Background information.

The removal of organic substances from process and waste streams poses a significant challenge for many industries. Treatment of organic substances containing waste streams is always an integral part of the operation of any industrial plant in order to meet local discharge standards. Depending on the nature of the industry, organic substances in industrial process and waste streams can be a complex mixture of different compounds ranging from simple low molecular weight hydrocarbons such as alcohols, aldehydes, ketones, carboxylic acids, low molecular weight fatty acids etc... to a very high molecular weight hydrocarbons such as fulvic and humic acids. Organic substances in industrial waste effluents may contain nitrogen, chloride, and sulphur and some effluents contain process inorganic chemicals which need to be recycled, e. g. black liquor from Bayer process and pulp mill.

Conventional methods for the treatment of organic containing industrial effluent involve either the use of microorganisms, wet oxidation or energy intensive evaporation, concentration followed by incineration.

Activated sludge treatment is a widely used method for the treatment of various kinds of aqueous wastes because of its simplicity and low cost. However, the microorganisms used for this process can only be effective for low organic content wastes.

It is known from the prior art that wet oxidation (wet-air oxidation) is a process in which organic substances in aqueous streams are oxidized by strong oxidants. Depending on reaction conditions and the type of organic compound to be oxidized, both non-catalytic and catalytic wet oxidation can be used to convert organic substances into C02 and biodegradable low molecular substances, e. g. mono or dicarboxylic acids.

Wet oxidation has been studied for sulphide liquor treatment in the pulp and paper industry since 1911 (Swed. Pat 34 941,1911). The patented technique was first commercialised in 1958 by Borregaard-Norway for the treatment of sulphide containing liquor. However the operation of the plant proved uneconomical and the plant subsequently closed. The non-catalytic wet oxidation process normally takes place at temperatures of up to 350°C and pressures of up to 200 bars. These extreme conditions cause severe technical difficulties as well as increased capital costs.

Catalytic wet oxidation is a promising alternative technique, which can operate at lower temperatures and pressures. Catalytic wet oxidation can be carried out by means of homogeneous or heterogeneous catalysis. There are a number of homogeneous catalytic systems reported which effectively oxidise organic substances from aqueous streams.

Examples of such catalytic systems are CuS04 and Cu (N02) 2. However, the need for down stream processing to remove the spent catalyst is a distinct disadvantage making homogeneous technology commercially infeasible.

Heterogeneous catalysis appears to be a better alternative. In heterogeneous oxidation catalysis, the catalytic activity is attributable to surface oxygen available on the solid catalyst. A good catalyst is characterised by high surface oxygen availability and fast oxygen transfer ability.

Industrial interest has stimulated numerous investigations into catalysts for wet oxidation of organic substances from process and waste streams. Although a number of catalytic systems have been reported in the open and patent literatures, most of the catalytic systems require severe conditions, namely high pressures and high temperatures, in order to achieve significant oxidation rate. In some catalytic systems, strong oxidants such as ozone and H202 are required.

Accordingly, it is an object of the present invention to overcome, or at least alleviate, the difficulties presented by prior art.

Summary of the Invention The present invention provides a catalytic system for the oxidation of organic substances present in process and waste streams. The catalyst system comprises red mud, a waste from Bayer process for extraction of alumina from bauxite, as a catalyst support, and at least one catalytically active component selected from Ag, Mn, Cr, Ce, Zr, Ni, Pt, Ru, Cu, V and Co.

The said active components may be present in any form such as metallic, oxide, or carbonate and in any oxidation state. The active components may be chemically bound to the support surface or present as separate crystalline phases.

The weight ratio of the active components to red mud as support catalyst may be in the range of approximately 0.01: 99.99 to 99.9: 0.01 but preferably in the range of 0.01: 99.99 to 50: 50.

The catalyst may contain one or more additional components, which alter its activity and selectivity. These additional components may be selected from oxides or carbonates of metal of groups IA, IIA, transition metals and lanthanides or mixtures thereof.

Red mud as hereinafter referenced, is a waste material remaining from the Bayer process for extraction of alumina from bauxite. The exact composition of red mud varies depending on the bauxite used but it contains Al, Si, Fe as the major components. The iron is usually present in red mud in the form of hematite, sideride and goethite. The aluminum is usually present in the form of gibbsite, boehmite and sodalite. The silicon is usually present in the form of quartz and sodalite. The red mud also contains smaller amounts of Ti, K, Ca, Na, Mn, Mg, P, S, Mn, Mg, P, S, Cr, Ni, Co, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Ba, Pb, V, U, Th, Ag and significant amounts of residue caustic used in the Bayer process.

The red mud may be used as received from industry but is preferably pretreated by any known technique such as washing, drying, calcining, neutralizing, size reducing/uniforming or combination thereof.

Accordingly the invention provides a method of preparing a catalyst comprising the steps of : grinding a red mud slurry to achieve a reduction in the particle size and a more uniform particle size distribution, adding solutions containing ions of at least one catalytically active component selected from Ag, Mn, Cr, Ce, Zr, Ni, Pt, Ru, Cu, V and Co, increasing the pH of the mixture of catalyst support and ions of at least one catalytically active component to co-precipitate the hydroxides or carbonates of the at least one catalytically active component onto the catalyst support, separating the precipitate from the solution, washing and drying the precipitate, and heating the precipitate to a temperature sufficient to activate the catalyst.

According to a further aspect of the invention, there is provided a process for the treatment of industrial effluents, such as paper pulp mill effluents, tannery effluents, Bayer back liquor, textile effluents, etc... which involve the use of a catalyst defined above.

In another aspect of the invention, there is provided a process for improving the sodium recovery and/or silicon removal from a Bayer black liquor including the steps of contacting the catalyst of the invention with a Bayer black liquor in the presence of an oxidising agent and then separating the catalyst from the effluent stream.

Further features, objects and advantages of the present invention will become apparent from the following description of the preferred embodiment and examples Description of the Preferred Embodiment According to the first aspect of the present invention, there is provided a novel catalyst system for the wet oxidation of organic substances present in industrial process and waste streams which comprises catalytic active components selected from Ag, Mn, Cr, Ce, Zr, Ni, Pt, Ru, Cu, V and Co supported on a readily abundant red mud, a solid waste from Bayer process for extraction of alumina from bauxite. Red mud received from industry may be as a dry solid or as slurry containing 10-90% solids. Red mud may be subjected to a size reducing/uniforming process such as milling using high-speed stirring and sintered ceramic as the milling media. Other types of grinding may be used provided the primary objective of the grinding is to reduce the larger particles which may be found in the red mud slurry to a particle size less than 10 microns thereby providing a more uniform particle size in the slurry A solution containing ions of at least one catalytically active component selected from the group of Ag, Mn, Cr, Ce, Zr, Ni, Pt, Ru, Cu, V, Co or mixtures thereof is then prepared and added to the red mud slurry while milling is continued. The solution of active components may be prepared from any suitable source including the chloride, sulphate or nitrate of the metals or carboxylic acid anions containing sources of the metals such as acetates or the like. The solution may also contain ions of a secondary catalytic component, which affect the activity or selectivity of the main catalytic component.

These secondary catalytic components may be selected from the groups IA, IIA, transition metals and lanthanide or mixtures thereof.

A caustic solution prepared from the hydroxide or carbonate of sodium is then added to the red mud-catalyst mixture while the milling is continued to co-precipitate hydroxides or carbonates of the catalytically active components from the solution onto the red mud catalyst support. The resulting catalyst is then separated from the liquid phase, washed, dried and calcined at a suitable temperature.

The prepared catalyst system is then used to catalyse the wet oxidation of organic substances including chlorine-, sulphur-and nitrogen-containing hydrocarbons to less harmful products as well as affecting the colour and odour removal.

The catalyst system and process described in the present invention may also be used as a means of process improvement such as oxidation of the sulphide containing green liquor in pulp and paper industry, improvement of sodium recovery and removal of silicon content in Bayer black liquor as consequences of organic substance destruction.

The process of the present invention may be carried out in the presence of oxygen or oxygen-containing gases or any other oxidants such as ozone and hydrogen peroxide, and in any suitable reactor. Additives such as surfactants may be added to enhance the reaction rate. The surfactant may be selected from amines or sodium salts of alkyl sulphonates or alkyl sulphates or any suitable commercial surfactant. The weight ratio of the surfactant to liquor may be in the range of approximately 0.01: 99.99 to 99.99: 0.01 but preferably in the range of approximately 0.01: 99.99 to 10: 90.

Examples To further illustrate the process of this invention, the following examples are provided. It should be understood that the details thereof are not to be regarded as limitations of the invention.

1. Preparation of supported mixed oxides on red mud: Example 1: 0.4wt% Ag20,21.2wt% NiO and 4.4wt% CeO2 supported on red mud A 100mL solution containing 0.39 g AgN03,35.2 g Ni (N03) 2.6H20 and 9.6 g (NH4) 2Ce (NO3) 6 was added to a 100g of commercial red mud slurry (50% solid), which was attrition milled with ceramic milling media (TZP, lmm, 300g) at room temperature for 20 minutes. A 50mL solution containing 5g sodium hydroxide was then added and the milling was continued for another 20 min. The resulting catalyst was separated by centrifugation, washed with water, dried at 100°C and then calcined in air at 400°C for 4h.

Example 2: 0.49wt% Ag20,8.2wt% CuO and 8.8wt% Mn304 supported on red mud A 200 mL solution containing 0.78 g of AgN03,23.4 g of Cu (N03) 2.3H20 and 70 mL of commercial Mn (N03) 2 solution at concentration of 2M was added to 200 g of red mud slurry (50% solid), which was attrition milled for 20 minutes. A 100mL solution containing lOg NaOH was then added and the milling was continued for another 20 minutes. The resulting catalyst was separated by centrifugation, washed with water, dried at 100°C and then calcined in air at 400°C for 4h.

Example 3: l. lwt% Ag20 and 4.7wt% Cr203 supported on red mud The procedure of example 2 was followed except that a 200mL solution containing 1.56g AgN03 and 15.6g Cr (N03) 3.9H20 was added to the red mud slurry Example 4: 0.49wt% Agio, 8.2wt% CuO, 8.8wt% Mn304 and 1.6wt% Ce02 supported on red mud The procedure of example 2 was followed except that a 100mL solution containing 6.4g of (NH4) 2Ce (NO3) 6 was added to the red mud slurry followed by addition of 100mL solution containing 0.78 g of AgN03,23.4 g of Cu (N03) 2.3H20, and 70 mL of commercial Mn (N03) 2.

2. Catalytic wet oxidation: Treatment of Pulp mill effluents Example 5: 2 grams of the catalyst prepared in accordance to example 1 were added to 100mL of sewer waste stream from pulp and paper industry. The catalyst and effluent were allowed to react in an autoclave at 70°C for 2 hours in the presence of 200kPa gaseous oxygen. A reduction in colour (measured by PtCo technique) of 84% was achieved.

Example 6: 2 grams of the catalyst prepared in accordance to example 1 were added to 100mL of bleach waste stream from a pulp and paper mill. The catalyst and effluent were allowed to react in an autoclave at 90°C for 1 hours in the presence of 200kPa gaseous oxygen. A reduction in colour (measured by PtCo technique) of 84% and a reduction in chlorinated hydrocarbon content (measured as AOX) of 50% were achieved.

Example 7: 2 grams of the catalyst prepared in accordance to example 1 were added to 200mL of green liquor from a pulp and paper mill. The catalyst and effluent were allowed to react at 90°C for 1 hours with sparging oxygen flowing at a rate of 30 mL/min. The following table shows the reduction of sulphide content as a function of time, in comparison with the same experiment run without catalyst Time (h) % sulphide reduction % sulphide reduction (with catalyst) (without catalyst) 0. 5 13. 1 <1. 0 1.0 31.8 4.6 2. 0 49. 7 9. 8 4. 0 62. 9 17. 6 Treatment of tannery effluents Example 8: 2 grams of the catalyst prepared in accordance to example 1 were added to 100mL of general effluent (COD=94000mg/L) from tannery industry. The catalyst and effluent were allowed to react at 90°C for 4 hours with sparging air flowing at a rate of 30 mL/min. The COD of the treated effluent was reduced to 26000 mg/L. This represents a 72% reduction in COD.

Example 9: 2 grams of the catalyst prepared in accordance to example 1 were added to 100mL of lime drain effluent (COD=52500mg/L, Sulphide= 2000ppm) from tannery industry. The catalyst and effluent were allowed to react at 90°C for 4 hours with sparging air flowing at a rate of 30 mL/min. The COD and sulphide content of the treated effluent was reduced to 28400 mg/L and 100ppm respectively. These represent a 46% reduction in COD and 95% reduction in sulphide content.

Treatment of Textile effluent Example 10: 1 grams of the catalyst prepared in accordance to example 1 was added to 1 OOmL of waste effluent from textile industry. The catalyst and effluent were allowed to react in an autoclave at 70°C for 1 hours in the presence of 200kPa gaseous oxygen. A reduction in colour (measured by PtCo technique) of 92% was achieved.

Treatment of Bayer black liquor Example 11: 7.5 grams of the catalyst produced in accordance with example 1 was added to 150mL of Bayer black liquor (COD=76500mg/L). The black liquor and catalyst was allowed to react at 140°C in an autoclave for 4 hours in the presence of 750kPa gaseous oxygen. The COD of the treated liquor was reduced to 70600 mg/L. This represents a 7.7% reduction in COD.

Example 12: 7.5 grams of the catalyst produced in accordance with example 2 and 0.165g of sodium dodecylbenzenesulphonate (SDBS) were added to 150mL of Bayer black liquor (COD=76500mg/L) giving the concentration of SDBS of 0.11%. The black liquor and catalyst was allowed to react at 140°C in an autoclave for 4 hours in the presence of 750kPa gaseous oxygen. The COD of the treated liquor was reduced to 56300mg/L. This represents a 26.4% reduction in COD.

Example 13: The testing procedure in example 12 was followed except that the concentration of sodium dodecylbenzenesulphonate (SDBS) used was varied. The following table shows the effect of the amount of sodium dodecylbenzenesulphonate used on the COD reduction. SDBS (%) % COD reduction 0 17.1 0.11 26.4 0.33 24.5 0.50 24.6 0.33 24.5 0. 50 24. 6 Example 14: The testing procedure in example 12 was followed. Catalysts produce in accordance with examples 1,2,3 and 4 and concentration of SDBS of 0.33% were used. The following table summarizes the result for COD reduction using different catalysts. Catalyst % COD reduction Example 1 7.7 Example 2 31. 0 Example 3 6. 2 Example 4 31. 0 Example 15: 7.5 grams of the catalyst produced in accordance with example 2 and 0.165g of sodium dodecylbenzenesulphonate (SDBS) were added to 150mL of Bayer black liquor (COD=79000mg/L) giving the concentration of SDBS of 0.11%. The black liquor and catalyst was allowed to react at 140°C in an autoclave for 0.5 hours in the presence of 750kPa gaseous oxygen. The COD of the treated liquor was reduced to 56560mg/L. This represents a 28.4% reduction in COD.

Example 16: The testing procedure in example 15 was followed except that the reaction time was changed. The following table shows the effect of reaction time on the COD reduction Reaction time (h) % COD reduction 0. 5 28. 4 1. 0 29. 4 2.0 33. 7 4. 0 35. 2 Example 17: 7.5 grams of the catalyst produced in accordance with example 2 and 0.165g of sodium dodecylbenzenesulphonate (SDBS) were added to 150mL of Bayer black liquor giving the concentration of SDBS of 0.11%. The Na2C03 and Si02 content in the black liquor were 46.2g/L and 660g/L respectively. The black liquor and catalyst was allowed to react at 140°C in an autoclave for 4 hours in the presence of 750kPa gaseous oxygen. The Na2C03 content in the treated liquor increased to 89.2g/L and the Si02 content decreased to 175g/L. This represents a 193% increase in the soda (Na2C03) content and a 73% decrease in the silica content.

Example 18: The testing procedure in example 17 was followed except that the catalyst prepared in example 4 was used. The Na2C03 content in the treated liquor increased to 90.5g/L and the Si02 content decreased to 173g/L. This represents a 195% increase in the soda content and a 74% decrease in silica content.

Examples 16 and 17 show the effectiveness of the catalyst described in the present invention for increasing the sodium recovery and for reducing the silicon content in the Bayer process. Example 16 also shows that a high proportion of the COD reduction occurs upon initial exposure of the catalyst and support to the effluent to be treated.

The above Examples 11-18 demonstrate that red mud from the Bayer process extraction of alumina from bauxite which is usually a solid waste product can be used effectively as a catalyst support in a catalyst system for the treatment of industrial effluents. This represents not only the utilisation of an otherwise waste product but also an inexpensive catalyst system for the treatment of effluents.