The present invention relates to treatment and purifying of fly ash coming from an electrostatic precipitator for sodium sulfate -containing material streams of a chemi- cal pulp mill, especially a recovery boiler, such as a kraft recovery boiler

As a result of the operation of a recovery boiler, part of the inorganic substance of black liquor is removed from the furnace of the boiler entrained in flue gases in solid form, i.e. as so-called fly ash, by means of electrostatic precipitators. Fly ash contains mainly sodium sulfate (typically 80-90%), sodium carbonate (typically 5-15%) and sodium chloride and potassium chloride. Normally said ash is returned to the recovery cycle via a mixing tank of the recovery boiler. The amount of ash being returned depends on the size of the boiler, but typically it is approximately 10% of the amount of dry solids to be combusted. In some cases it is inevitable to remove fly ash of a recovery boiler from the recovery cycle for controlling the sulfur/sodium (S/Na) balance of the chemical pulp mill or for removing excess potassium (K) and chloride (CI).

Traditionally the fly ash removed from the chemical cycle has been dissolved in water or condensate and the solution has been led to an effluent plant. Due to tightening environmental regulations it is obvious that dissolving of ash will be prohibited at some time interval, whereby alternative methods for controlling the S/Na-balance of the mill, and the amount of potassium and chloride, will be needed. Fly ash, the main components of which are sodium sulfate and sodium carbonate, is very light, so that its transportation somewhere else is not economically profitable.

The sulfur balance of a chemical pulp mill is burdened, especially for soft wood mills, by the sulfuric acid (H ₂ S0 ₄ ) used for tall oil splitting that brings excess sulfur into the balance of the mill, even up to 5.0 kg per produced ton of pulp (ADt). This excess sulfur is typically removed by removing from the chemical cycle part of the fly ash of the recovery boiler. A problem in this method is that simultaneously also a valuable cooking chemical, i.e. sodium (Na) is removed, which has to be replaced by adding sodium hydroxide (NaOH) into the chemical cycle of the chemical pulp mill.

Another component burdening the sulfur balance of a chemical pulp mill is acid salt, so-called sodium sesquisulfate (Na ₃ H(S0 ₄ ) ₂ , that comes from the chloride dioxide plant and contains sodium sulfate (Na ₂ S0 ₄ ), which acid salt is usually returned either to the black liquor evaporation plant and there through into the chemical cycle, or it is used in bleaching in pH adjustment or dissolved in effluents. Usually these sulfate containing salts are generated in the mill approximately in the amount of 50-60 tons per day, i.e. approximately 30-40 kg/ADt. Sesquisulfate is formed in the amount of approximately 1.5 kg per each produced kg of chloride dioxide (CI0 ₂ ). In some mills the sesquisulfate of the chloride dioxide plant is dissolved and led into effluent treatment, whereby it does not burden the S/Na balance of the mill any more, but on the other hand increases effluent emissions from the mill. Acid salt of a chloride dioxide plant is usually neutralized, and it is also referred to as waste salt or waste acid. An object of the present invention is to decrease or completely stop discharge of sodium sulfate containing material streams into effluent streams of the mill or into other waste streams and to provide a new method of treating them. Especially the invention relates to treatment of fly ash from the recovery boiler, which ash has to be removed from the chemical cycle. An object is also to improve the use of acid salt of the chloride dioxide plant.

In the new method, sodium sulfate (Na ₂ S0 ₄ ) contained in ash is used to produce Glauber's salt (decahydrate of sodium sulfate, Na ₂ SO ₄ *10H ₂ O) or anhydrous sodium sulfate. These can either be utilized internally at the mill or commercially and sold out from the mill, whereby the need for discharge of ash and its passing into effluent streams is substantially decreased or it can even be completely prevented.

In the new method, sodium sulfate-containing fly ash of the recovery boiler of a chemical pulp mill is purified. The method comprises at least the following steps: a) ash is dissolved in an aqueous solution and the pH of the solution is adjusted with alkali for precipitation of impurities,

b) the solution is filtered for removing impurities-containing precipitate, c) sodium sulfate is crystallized out from the solution and crystals are separated from the solution by filtering or by centrifugation, and

the crystallized sodium sulfate is used as initial material for producing sodium and sulfur containing chemicals or as process chemical.

The ash is dissolved in an aqueous solution, advantageously either in ion-exchanged water (demi water) or in clean condensate, having a temperature of 32-100°C, preferably over 32°C, most preferably between 35-50°C. The purpose in the dissolving is to produce a saturated or nearly saturated solution. The pH of the solution is ad- justed to a level of over 11 , preferably over 12, for precipitating inorganic impurities, such as magnesium, present in ash of the recovery boiler. For pH-adjustment alkali is used, typically sodium hydroxide. The water-fly ash mixture is mixed at a constant temperature, which is over 32°C, and mixed until the salt has been dissolved evenly. The formed solution is filtered for removing impurities-containing precipitate and crystallized. The crystallization can be performed either as cooling crystallization or as evaporation crystallization.

In cooling crystallization the filtered solution is cooled to a temperature of below 32.4°C, preferably to a temperature of 10-15°C, simultaneously mixing, whereby in cooling crystallization Glauber's salt crystals (Na ₂ SO4*10H ₂ O) are formed at temperatures below 32.4°C. After the crystallization the formed crystals are separated from the mother liquor either by filtering or by centrifugation.

Chloride and potassium have higher solubility in water than other components, and they are enriched in liquid phase, which can be removed from the process. In evaporation crystallization the filtered solution is concentrated by evaporation, simultaneously mixing, whereby sodium sulfate crystals are generated. By means of evaporation crystallization clean anhydrous sodium sulfate is produced. After the crystallization the formed crystals are separated from the mother liquor either by filtering or by centrifugation. In evaporation crystallization the temperature is most pre- rerably over 70°C, whereby potassium and chloride remain in the mother liquor most efficiently.

Separated sodium sulfate or Glauber's salt can be used as initial material in an elec- trodialysis process for producing sodium and sulfur chemicals, such as sodium hydroxide and sulfuric acid. For this purpose, the crystals are dissolved in a clean wa- ter fraction, such as in clean secondary condensate from an evaporation plant, or in demineralized water.

If Glauber's salt is used at the mill e.g. as initial salt for an electrolysis plant or an electrodialysis (ED) plant, when fly ash is used as raw material, a calcium removal unit is typically required after dissolving of the crystals, preferably an ion-exchange column, in order to obtain an adequately low calcium content of the feed solution, preferably below 1 ppm. Calcium can also be separated by lowering the pH of the crystal solution in connection with the dissolving of the crystals. Further, the crystals being fed into the electrodialysis equipment are to be carbonate- free, because the cells used in electrodialysis devices break down due to the impact of carbon dioxide (C0 ₂ ) being released from carbonate under acid conditions. Ash can contain 5-15% of sodium carbonate. Carbonate can be removed from Glauber's salt crystals by means of a suitable acid by adjusting the pH of the Glauber's salt water solution below 6, most preferably to a level of below 4. The acid is preferably acid salt from a chloride dioxide plant, which does not require precleaning and which typically contains sulfuric acid approximately 26%. Alternatively also another acid can be used, such as sulfuric acid, but this increases the consumption of fresh acid at the mill. The acid can be fed directly into the mixing tank for crystals, whereby the pH decreases and it is adjustable. Calcium precipitates and can be removed as precipitate. Simultaneously carbonate is released as carbon dioxide, the separation of which can be improved by efficient mixing or by leading air into the solution.

When calcium is separated in connection with dissolving of the crystals, the ion- exchange unit can typically be passed. Nevertheless, an ion-exchange unit is a useful process device, since it can be needed as a polishing filter.

Potassium (K) and chloride (CI) present in the ash can be removed together with the discharge liquid, mother liquor, coming from the filtering or centrifugation after the crystallization, whereby preferably no separate potassium or chloride removal plant is needed at the mill.

The chemical recovery cycle of a chemical pulp mill substantially comprises combustion of black liquor in the recovery boiler, which black liquor is separated from chemical pulp and contains cooking chemicals, dissolving the chemical smelt formed in the combustion into green liquor, causticizing of the green liquor into white liquor, which is used in chemical pulping. In the new method, ash generated in the combustion is removed from the chemical recovery cycle, and its sodium sulfate is not recycled back into the recovery boiler. Ash is typically treated in the amount that has to be removed from the chemical cycle and which would have to be dumped from the mill into waste waters. This amount can be tens of tons per day. Due to the new method, leading sodium sulfate into the effluent streams of the mill can be substantially decreased or even stopped.

The final product generated in the crystallization is advantageously Glauber's salt of Pro Analysis (PA) quality or anhydrous sodium sulfate of Pro Analysis quality. The sodium sulfate obtained in evaporation crystallization or cooling crystallization can be used instead of electrolysis also in other production processes as initial material at the chemical pulp mill, where it has been produced, or alternatively outside the mill. It can also be used as process chemical outside the chemical pulp mill, e.g. for fertilizers, detergents, in textile industry or in glass industry. The invention is described in more detail with reference to the appended Figure 1 , which is a schematic illustration of a preferred embodiment for performing the new method.

Fly ash from a recovery boiler is introduced via line 1 into a vessel 2, wherein the ash is dissolved in an aqueous solution from line 3. The aqueous solution is typically either ion-exchanged water (demi water) or clean condensate. The dissolving takes place at a temperature of between 32-100°C, preferably between 35-50°C, whereby the aim is to produce a saturated or almost saturated solution. The pH of the solution is adjusted with alkali, e.g. sodium hydroxide, to a level of approximately 12-13 for precipitating impurities, mainly magnesium (Mg). The alkali is introduced via line 4 into an ash dissolving vessel 2.

The water-fly ash mixture is mixed at a constant temperature, which is over 32°C, and mixed until the sodium sulfate has been dissolved evenly. The solution is filtered e.g. in a cross flow filter 6, into which it is led via line 5. Thereby the magnesium- containing precipitate is separated from the sodium sulfate-containing solution. The precipitate is taken via line 7 into a mixing tank (not shown) at the recovery boiler plant, where ash of the electrostatic precipitator of the recovery boiler is dissolved in liquor.

The filtered solution is led via line 9 into a cooling crystallization vessel 8, where the sodium sulfate is crystallized by cooling crystallization in a way known per se. In cooling crystallization, Glauber's salt crystals (Na ₂ SO4*10H ₂ O) are generated at temperatures below 32.4°C. The Glauber's salt crystals (dehydrate of sodium sulfate) are separated from the mother liquor in line 19 by a centrifuge 24. The final product generated in the crystallization is Glauber's salt of Pro Analysis (PA) quality. The crystallization can also be performed by evaporation crystallization, whereby clean, Pro Analysis quality anhydrous sodium sulfate is produced.

Potassium and chloride present in the ash can be removed together with the discharge liquid, mother liquor, coming from the filtering or centrifugation after the crystallization, whereby no separate potassium or chloride removal plant is needed at the mill. The discharge solution from line 22 is precipitated in a clarifier 10, from the bottom of which the potassium and chloride containing solution is led via line 1 1 out of the chemical cycle of the mill.

The sodium sulfate crystals can be used in an electrodialysis as initial material at the mill. The crystals are led from the centrifuge 9 via line 23 into a subsequent process step, where they are dissolved preferably in ion-exchanged water or in clean condensate in a mixing tank 12. Water or condensate is introduced via line 18. When fly ash is used as raw material, a calcium removal unit, preferably an ion-exchange column 15, is typically needed after dissolving of the crystals, in order to reach an ade- quately low calcium content of the feed solution, preferably below 1 ppm. The aqueous crystal solution is led via line 13 into the ion exchange column 15, from where the separated calcium is led via line 16 into an ash mixing tank (not shown) of the recovery boiler.

Further, the crystals being fed into the electrodialysis equipment are to be carbonate- free, because the cells used in electrodialysis devices break down due to the impact of carbon dioxide (C0 ₂ ) being released from carbonate under acid conditions. Carbonate can be removed from Glauber's salt crystals by means of a suitable acid by adjusting the pH of the Glauber's salt water solution below 6, most preferably to a level of below 4. Most advantageous to be used is waste acid of the chloride dioxide plant, which is introduced via line 20. Also another acid can be used, such as sulfuric acid, but this increases the consumption of fresh acid at the mill. When the pH is decreased, also calcium is precipitated and it can be removed as precipitate from the bottom of crystal mixing tank 12. This calcium precipitate is led via line 21 into the recovery boiler ash mixing tank. Since especially the initial salt of an electrodialysis plant must be carbonate-free, the solution can be efficiently mixed or air can be introduced into the solution for releasing carbon dioxide from the solution. When calcium is separated in connection with dissolving of the crystals in the mixing tank 12, the ion-exchange unit 15 can typically be passed via line 14. Nevertheless, the ion- exchange unit 15 is a useful process device, since it can be needed as a polishing filter.

The solution containing Glauber's salt is led into an electrolysis or electrodialysis (ED) plant 17, where it is used as initial salt for producing sodium and sulfur chemicals for the needs of the mill or for use outside the mill. Crystallized sodium sulfate can be used outside the chemical pulp mill as a chemical e.g. for fertilizers, detergents, textile dyeing and for rhe needs of glass industry.

The operation of an electrodialysis plant is based on the potential difference between a cathode and an anode and the movement of ions through an ion exchange mem- brane caused by the difference. When there is an electrical field between a cathode and an anode, ions move based on their electric charge, i.e. positively charged ions (cations) are passed to a cathode and respectively negatively charged ions (anions) are passed to an anode. In an electrodialysis unit, always two different diaphragms/membranes are used, cation and anion membranes. Only positively charged ions (cations) pass through a cation membrane and respectively only negatively charged anion pass through an anion membrane. The cells of electrodialysis equipment are formed of successively arranged ion-exchange membranes (cation- anion-membranes).

The new method according to the invention provides following advantages: - sodium sulfate -containing ash does not need to be led into the effluent streams of the mill anymore, but it can be used for producing chemicals to be used at the mill or outside the mill, or sodium sulfate can be used as such as process chemical typically outside the mill;

- waste acid of the chloride dioxide plant can be utilized in a novel way, and thus further decrease the effluent stream of the mill.