroxides, for example inorganic or organic peroxides, also inorganic peroxo salts. In an embodiment of the present invention, substances which can release oxygen in an aqueous medium are selected from among H 2 O 2 , inorganic peroxides, for example Na 2 O 2 , inorganic persulfates, for example Na 2 S 2 O 8  or K 2 S 2 O 8 , and inorganic percarbonates, for example sodium percarbonate. 
     In an embodiment of the present invention, the treatment is carried out using a combination of a gas selected from among oxygen and oxygen-comprising gas and a substance which can release oxygen in an aqueous medium. 
     In an embodiment of the present invention, step (b) of the process of the invention is carried out at a temperature in the range from 80 to 110° C., preferably from 90 to 100° C. Here, it is possible to work under superatmospheric pressure, for example from 1.1 bar to 5 bar, when step (b) is to be carried out at temperatures of 100° or more. Step (a) is preferably carried out with evaporative cooling and pressure and temperature are automatically set as a result. 
     In an embodiment of the present invention, step (b) of the process of the invention is carried out at atmospheric pressure. In another embodiment of the process of the invention, step (b) is carried out at superatmospheric pressure, for example from 1.1 bar to 5 bar. Step (a) is preferably carried out with evaporative cooling and pressure and temperature are automatically set as a result. 
     The duration of step (b) can be in the range from 30 seconds to 30 minutes, preferably from 40 seconds to 5 minutes. In an embodiment of the present invention, step (b) is carried out a plurality of times by treating the acidic cobalt sulfate solution a plurality of times with oxygen or oxygen-comprising gas, preferably in the presence of metallic cobalt. In this case, each individual treatment step can have a duration in the range from 30 seconds to 30 minutes. 
     Step (b) of the process of the invention can be carried out in the same vessel as step (a), or the acidic cobalt sulfate solution obtained by carrying out step (a) is transferred to another vessel, for example a stirred vessel or a tube reactor, and oxidized there. 
     In an embodiment of the present invention, mixing is carried out, for example by stirring by means of a stirrer. In another embodiment, mixing is carried out by pneumatic stirring, for example when oxygen or an oxygen-comprising gas is used as oxidant. 
     In a preferred embodiment of the present invention, step (b) is carried out in the presence of metallic cobalt. 
     Carrying out the process of the invention gives aqueous solutions of cobalt sulfate which can have a concentration in the range from 2 to 15% by weight, preferably from 6 to 9% by weight, of Co 2+ . The concentration of cobalt sulfate in step (b) can alter as a function of the way in which step (b) is carried out, for example as a result of addition of an aqueous solution of H 2 O 2  or evaporation of water. Particularly when oxygen or oxygen-comprising gas is used in step (b), the cobalt sulfate concentration does not change significantly in step (b). 
     Carrying out the process of the invention can give aqueous solutions of cobalt sulfate which comprise only small amounts of impurities which would interfere in batteries. 
     In an embodiment of the present invention, an aqueous cobalt sulfate solution having a content of iron ions of not more than 50 ppm, preferably from 0.1 to 10 ppm, based on cobalt, is obtained. Here, iron ions are present as Fe(II) or preferably as Fe(III) or as a mixture of Fe(III) and Fe(II). 
     In an embodiment of the present invention, an aqueous cobalt sulfate solution having a content of aluminum ions of not more than 50 ppm, preferably from 0.1 to 10 ppm, based on cobalt, is obtained. 
     In an embodiment of the present invention, an aqueous cobalt sulfate solution having a content of calcium ions of not more than 50 ppm, preferably from 1 to 10 ppm, based on cobalt, is obtained. 
     In an embodiment of the present invention, an aqueous cobalt sulfate solution having a content of copper(II) ions of not more than 50 ppm, preferably from 1 to 10 ppm, based on cobalt, is obtained. 
     In an embodiment of the present invention, an aqueous cobalt sulfate solution having a content of chromium(III) ions of not more than 50 ppm, preferably from 1 to 10 ppm, based on cobalt, is obtained. 
     In an embodiment of the present invention, an aqueous cobalt sulfate solution having a content of calcium ions, copper(II) ions and chromium(III) ions of in each case not more than 50 ppm, preferably from 1 to 10 ppm, based on cobalt, and a content of iron ions and aluminum ions of in each case not more than 50 ppm, preferably from 0.1 to 10 ppm, is obtained. 
     The proportion of copper(II) ions, iron ions, calcium ions, aluminum ions and chromium(III) ions can, for example, be determined by optical emission spectrometry with inductively coupled plasma, referred to as ICP-OES (inductively coupled plasma-optical emission spectrometry) for short. 
     In an embodiment of the present invention, an aqueous cobalt sulfate solution having a content of sodium ions of not more than 1000 ppm, for example from 100 to 1000 ppm, determined, for example, spectroscopically, based on cobalt, is obtained. 
     Nickel ions generally do not interfere when cobalt sulfate solution obtained according to the invention is to be used for producing electrode materials. In an embodiment of the present invention, an aqueous cobalt sulfate solution comprising not more than 2% by weight of nickel ions, based on cobalt, is obtained. Nickel can, for example, be determined by ICP-OES. 
     In an embodiment of the present invention, a further step in which
         (c) the resulting aqueous cobalt sulfate solution is purified by means of a filter can be carried out after step (b).       

     Examples of suitable filter materials are filter cloths, for example made of wool, cotton, paper, synthetic or mineral fibers. Other suitable filter materials are, for example, porous filter compositions, for example ceramic. Candle filters are also suitable. 
     Step (c) can, for example, be assisted by pressure, for example by application of a reduced pressure on the side of the filtrate, for example by use of a suction filter, or by application of superatmospheric pressure on the side of the filter cake, for example by means of suitable pumps. 
     In an embodiment of the present invention, filters which have an average pore diameter in the range from 0.1 to 100 μm, preferably from 0.3 to 50 μm, are used. 
     In one variant, one or more filter aids, for example kieselguhr or Celite®, can be added. 
     Impurities of Cr(III), AI(III) and Fe(III), in particular, which are present as hydrated oxides or hydroxides in the form of precipitates in cobalt sulfate solutions obtained according to the invention can be removed by filtration. 
     In an embodiment of the present invention, a further step in which
         (d) the resulting aqueous cobalt sulfate solution is purified by means of an ion exchanger can be carried out after step (b), optionally after step (c).       

     In an embodiment of the present invention, ion exchangers are selected from among crosslinked, preferably divinylbenzene-crosslinked, polystyrenes which bear primary, secondary or preferably tertiary amino groups. Primary, secondary or preferably tertiary amino groups can be bound via a spacer to the aromatic groups of the polystyrene, for example via methylene or ethylene groups. Examples of tertiary amino groups are —CH 2 —N(CH 3 ) 2  groups and —CH 2 —N(C 2 H 5 ) 2  groups. 
     In another embodiment of the present invention, ion exchangers are selected from among divinylbenzene-crosslinked polystyrenes comprising sulfonic acid groups. 
     The ion exchanger can be located in an ion-exchange apparatus. An ion-exchange apparatus comprises at least one column filled with ion-exchange resin, preferably two such columns. It is industrially advantageous to connect two or more ion-exchange columns to one another in such a way that it is possible to regenerate one or more ion-exchange columns without having to interrupt the preferred continuous operation of the ion-exchange apparatus. The ion exchanger can, for example, be regenerated using dilute sodium hydroxide solution. Before and after the treatment with sodium hydroxide, water or dilute sulfuric acid can optionally be used as flushing medium. 
     In an embodiment of the present invention, step (d) is carried out at a temperature in the range from 15 to 40° C., preferably in the range from 20 to 35° C. 
     In an embodiment of the present invention, the pressure drop in step (d) is in the range from 0.1 to 0.4 bar. 
     Calcium ions, for example, can readily be removed from aqueous cobalt sulfate solutions by purification of the cobalt sulfate solution by means of an ion exchanger. 
     The process of the invention makes it possible to prepare aqueous cobalt sulfate solutions which have a pH of at least 4 and comprise only extremely small proportions of ions which interfere in lithium ion batteries. Furthermore, the process of the invention can be operated simply and safely. 
     The invention is illustrated by examples. 
     General note: standard I: standard liters 
     The pH was determined in each case at 23° C. using a glass electrode (combination electrode) which was calibrated by means of buffer solutions at pH values of from 4 to 7. 
     Reaction apparatus: double-wall tube reactor (internal diameter: 50 mm, height 500 mm, of which 300 mm are utilized, with sieve plates). The double-wall tube reactor was connected via a pump circuit to a reservoir, so that solutions can be taken off from the double-wall tube reactor at the bottom and conveyed into the reservoir and solutions can be pumped into the double-wall tube reactor at the top. A heat exchanger was located at the inlet of the double-wall tube reactor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plot of pH versus time of an acidic cobalt solution prepared by the process of inventive Example 2, with oxygen treatment. 
         FIG. 2  is a plot of pH versus time of an acidic cobalt solution prepared by the process of Comparative Example C-3, without oxygen treatment. 
     
    
    
     EXAMPLE 1 
     Step 1(a) 
     1.3 kg of cobalt metal (electrode fragments, dimensions: thickness from 1 to 2 mm, diameter 20 mm, circular cross section) were placed in the double-wall tube reactor of the reaction apparatus. The reservoir was charged with 221 g of 20% strength aqueous sulfuric acid. 
     After setting an internal temperature of 98.5° C. in the double-wall tube reactor, a stream of 100 standard I/h of nitrogen was introduced into the double-wall tube reactor via a sieve plate and the pump of the pump circuit was set into operation at a pumping rate of 1.06 I/h. The aqueous sulfuric acid/acidic cobalt sulfate solution from the reservoir was continually fed at a temperature of about 98° C. into the double-wall tube reactor. 
     After 7.6 hours, the acidic cobalt sulfate solution in the reservoir had a pH of 1 (residual amount of sulfuric acid: about 5 g/l). 
     Step 1(b) 
     Subsequently, 100 standard I/h of air were introduced instead of the nitrogen into the double-wall tube reactor. After 33 hours of operation of step (b), the cobalt sulfate solution was taken from the reservoir and the pH and concentration were determined, giving the following results: pH 5.42; concentration: 8.8 g of Co 2+ /100 g of solution. 
     EXAMPLE 2 
     Step 2(a) 
     1.3 kg of cobalt metal (electrode fragments, dimensions: thickness from 1 to 2 mm, average diameter 20 mm, irregular polygonal cross section, maximum diameter 40 mm) were placed in the double-wall tube reactor of the reaction apparatus. The reservoir was charged with 221 g of 20% by weight aqueous sulfuric acid. 
     After setting an internal temperature of 98.5° C. in the double-wall tube reactor, a stream of 100 standard I/h of nitrogen was introduced via a sieve plate into the double-wall tube reactor and the pump of the pump circuit was set into operation at a pumping rate of 1.06 I/h. The aqueous sulfuric acid/acidic cobalt sulfate solution from the reservoir was continually fed at a temperature of about 98° C. into the double-wall tube reactor. 
     After 7.6 hours, the acidic cobalt sulfate solution in the reservoir had a pH of 1.97 (residual amount of sulfuric acid: about 2.5 g/l) and the Co 2+  content was 8 g/100 g of solution. 
     Step 2(b) 
     At a reactor temperature of 98.5° C., 25 standard I/h of air were introduced via a sieve plate into the double-wall tube reactor and the acidic cobalt sulfate solution was circulated at a pumping rate of 1.1 I/h. The acidic cobalt sulfate solution from the reservoir was continually fed at a temperature of about 98° C. into the double-wall tube reactor. After a period of operation of 6 hours, the solution was drained (pH 6.71; concentration: 8.4 g of Co 2+ /100 g of solution). 
     C-3: Comparative Example 
     1.3 kg of cobalt metal (electrode fragments, dimensions: thickness from 1 to 2 mm, diameter 20 mm, circular cross section) were placed in the double-wall tube reactor of the reaction apparatus. The reservoir was charged with 221 g of 20% strength by weight aqueous sulfuric acid. 
     After setting an internal temperature of 98.5° C. in the double-wall tube reactor, a stream of 100 standard I/h of nitrogen was introduced via a sieve plate into the double-wall tube reactor and the pump of the pump circuit was set into operation at a pumping rate of 1.06 I/h. The aqueous sulfuric acid/acidic cobalt sulfate solution from the reservoir was continually fed at a temperature of about 98° C. into the double-wall tube reactor. 
     After 7.6 hours, the acidic cobalt sulfate solution in the reservoir had a pH of 1.97 (residual amount of sulfuric acid: about 2.5 g/l), after 27 hours it had a pH of 2.5 and after 40 hours it had a pH of 3.0. Subsequently, the pH increased at a rate of only 0.04 units per hour.