GB2133039A - Recovery of cobalt oxidation catalyst - Google Patents

Abstract

Cobalt catalyst is recovered from the residue of an organic compound oxidation process by passing an electric current between electrodes immersed in the residue so as to deposit cobalt on one of the electrodes.

Description

SPECIFICATION Recovery of cobalt oxidation catalyst The present invention relates to the recovery of cobalt which has been used as catalyst in an organic compound conversion process.

Cobalt, either as metal or in the form of a compound is widely used as a catalyst in organic chemical processes particularly in the oxidation of organic compounds. For example, a cobaltcatalyst maybe used in the oxidation of cyclohexane to cyclohexanolcyclohexanone and paraxylene to terephthalic acid. At the end of the process after recovery of the product and any unchanged reactants there is usually left a liquid residue which contains the cobalt in conjunction with relatively high-boiling organic by-products.

Various methods have been proposed to recover the cobalt from such a by-product-containing liquid, firstly because cobalt is expensive and secondly because its disposal presents an environmental hazard. The methods heretofor proposed however generallysufferfrom phase separation problems because they involve various forms of extraction with consequent intractable problems when the extractant, which is usually water-based, comes to be separated from the organic by-products.

We have nowfound a method of recovery of cobalt from the residue of an organiccompound conversion process which dispenses with the need for a phase separation step and which recovers the cobalt in a form sufficiently pure for it to be reusable in the process.

According to the invention therefore a processfor the recovery of cobalt from the residue of an organic compound conversion process which uses cobalt our a cobalt compound as catalyst comprises passing an electric current between electrodes immersed in said residue so asto deposit metallic cobalt on one ofthe electrodes and recovering cobalt from said electrode.

The process of the invention is carried out in a cell which, in its simplest form, comprises an anode, a cathode on which the cobalt is deposited, and a containerforthe electrolyte which may be the untreated liquid residue of the oxidation process but which is usually diluted by an added diluent eg by water. For example, in the oxidation of p-xylene to terephthalicacid using a cobaltcatalystthe cobalt remains in a distillation residue which is preferably diluted with 1 to 15, particularly 1 to 5, eg 3 parts of water before being submitted to the process of the present invention. Ifthe conductivity ofthe electrolyte is inadequate then a conductivity promoter, eg a salt such as sodium sulphate, may be added.

The electrodes which are used in the process may be madefrom material commonly used in the electrochemical arteg carbon and graphite. Preferably howeverthe electrodes are ofthe dimensionally stable anode type which comprise a layer of platinum bonded to a tantalum or, preferably, titanium surface.

Such an electrode serves as the anode while the cathode is tantalum or titanium metal. Such electrodes are characterised by an extended life. The process ofthe invention may be carried out using two electrodes ie an anode and a cathode or a plurality of electrodes may be present. The physical shape of the electrodes is chosen so asto provide a large surface area eg by using sheets of expanded metal.

In order to maximise the efficiency ofthe process it is desirableto maintain as high a concentration of cobalt ions as possible around the cathode. To this end it is helpful to create turbulence in the electrolyte eg by providing a circulation system in the cell optionally with baffles to interuptthe flow or, preferably, with a bed of glass beads between the elec trodes, the circulation oftheelectrolytefluidisingthe bed and hence promoting turbulent conditions.

The process may be carried out at ambient or elevated temperature. If undiluted residues are used or if a high boiling electrolyte is added then temperatures up to 200"C may be used. If water is added as diluent however then the operable temperature range is preferably ambient to 100"C. e.g. ambient to 80"C.

The cobalt may be recovered from the cathode by means known for such a purpose. As it is generally intended that the cobalt be recycled as a catalyst it is preferred that the cobalt be recovered in soluble form.

The preferred method of recovery is electrolytic in which the cathode carrying the cobalt becomes the anode of a cell and by passing a current between the new anode and a cathode the cobalt passes into solution. The simplest method of performing this operation isto reverse the current flow in the cell in which the cobalt has been deposited but such a technique presents problems as the cobalttends to redeposit on the new cathode (iethe original anode). It is preferred therefore to use a separate cell to recover the cobalt in which, by using a very small cathode or by surrounding the cathode buy a porous membrane, redisposition ofthe cobalt may be minimised or prevented altogether.The electrolyte used in the recovery ofthe cobalt may comprise water but in certain instances eg when the cobalt is catalyst in an aromatic compound oxidation process using a C2to C8 aliphatic monocarboxylicacid solventthe latter acid may form the basis oftheelectrolyte so facilitating the recycle ofthe cobalt.

The organic compound conversion process in which the cobalt is used as catalyst is generally an oxidation process. In particular, cobalt is widely used as catalyst in the production of aromatic carboxylic acids by the oxidation with molecular oxygen of a substituted aromatic compound. Other metals eg manganese or nickel may also be present as cocatalysts as may bromine or a bromine containing compound such as hydrogen bromide. The aromatic compound is preferablysubstituted by an alkyl, hydroxyalkyl, or a formyl group. Particularly suitable alkyl groups are lower (C1 to C8) alkyl groups eg methyl, ethyl and isopropyl groups. Particularly suitable hydroxymethyl groups. One, two or more such groups may be present in the aromatic nucleus and the groups may be the same or different. The aromatic nucleus may, for example, be a benzene or napthalene nucleus. Particularly suitable aromatic compounds to be oxidised are toluene, ethylbenzene, isopropylbenzene, o-, m- and p-xylene, cumene, pseudocumene, the isomeric diisopropylbenzenes, durene, mesitylene, hydroxymethylbenzene, hyd roxyethylbenzene, bis-hydroxymethylbenzenes, benzaldehyde and2, 6, -dimethylnaphthalene. Suitable aromatic compounds also include those which are already partially oxidised to carboxylic acids and their corresponding esters, for example,theisomerictolualdehydes, p-toluicacid, methyl p-toluate and p-carboxybenzaldehyde. The process of the invention is particularly applicable to the oxidation of p-xylene, p-tolualdehyde or p-toluic acid to terephthalic acid.

The aromatic compound oxidation process is generally operated in a solvent which may be a lower (C2 to C8) aliphatic monocarboxylic acid, particularly acetic acid. At the end of the oxidation process after separation ofthe acid product and any unchanged starting material the solvent is recovered by distillation leaving the cobalt catalyst together with any other metals in the distillation residue which comprises organic by-products which are liquid atthe distillation temperature butwhich solidify at ambienttemperature. For example, in the oxidation of p-xylene to terephthalic acid the main organic constituents of the residue are benzoic acid, p-toluic acid and the three phthalic acids.Water may be added to these residues to make them more handlable and it is this mixture of organic compounds and water which is used in the process ofthe invention. The residue besides containing cobalt mayalso include manganese or nickel co-catalyst as well as sodium and/or iron. The latter two metals are deleterious to the efficiency of the catalyst in the oxidation process so they should not be recycled. It is an advantage of the process of the present invention that any sodium and iron remain in the electrolyte and are discarded with the latter.

The invention will now befurtherdescribed with reference to the following Examples.

EXAMPLE 1 The apparatus consisted of a beaker in which were immersed a platinum anode and platinum cathode each of 1 cm2 surface area. The source of cobalt was a p-xylene oxidation process and consisted of a distillation residue which contained benzoicacid, p-toluic acid, the three phthalic acids, 4-carboxybenzaldehyde, 4-hydroxybenzaldehyde, the acetic acid ester of 4hydroxybenzaldehyde and trimellitic acid together with cobalt, manganese, iron, sodium and chromium.

This residue was diluted threefold with water before being electrolysed at 600C, the voltage used being approximately 10 volts and the current density 0.15 ampsicm2. Cobalt was deposited on the cathode with a current efficiency of approximately 15% and could be recovered therefrom by converting the cathode to an anode in a cell with an aqueous acetic acid electrolyte.

EXAMPLE2 In this Example a cell provided with a circulation system was used. The cell comprised three pairs of expanded metal anodes and cathodes, each of geometric area 200 cm2, mounted in a polypropylene container provided with a circulation system compris ingareservoiranda pumptakingsuctionfromthe reservoir and delivering to the bottom of the contain er. The electrolyte in the container overflowed from the top ofthe container back into the reservoir. The containerwas partially filled with 2.0 mm diameter glass beads and the anodes were made of platinised titanium and the cathodes oftitanium.

A sample ofthe residue described in Example 1 was diluted threefold with water, and filtered priorto its pH being adjusted to 5.8 with caustic soda solution. This mixture was then used as electrolyte in the electrolysis at 65"C. The cell operated for 1 hourwith a current of 8 amps and a voltage of 4-5 volts. During this time, 1.97 gram of cobalt was deposited on the cathode, indicating a current efficiency of 22%. An identical experiment was performed using cathodes which were previously flash coated with cobalt by cathodic polarisation at a current density of 25 amps/m2 in 10 wt % cobalt sulphate solution, pH 6.0,for 30 minutes.

In this case, a cell current of 8 amps for 1 hour yielded 2.27 grams of cobalt, indicating a current efficiency of 26%.

EXAMPLE3 Asample ofthe residue described in Example 1 was diluted ninefold with water, and its pH adjusted to 5.8 with caustic soda solution. This mixture was then used as electrolyte in the electrolysis at 65"C. The cell operated for 2 hours with a currentof 10 amps, the beads and distributor having been removed. During this time, 2.4 grams of cobaltwere deposited on the cathode equivalent to a current efficiency of approximately 12%.

CLAIMS (Filed on 6 April 1984) 1. Aprocessforthe recovery of cobaltfrom the residue of an organic compound conversion process which uses cobalt or a cobalt compound as a catalyst, which comprises passing an electric current between electrodes immersed in the said residue so asto deposit metallic cobalt on one ofthe electrodes, and recovering cobalt from the said electrode.

2. A process as claimed in claim 1 in which the organic compound conversion process is the oxidation of para xylene to terephthalic acid.

3. A process as claimed in claim 2 in which a conductivity promoter is added.

4. A process as claimed in claim 2 or 3 in which water is added.

5. A process as claimed in any preceding claim in which the electrolyte is turbulent.

6. A process as claimed in claim 1 whenever carried out substantially as described in any of the Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   -dimethylnaphthalene. Suitable aromatic compounds also include those which are already partially oxidised to carboxylic acids and their corresponding esters, for example,theisomerictolualdehydes, p-toluicacid, methyl p-toluate and p-carboxybenzaldehyde. The process of the invention is particularly applicable to the oxidation of p-xylene, p-tolualdehyde or p-toluic acid to terephthalic acid.

   The aromatic compound oxidation process is generally operated in a solvent which may be a lower (C2 to C8) aliphatic monocarboxylic acid, particularly acetic acid. At the end of the oxidation process after separation ofthe acid product and any unchanged starting material the solvent is recovered by distillation leaving the cobalt catalyst together with any other metals in the distillation residue which comprises organic by-products which are liquid atthe distillation temperature butwhich solidify at ambienttemperature. For example, in the oxidation of p-xylene to terephthalic acid the main organic constituents of the residue are benzoic acid, p-toluic acid and the three phthalic acids.Water may be added to these residues to make them more handlable and it is this mixture of organic compounds and water which is used in the process ofthe invention. The residue besides containing cobalt mayalso include manganese or nickel co-catalyst as well as sodium and/or iron. The latter two metals are deleterious to the efficiency of the catalyst in the oxidation process so they should not be recycled. It is an advantage of the process of the present invention that any sodium and iron remain in the electrolyte and are discarded with the latter.

   The invention will now befurtherdescribed with reference to the following Examples.

   EXAMPLE 1 The apparatus consisted of a beaker in which were immersed a platinum anode and platinum cathode each of 1 cm2 surface area. The source of cobalt was a p-xylene oxidation process and consisted of a distillation residue which contained benzoicacid, p-toluic acid, the three phthalic acids, 4-carboxybenzaldehyde, 4-hydroxybenzaldehyde, the acetic acid ester of 4hydroxybenzaldehyde and trimellitic acid together with cobalt, manganese, iron, sodium and chromium.

   This residue was diluted threefold with water before being electrolysed at 600C, the voltage used being approximately 10 volts and the current density 0.15 ampsicm2. Cobalt was deposited on the cathode with a current efficiency of approximately 15% and could be recovered therefrom by converting the cathode to an anode in a cell with an aqueous acetic acid electrolyte.

   EXAMPLE2 In this Example a cell provided with a circulation system was used. The cell comprised three pairs of expanded metal anodes and cathodes, each of geometric area 200 cm2, mounted in a polypropylene container provided with a circulation system compris ingareservoiranda pumptakingsuctionfromthe reservoir and delivering to the bottom of the contain er. The electrolyte in the container overflowed from the top ofthe container back into the reservoir. The containerwas partially filled with 2.0 mm diameter glass beads and the anodes were made of platinised titanium and the cathodes oftitanium.

   A sample ofthe residue described in Example 1 was diluted threefold with water, and filtered priorto its pH being adjusted to 5.8 with caustic soda solution. This mixture was then used as electrolyte in the electrolysis at 65"C. The cell operated for 1 hourwith a current of 8 amps and a voltage of 4-5 volts. During this time, 1.97 gram of cobalt was deposited on the cathode, indicating a current efficiency of 22%. An identical experiment was performed using cathodes which were previously flash coated with cobalt by cathodic polarisation at a current density of 25 amps/m2 in 10 wt % cobalt sulphate solution, pH 6.0,for 30 minutes.

   In this case, a cell current of 8 amps for 1 hour yielded 2.27 grams of cobalt, indicating a current efficiency of 26%.

   EXAMPLE3 Asample ofthe residue described in Example 1 was diluted ninefold with water, and its pH adjusted to 5.8 with caustic soda solution. This mixture was then used as electrolyte in the electrolysis at 65"C. The cell operated for 2 hours with a currentof 10 amps, the beads and distributor having been removed. During this time, 2.4 grams of cobaltwere deposited on the cathode equivalent to a current efficiency of approximately 12%.

   CLAIMS (Filed on 6 April 1984) 1. Aprocessforthe recovery of cobaltfrom the residue of an organic compound conversion process which uses cobalt or a cobalt compound as a catalyst, which comprises passing an electric current between electrodes immersed in the said residue so asto deposit metallic cobalt on one ofthe electrodes, and recovering cobalt from the said electrode.
2. 2. A process as claimed in claim 1 in which the organic compound conversion process is the oxidation of para xylene to terephthalic acid.
3. 3. A process as claimed in claim 2 in which a conductivity promoter is added.
4. 4. A process as claimed in claim 2 or 3 in which water is added.
5. 5. A process as claimed in any preceding claim in which the electrolyte is turbulent.
6. 6. A process as claimed in claim 1 whenever carried out substantially as described in any of the Examples.