RECOVERY OF HEAVY METALS IN WASTE TREATMENT

Abstract: The invention concerns a method for the recovery and/or recirculation of a heavy metal in conjunction with the combustion of wood residue treated with a wood preservative, whereby the wood residue is crushed, burned by a combustion technique known in itself, the ashes produced in the combustion process are passed into a smelter where most of the heavy metal present in the ashes is recovered, and the combustion gases produced in the combustion process are cooled down and/or purified by separating the heavy metal contained in them. The invention is based on a principle whereby the combustion gases are cooled down, arsenic contained in the combustion gases is separated from the cooled combustion gases by washing, arsenic contained in the arseniferous wash water produced by the washing process and/or in the condensation water is oxidized by using an oxidizing agent, thus producing a solution containing oxidized arsenic, and the solution produced is treated so as to allow the solution and/or arsenic to be utilized in a wood preservative production process, e.g. directly as a working solution, at the same time preventing the generation of burdened waste water, and the washed combustion gases are heated again and paseed through a filter to recover the residual arsenic.

Description: RECOVERY OF HEAVY METALS IN WASTE TREATMENT The present invention relates to a method as defined in the preamble of claim 1 for the recovery and/or recirculation of heavy metal in conjunction with the combustion of waste wood treated with a wood preservative . In this context, 'heavy metal 1 refers to an element having a density exceeding 5 kg/dm 3 and regarded as having a detrimental effect on the environ- ment , humans or animals if encountered in excessive concentrations, e.g. As, Cu and Cr . In recent years, efforts have been made to discover economical, readily implementable and environmentally friendly methods for the treatment of pressure-impregnated wood residue, i.e. waste wood treated with a wood preservative. The amount of impregnating agent in the sap- wood portion of pressure- impregnated wood is typically about 3 - 24 kg/m 3 while the heartwood portion con- tains no impregnating agent. The composition of the impregnating agent may be e.g. 13.7 % CuO, 34.2 % Cr0 3 and 24.1 % As 2 0 5 . The wood preservative may alternatively consist of e.g. the following components: CuS0 4 • 5 H 2 0, Na 2 Cr 2 0 7 • 2 H 2 0 and As 2 0 5 • H 2 0. The composi- tion of the wood preservative may be as defined in the AWPA or the BS standard. The proportions of different substances in different wood preservatives vary. In prior art, methods for the treatment of pressure-impregnated wood are known whereby heavy met- als are extracted from the wood. Extraction methods have been found to be difficult to implement because heavy metals are closely combined with the wood. Further, methods for the treatment of pressure-impregnated wood e.g. in a copper smelter are known. The wood residue is crushed and burned in the copper smelter in an oxygen-rich atmosphere and at a high temperature. The problem is the crushing of the wood residue to a granular size below 10 mm. Moreover, treatment of wood material in the copper smelter reduces the capacity of the plant . Further, methods for the treatment of pres- sure- impregnated wood by combustion or carburization are known. These methods have been found to be applicable for the treatment of pressure-impregnated wood and for the separation of heavy metals from wood. Of heavy metals, e.g. arsenic evaporates mainly with com- bus ion gases while copper and chromium remain in the ashes produced. The ashes can be passed e.g. into a smelter. However, the problem is how to recover heavy metals e.g. from combustion gases without increasing the environmental stress, because heavy metals are difficult to remove. A further problem is the waste water produced in conjunction with the washing of combustion gases as it contains heavy metals, such as arsenic. Separating arsenic from waste water is a difficult and expensive process. The problem with generally known combustion methods is control of emissions. The object of the invention is to eliminate the problems referred to above and to disclose an environmentally friendly method for use in the treatment of pressure-impregnated wood. A specific object of the invention is to disclose a method for the recovery and/or recirculation of heavy metals that is easy to implement . The method of the invention is characterized by what is presented in the claims. The invention is based on recovering and/or recirculating heavy metals in conjunction with combustion of wood residue treated with a wood preservative, by crushing, e.g. chipping, and burning the wood resi- due by a combustion technique known in itself, e.g. in a gasification or grate type boiler or in a fluidized bed boiler. The ashes produced in the combustion proc- ess are passed into a smelter, where most of the heavy metal or heavy metals present in the ashes is recovered, and the combustion gases produced in the combustion process are cooled down and/or purified by sepa- rating the heavy metals contained in the combustion gases. According to the invention, the combustion gases are cooled down, arsenic contained in the combustion gases is separated from the cooled combustion gases by washing, i.e. by absorbing the arsenic into water. The arsenic contained in the arseniferous wash water produced by the washing process of the combustion gases and/or in the arseniferous condensation water condensed in a simultaneous cooling process is oxidized by using an oxidizing agent, thus producing a solution containing oxidized arsenic. The solution produced is treated so as to allow the solution and/or arsenic to be utilized in a wood preservative manufacturing process. The washed combustion gases are heated again and passed through a filter to recover the re- sidual arsenic. In a preferred embodiment, the ashes produced in the combustion process are exposed in the last stage of the process to oxidative conditions at a temperature of over 800 °C, thus minimizing the arsenic content of the ashes. The ashes are passed into a treatment process, e.g. into a copper smelter, where most of the heavy metal or heavy metals, e.g. copper and arsenic, is recovered. In this context, 'combustion gases' refers to all gaseous products appearing in conjunction with the combustion. The combustion gases contain various gaseous components. 'Wash water 1 in this context means any liquid or mixture of different liquids that is produced in the purification of the gases. The wash water preferably contains almost all of the arsenic evaporated in the combustion process. In an embodiment of the invention, the arsenic contained in the condensation water and/or wash water is oxidized, preferably using hydrogen peroxide, oxygen, air or any other oxidizer. Oxidization by means of oxygen can be accomplished e.g. in an autoclave by using e.g. iodine or active carbon as a catalyst . Arsenic occurs in the condensation water and/or wash water with a valence of +3 and/or +5. In an embodiment of the invention, the arsenic is oxidized to produce an arsenic compound in which arsenic occurs with a valence of +5, preferably arsenic acid, H 3 As0 4 . In this case, the oxidized solution produced is a weak arsenic acid solution. In an embodiment of the invention, the arsenic is precipitated from the solution produced. In an embodiment, the arsenic is precipitated from the solution in the presence of copper, a copper compound, an oxidizer and/or NaOH. The copper or copper compound may consist of e.g. crushed copper wire, metallic copper, a copper-arsenic compound, such as Cu 3 As, a copper sulfate compound, such as CuS0 4 • 5 H 2 0, or the like. In an embodiment of the invention, a large amount of a copper compound, such as Cu 3 As , or copper material, such as crushed copper wire, is loaded into a reactor in a stoichiometric proportion and a smaller amount of oxidized solution, i.e. condensation water or wash water, in stoichiometric proportions and oxi- dizer, e.g. hydrogen peroxide, is introduced into the reactor in order to precipitate the arsenic from the solution. In this case, the reaction is a rapid one, i.e. the reaction rate is high. A single charge of copper compound or copper material can be used to treat several batches of solution, in other words, a single copper compound or copper material charge is sufficient for several repeated precipitation treatments . In a preferred embodiment of the invention, precipitation of arsenic into copper arsenate, the chemical formula for which is Cu 3 (As0 ) 2 or equivalent, is achieved. The precipitated copper arsenate can be easily separated from the solution, and it can be utilized e.g. in the preparation of a CCA working solution or a CCA concentrate. The residual arsenic in the solution can be precipitated using ferrous sulfate and calcium. In an embodiment of the invention, the arseniferous solution is concentrated by evaporation. In an embodiment of the invention, copper, chromium and/or arsenic compounds, e.g. CuO, Cr0 3 , H 3 As0 4 , are added into the arseniferous solution and/or precipitate. In an embodiment of the invention, the arsenic, arsenic compound, arseniferous solution and/or arseniferous precipitate or the like are/is passed into a manufacturing process for the production of e.g. a commercial wood preservative to form a CCA working solution, a strong CCA concentrate and/or CCA paste or the like. A CCA wood preservative contains arsenic, copper and chromium compounds, preferably e.g. CuO, Cr0 3 , H 3 As0 4 , CuS0 4 ■ 5 H 2 0 ja Na 2 Cr 2 0 7 • 2 H 2 0. The concentration and viscosity of CCA paste are higher than those of CCA concentrate. In an embodiment of the invention, copper and chromium compounds are added into the solution containing oxidized arsenic, and the solution is utilized as a CCA working solution in a wood impregnating process . In an embodiment of the invention, the com- bustion gases are cooled down to a temperature of 200 300 °C by simultaneously recovering the heat. At this temperature, the partial pressure of arsenic, As 2 0 3 , is so high that it will not sublimate on the surface of the heat recovery equipment . The cooled combustion gases are passed into a wet gas washing apparatus known in itself, e.g. an Imatra Venturi washer, to separate particles and/or gaseous impurities from the combustion gases. The wet washing apparatus may preferably comprise a separating chamber, e.g. a cyclone, in which the gas is separated from the wash water. In the wet washing apparatus, the combus- tion gases are cooled further to about 40 - 60 °C . The temperature of the washed gas coming out of the wet washing apparatus is raised by 5 - 40 °C, preferably 10 - 25 °C, e.g. by means of a heat exchanger e.g. to a level of 50 - 90 °C and the gas is passed through a gas filter known in itself, e.g. a bag filter, to filter out the residual impurities. In the filter, the gas passes e.g. through a filter cloth which is cleaned at certain intervals e.g. by dissolving the impurities gathered in it. The solution used, which contains e.g. soluble arsenic, can be utilized in the manufacture of a wood preservative. Naturally, any device or filter for the separation of corpuscular or gaseous impurities can be used in the method. In the wet washing apparatus, an effective contact area between the combustion gas and the water drops is achieved, with the result that the particles and drops agglomerate and can therefore be separated from the gas. The wet washing apparatus, especially a Venturi washer, effectively separates even small cor- puscular and gaseous impurities from the gas. The method of the invention makes it possible to achieve a perfect recirculation of heavy metals, such as copper, chromium and arsenic, contained e.g. in a wood preservative, without producing environ- mental stress. This means a low level of emissions and a high degree of utilization of the waste. Further, the energy contained in wood residue treated with a wood preservative can be recovered and the heavy metals, such as arsenic, contained in the wood residue can be effectively recirculated. By the method of the invention, the wood residue is burned to recover the energy, the ashes produced in the combustion process are cleaned and/or stabilized, the combustion gases are cleansed of corpuscular and gaseous impurities and the condensation water produced in the process for cooling the combustion gases as well as the wash water produced in the purification of the combustion gases is purified and/or utilized. The heavy metals, especially copper and arsenic, recovered in different stages of treatment can be utilized in the production of a wood preservative. The environmentally friendly nature of the method is a great advantage, especially as the environmental laws and restrictions on emissions are continuously becoming stricter. The solution produced in the treatment of condensation water and/or wash water can be utilized e.g. directly in wood preservative production e.g. as a working solution, which means that no waste solution or waste water is produced at all. On the other hand, arsenic can be recovered from the solution and recir- culated into a wood preservative production process, so that the waste water is substantially clean. Furthermore, residual arsenic is effectively separated from the washed combustion gases. The invention discloses a significant alter- native for the utilization of wood treated with a wood preservative, making it possible to reduce the amount of waste wood transported to dumping places, thus reducing the risk of heavy metals dissolved from the wood getting into the ground and ground water. A fur- ther advantage is a reduction in the amount of waste wood that may be classified as problem waste, which reduces the waste treatment costs. The method of the invention is effective and it can be easily implemented in conjunction with existing equipment. The method is also economical in respect of operating costs . In the following, the invention will be described in detail by the aid of a few examples of its embodiments with reference to the drawings, wherein Fig. 1 presents a block diagram representing the method of the invention, and Fig. 2 and 3 present block diagrams representing methods according to the invention for the treatment of condensation water and/or wash water. Example 1 An experiment was carried out to study the combustion of wood residue treated with a wood preservative and the recovery of heavy metal emissions. The wood residue was burned in a gasification-type heating boiler for the production of heat. The boiler was situated in a hall. Fig. 1 presents a block diagram representing the treatment of wood impregnated with a wood preservative and the recovery of detrimental substances. The impregnated waste wood was chipped, producing 96 solid m 3 of wood chips. The wood material contained heavy metals as follows: Cu 0,74 kg/solid m 3 As 1,04 kg/solid m 3 Cr 1,03 kg/solid m 3 The wood chips were burned in the boiler. The amount of ashes produced as a result of the combustion was about 1000 kg, which contained 66 kg of copper, 30 kg of arsenic and 99 kg of chromium. The ashes were fed into a copper smelter, where they were processed together with concentrates. Most of the copper was recovered as metallic copper via copper electrolysis. The chromium and part of the arsenic was bound in slag. Most of the arsenic was recovered in the form of Cu 3 As precipitate in conjunction with the copper electrolysis. The precipitate was utilized by a method disclosed in specification US 5141753. By this method, a CCA concentrate needed in wood preservative manufacture is produced from Cu 3 As precipitate . The combustion gases produced in the combus- tion process were cooled down to 200 - 300 °C and washed in a high-pressure Venturi washer, where most of the arsenic was absorbed in water. The arsenic (3.56 kg/m 3 ) contained in the wash water and in the condensation water condensed as a result of the cool- ing of the combustion gases was oxidized using hydrogen peroxide at a temperature of 50 °C according to reaction (I) . About 95 % of the arsenic occurred in the wash water and condensation water with a valence of +3 (As 2 0 3 ) and the rest with a valence of +5 (As 2 0 5 ) . After the oxidation, arsenic mainly occurred with a valence of +5. As 2 0 3 + 2 H 2 0 2 + H 2 0 → 2 H 3 As0 4 (I) The arsenic acid produced as a result of oxidation and the wash/condensation water formed a weak arsenic acid solution, which could be used directly as a weak working solution in the production of wood preservative, by adding to it some copper chromate, which was obtained by mixing CuO and Cr0 3 together. The composition of the working solution obtained was CuO 2.81 g/dm 3 , Cr0 3 6.76 g/dm 3 and As 2 0 5 4.85 g/dm 3 . The solution was suited for use in class A impregnation in accordance with NTR norms . Based on the experiment, it was possible to recover the arsenic from the combustion gases with a good yield and in an environmentally friendly manner. Measurements pertaining to industrial hygiene performed in the hall indicated that the workers were not exposed to heavy metals during the experiment. Example 2 The flow of combustion gases produced by the combustion of wood treated with a wood preservative was 5000 Nm 3 /h and its temperature was 1100 °C . The combustion gas contained 5648 kg of dry gas/m 3 . The water content of the gas was 153 g/kg of dry gas and the As content 230 mg/kg of dry gas. The combustion gas was cooled down to 250 °C via indirect cooling and passed into a Venturi washer, where it was cooled further to a temperature of 50 °C . The gas was purified in the Venturi washer and the washed gas was heated again to a temperature of about 64 °C via indirect heating. The water content of the gas was 90 g/kg of dry gas and the As 2 0 3 content 3 mg/kg of dry gas. The gas was passed into a fabric filter, in which residual heavy metals were removed from the gas (Fig. 1) . After the filtering, the As 2 0 3 content of the gas was 0.04 mg/kg of dry gas. The dust emissions and the amount of PAH compounds in the gas were also low. During 5 h, 1.95 m 3 of condensation and wash water was collected, in which the average As 2 0 3 content was 3.55 g/dm 3 . The arsenic contained in the water was oxidized into arsenic acid in the manner described in Example 1. The solution was concentrated by evaporating it until its volume was 1.63 m 3 and the As 2 0 5 content of the solution was 4.93 g/dm 3 . 8.9 kg of CuO and 32.1 kg of Cr0 3 was added into the solution in the manner described in Example 1. The final volume of the solution was 1.75 m 3 and the composition of the solution was CuO 5.07 g/dm 3 , CrQ 3 18.3 g/dm 3 and As 2 0 5 4.59 g/dm 3 . During 24 h, no precipitate was formed in the solution, so the solution was appropriate for use as a working solution according to the AWPA standard (P5 type A) in the manufacture of a wood preservative, the solution contain- ing a total of 28 g/dm 3 of metal oxides. The solution thus produced can be diluted by adding water into it if desirable. Example 3 The combustion of CCA- impregnated wood produced 10 dm 3 of condensation/wash water. The condensation water contained As 2 0 3 3.29 g/dm 3 , As 2 0 5 0.26 g/dm 3 , Cu 2+ 1 mg/dm 3 and Cr 3+ 0.5 mg/dm 3 . The solution was oxidized as in Example 1. Into the solution was added 77 g of CuS0 4 • 5 H 2 0 and 99 g of Na 2 Cr 2 0 7 • 2 H 2 0. The volume of the solution was 11 dm 3 and its composition was CuS0 4 • 5 H 2 0 7.03 g/dm 3 , Na 2 Cr 2 0 7 • 2 H 2 0 8.94 g/dm 3 ja As 2 0 5 • H 2 0 4.02 g/dm 3 . During 24 h, no precipitate was formed in the solution. The solution was suited for use as a working solution according to the BS standard (4072, part 1, type 2) , concentration 20 g/dm 3 , in a wood impregnating plant. A corresponding solution can also be used as a weaker or stronger solution. Example 4 The arsenic (As 2 0 3 ) contained in the condensa- tion/wash water was oxidized as in Example 1. Arsenic occurring in the solution with a valence of +5 was precipitated from the solution according to reaction (ID . As 2 0 5 + 3 CuS0 4 + 6 NaOH → Cu 3 (As0 4 ) 2 + 3 Na 2 S0 4 + 3 H 2 0 ( I I ) The arsenic was precipitated almost completely. The Cu 3 (As0 4 ) 2 precipitate produced was utilized in the manufacture of CCA wood preservative. The residual arsenic was precipitated in a manner known in itself, e.g. using ferric sulfate and calcium, and via oxidation, producing as a result a FeAs0 • CaS0 precipitate and purified condensation water (Fig. 2). Example 5 Fig. 2 presents a block diagram representing a method for the treatment of condensation/wash water. The arseniferous condensation water, 10 dm 3 , which had an As 2 0 3 content of 3.55 g/dm 3 and an As 2 0 5 content of 0.5 g/dm 3 , was oxidized at a temperature of 98 °C using oxygen, producing arsenic acid. The solution was passed through a 1 dm 3 reactor, in which it was intensively agitated. The reactor had been filled with 2 kg of chopped copper wire mate- rial. Also, 50 % hydrogen peroxide was supplied into the reactor at 8 ml/h. The arsenic was precipitated according to reaction (III) . The temperature during the reaction was 55 °C. 2 H 3 As0 4 + 3 Cu + 3 H 2 0 2 → Cu 3 (As0 4 ) 2 + 6 H 2 0 (III) The copper arsenic precipitate produced was removed from the upper part of the reactor, leaving the copper wire material in the reactor. After the precipitation, the pH of the solution was 3.5. By adding a small amount of NaOH, the pH value of the solution was increased to 5.5. The arsenic content of the solution was 30 mg/dm 3 and its copper content was 40 mg/dm 3 . The final solution was purified in the manner described in Example 4 and Fig. 2, i.e. the residual arsenic was precipitated. The copper arsenic precipitate was separated by filtering. The precipitate was mixed with Cr0 3 and H 3 As0 4 , thus obtaining a strong CCA concentrate in which the amount of active material was 800 g/dm 3 and which contained 13.7 % CuO, 34.2 % Cr0 3 and 24.1 % As 2 O s and 28 % H 2 0. Example 6 Fig. 3 presents a block diagram representing a method for the treatment of condensation/wash water. Condensation/wash water oxidized as in Example 1 was supplied into a 3 dm 3 reactor which contained 2 kg of Cu 3 As precipitate. Cu 3 As precipitate is a waste material which is produced as a side product in copper industry (Fig. 1) . 50 % hydrogen peroxide in an amount of 80 ml at a temperature of 50 °C was supplied into the reactor. The Cu 3 As precipitate was dissolved in the oxi- dized condensation/wash water. The arsenic was precipitated according to reaction (IV) . Cu 3 As + 5,5 H 2 0 2 + H 3 As0 4 → Cu 3 (As0 4 ) 2 + 7 H 2 0 (IV) At the end of the precipitation process, a small amount of NaOH was added into the solution, with the result that the pH value of the solution rose to 5.5. The precipitate was sedimented at a temperature of 60 °C and the clear solution was removed from the reactor. The arsenic content of the clear solution was 30 mg/dm 3 . It was possible to treat the clear solution by the method presented in Fig. 2. Oxidized condensation/wash water was again added into the reactor. The above-described process steps were repeated a suffi- cient number of times until the Cu 3 As precipitate had been almost entirely converted into Cu 3 (As0 4 ) 2 precipitate, which was utilized in the production of a strong CCA concentrate, paste or working solution in a wood impregnating plant. The same charge of Cu 3 As precipitate could be used to treat a plurality of repeated single condensation/wash water charges, e.g. 50 - 100 charges, because the reaction took place quickly and there was a stoichiometric excess of Cu 3 As precipitate in relation to arsenic acid. In all the examples presented above, instead of hydrogen peroxide, it is possible to use e.g. oxygen or air as an oxidizer, only at a higher temperature . Different embodiments of the method of the invention are applicable for the combustion of various kinds of waste wood and/or for the recirculation, recovery and/or utilization of heavy metals, especially arsenic and copper. The embodiments of the invention are not restricted to the examples presented above; instead, they may be varied in the scope of the following claims .

Claims: CLAIMS 1. Method for the recovery and/or recirculation of a heavy metal in conjunction with the combustion of wood residue treated with a wood preservative, whereby the wood residue is crushed, burned by a combustion technique known in itself, the ashes produced in the combustion process are passed into a smelter where most of the heavy metal present in the ashes is recovered, and the combustion gases produced in the combustion process are cooled down and/or purified by separating the heavy metal contained in them, char ac t e r i z ed in that the combustion gases are cooled down, arsenic contained in the combustion gases is separated from the cooled combustion gases by wash- ing, arsenic contained in the arseniferous wash water produced by the washing process of the combustion gases and/or in the condensation water is oxidized by using an oxidizing agent, thus producing a solution containing oxidized arsenic, and the solution produced is treated so as to allow the solution and/or arsenic to be utilized in a wood preservative production process, and the washed combustion gases are heated again and passed through a filter to recover the residual arsenic . 2. Method as defined in claim 1, charac t e r i z ed in that the arsenic contained in the condensation and/or wash water is oxidized using hydrogen peroxide, oxygen, air or a similar oxidizer. 3. Method as defined in claim 1 or 2, charact eri zed in that the arsenic is oxidized to an arsenic compound in which arsenic occurs with a valence of +5. 4. Method as defined in any one of claims 1 - 3, charac t e ri z ed in that the arsenic is pre- cipitated from the solution produced. 5. Method as defined in any one of claims 1 - 4 , c h a r a c t e r i z e d in that the arsenic is pre- cipitated from the solution in the presence of copper, a copper compound, an oxidizer and/or NaOH. 6. Method as defined in any one of claims 1 - 5, c h a r a c t e r i z e d in that the reactor is loaded with a copper compound, such as Cu 3 As, or copper material, such as crushed copper wire material, and oxidized solution and oxidizer are supplied into the reactor to precipitate the arsenic from the solution. 7. Method as defined in any one of claims 1 - 6, c hara c t e r i z ed in that the arsenic is precipitated to form copper arsenate. 8. Method as defined in any one of claims 1 - 7, chara c t e r i z ed in that the arseniferous solution is concentrated by evaporating. 9. Method as defined in any one of claims 1 - 8, chara c t e ri zed in that copper, chromium and/or arsenic compounds are added into the arseniferous solution and/or precipitate. 10. Method as defined in any one of claims 1 - 9, c h a r a c t e r i z e d in that the arsenic, arsenic compound, arseniferous solution and/or arseniferous precipitate or the like is passed into a wood preservative production process to form a CCA working solution, a strong CCA concentrate and/or CCA paste or the like. 11. Method as defined in any one of claims 1 - 10, charac t er i zed in that copper and chromium compounds are added into the solution containing oxidized arsenic, and the solution is utilized as a CCA working solution in a wood impregnation process. 12. Method as defined in any one of claims 1 - 11, c h a r a c t e r i z e d in that the combustion gases are cooled down to at a temperature of 200 - 300 °C, the combustion gases are washed in a wet washing apparatus, causing them to cool down to a temperature of 40 - 60 °C, and the washed combustion gases are heated to raise their temperature by 5 - 40 °C, preferably 10 - 25 °C, before being passed through a filter.

Inventors: LINDROOS LEO, LINDROOS, LEO
Applicants: LINDROOS LEO, LINDROOS, LEO
Application year: 2000
Main classification: B09B3
IPC codes: B01D53, C02F1, C22B7, C22B30