SE537954C2

SE537954C2 - Process for production of a fertilizer comprising potassium sulfate

Info

Publication number: SE537954C2
Application number: SE1451569A
Authority: SE
Inventors: Jakob Liedberg
Original assignee: Aprotech Engineering Ab
Priority date: 2014-12-17
Filing date: 2014-12-17
Publication date: 2015-12-01
Also published as: CA3004882C; WO2016099392A1; SE1451569A1; FI20175682A; FI128696B; CA3004882A1

Abstract

Abstract The present invention relates to a process for producing a fertilizer comprising potassium sulfate, K2SO4, from precipitator ash from a pulp mill, wherein precipitator ash from a pulp mill is provided; water is provided; potassium chloride is provided; and a reaction mixture is provided comprising said water, potassium chloride and precipitator ash, and is allowed to react, wherein potassium sulfate is obtained.

Description

The present invention relates to a process for producing a fertilizer comprising potassium sulfate, K2SO4, from precipitator ash from a pulp mill, wherein precipitator ash from a pulp mill is provided; water is provided; potassium chloride is provided; and a reaction mixture is provided comprising said water, potassium chloride and precipitator ash, and is allowed to react, wherein potassium sulfate is obtained.

Ka Fri Water va ur :)K2SO4 NaCH/KOH> Ash vv.ter KCI ' —Er —0- 0— 0— NCI 537 954 Abstract The present invention relates to a process for producing a fertilizer comprising potassium sulfate, K2SO4, from precipitator ash from a pulp mill, wherein precipitator ash from a pulp mill is provided; water is provided; potassium chloride is provided; and a reaction mixture is provided comprising said water, potassium chloride and precipitator ash, and is allowed to react, wherein potassium sulfate is obtained. 537 954 PROCESS FOR PRODUCTION OF A FERTILIZER COMPRISING POTASSIUM SULFATE Field of the invention The present invention relates to a process for providing value adding products from precipitator ash from a pulp mill.

Background Pulp mills are working continuously to minimize the emissions to air and water and at the same time they aim at recycling process essential chemicals like sodium sulfate which helps to reduce the pulp mills' operating costs. Therefore the pulp mills preferably recycle the precipitator ash containing sodium sulfate to the mill's "caustic cycle". However, as the ash contains also non-desirable elements like chlorides and potassium these are accumulated in the mill and at too high levels they negatively influence the process and therefore it is not possible to recycle all precipitator ash to the pulp mill. Pulp mills continuously monitor the sodium/sulfur balance. The sodium/sulfur balance and the levels of chloride and potassium in the mills are controlled by recycling or discharging precipitator ash, or forwarding the precipitator ash to an ash treatment process if present in the mill.

The balance of chemicals in the mill is important as too high levels of potassium and chloride results in process related problems such as corrosion due to chlorides and fouling in the boiler due to potassium.

The precipitator ash often removed from the pulp mills and is deposited to landfills or dissolved in water and forwarded to drains. Since the precipitator ash contains high levels of sodium sulfate and sodium carbonate which are chemicals used in the pulp mill discharging too much ash will directly affect the pulp mill's operating cost since the loss of these chemicals has to be replaced with new chemicals.

Today there are processes that treat the precipitator ash and remove non-desirable process elements such as chloride and potassium in order to be able to recycle the sodium sulfate content of the ash back to the process. However, such solutions are not able to recycle all sodium sulfate as there always is a need to discharge a certain amount of the sodium sulfate containing reject e.g. to the drain and therefore not all the process valuable 1 537 954 chemicals in the ash can be recycled and large amounts are e.g. forwarded to the drain through a bleed stream or reject. For pulp mills that do not have an ash treatment system the levels of non-desirable process elements are controlled by means of forwarding the precipitator ash to e.g. the drain.

Below are different known processes for precipitator ash handling or treatment disclosed.

The precipitator ash may be recycled to the pulp mill directly. The amount of ash that is possible to recycle is limited by the amount of non-desirable process elements, such as potassium and chlorides, and the mill's sodium/sulfur balance. The pulp mills try to recycle as much precipitator ash as possible but due to the non-desirable process elements in the ash, recycling all the ash will accumulate these undesirable elements. Too high levels of chlorides will result in corrosion of the equipment and too high levels of potassium results in undesirable fouling/scaling in the black liquor boiler.

If the pulp mill is not able to recycle the precipitator ash, it may be forwarded to landfill. This is of cause an additional cost in addition to the lost of valuable process chemicals.

If the pulp mill is not able to recycle the precipitator ash, it may be dissolved in water and forwarded to the wastewater system.

One alternative is also to partly dissolve the precipitator ash in water, possibly including a pH adjustment using sulfuric acid, and separate the non-dissolved ash and the liquid. The non dissolved ash free from chlorides and potassium is then returned to the pulp mill and the liquid rich in chlorides and potassium, i.e. reject, is sent to the wastewater system.

Another alternative is to dissolve the precipitator ash and then cool it with ice and where after separation of sodium salts together with crystalline water may be made. This process primarily removes chlorides from the ash. Since the solubility of sodium sulfate decrease with a lower temperature a solid phase consisting of sodium sulfate and crystalline water is formed. The solid phase is removed and recycled back to the pulp mill and the liquid phase containing most of the chlorides is sent to the wastewater system.

Another alternative is to dissolve the precipitator ash in water and then evaporating/crystallizing the dissolved sodium salts and separation of the salts. With this process chlorides and potassium are removed from the precipitator ash by first dissolving the ash in water and then forward the solution to an evaporator/crystallizer in order to remove water and increase 2 537 954 the concentration to precipitate sodium salts which forms a solid phase which are removed and returned to the pulp mill. The liquid phase containing most of the chlorides and potassium together with dissolved sodium salts is continuously discharged in order to keep the levels of chlorides and potassium at a level inside the crystallizer which is economically viable and not resulting in corrosion.

A problem with the presently known technologies is that none of the techniques utilize all the ash. In reality a large amount of chemicals is always discharged to landfill or wastewater system even for solutions where the precipitator ash is treated and cleaned from non-desirable process elements and is recycled. At optimum conditions and in theory it is possible to recover about 90% of the precipitator ash by means of such an ash treatment system but in reality the values are much lower, close to about 50% is mentioned in literature and expressed by users.

Today also much focus is put on obtaining environmentally sustainable processes and obtaining as much value adding products or recyclable products out of a process as possible, in order to avoid as much waste and losses as possible.

Thus, there is a need to obtain more efficient processes. There is a demand for processes which reduces the need for putting material on landfills and discharging valuable chemicals to wastewater system. There is also a need for providing additional value adding products from waste material from pulp mills which improves the economy of the total pulp mill.

Summary With the present process high value products are obtainable and at the same time as much chemicals as possible form the process may be recycled back into the pulp mill process to provide an environmentally more sustainable solution to waste handling. By reusing chemicals and also providing an added value product that may be sold the total economy of a pulp mill is improved and the recourses of Mother Nature are used with case.

Precipitator ash which today is forwarded to landfill or the wastewater system may be treated with potassium chloride in order to create a high value fertilizer, K2SO4, and a byproduct, NaCI, which may be used in different applications e.g. road salt. 3 537 954 With the invention almost all chemicals present in the precipitator ash can be used and the negative environmental impact from the pulp mill precipitator ash can be eliminated. Since a high grade fertilizer is obtained by the present invention is it also possible to forward the nutrient chemicals back to the plants where they are needed instead of letting them out to a drain. This way, nutrients which are removed from the land, e.g. in view of the logging, may be reintroduced with the obtained fertilizer.

The invention may be applied and implemented to any pulp mill that bleed out ash or treat ash in an ash treatment system and the present invention does not negatively affect the pulp mill in any way since it is an "end of pipe solution".

The present invention relates to a process for producing a fertilizer comprising potassium sulfate, K2SO4, from precipitator ash from a pulp mill, wherein precipitator ash from a pulp mill is provided; water is provided; potassium chloride is provided; and a mixture is provided comprising said water, potassium chloride and precipitator ash, and is allowed to react, wherein potassium sulfate is obtained.

According to one embodiment the potassium chloride and the precipitator ash of the mixture are in any order or simultaneously. Preferably the water and precipitator ash is added before the potassium chloride.

According to one embodiment acid is admixed to the mixture. Preferably sulfuric acid and/or hydrochloric acid is used, more preferably sulfuric acid. Preferably the acid is added before the addition of the potassium chloride.

According to one embodiment the precipitator ash has been pretreated with the water in an ash treatment process and the reject of the ash treatment process is contacted with the potassium chloride.

According to one embodiment sodium hydroxide and/or potassium hydroxide is added to the water, potassium chloride, and precipitator ash mixture. This is done to adjust the pH, e.g. if acid has been added.

According to one embodiment glaserite is obtained by the reaction of the water, the potassium chloride and the precipitator ash, said glaserite is removed and admixed with additional potassium chloride and/or is leached with water to provide potassium sulfate. The potassium sulfate may then be removed for further use or sold. It is to be noted that the admixing of potassium chloride and leaching with water may be done in any order. 4 537 954 However, in a preferred embodiment the reaction with potassium chloride is performed first followed by leaching with water.

According to one embodiment the remaining mixture after removal of potassium sulfate is concentrated, where after any sodium chloride present is removed for further use.

According to one embodiment the removed sodium chloride is forwarded to a cell membrane process converting it to sodium hydroxide, hydrogen and chlorine.

The present invention also relates to use of the present process for the production of a fertilizer comprising potassium sulfate.

Short description of the drawings Figure 1 discloses a schematic embodiment of the present process.

Detailed description Precipitator ash is mixed with and at least partially dissolved in water. Preferably all precipitator ash is dissolved. The aqueous mixture of the precipitator ash may optionally be treated with an acid, preferably sulfuric acid. The optional use of acid may depend on the composition of the precipitator ash. The acid may be used to decrease the amount of carbonates present in the end product. It is to be noted that it is not necessary to remove all carbonates present. If some carbonates are removed using acid the final end product has a higher degree of potassium sulfate, i.e. more pure product. The amount of sulfuric acid which may be added to the mixture in order to convert carbonates of the precipitator ash into carbon dioxide (CO2), sulfates and water depends on the amount of carbonates in the ash and the requested quality of the end product, potassium sulfate (K2SO4). Acid may be used in an equivalent molar ratio with regards to the carbonates present. As an example a molar ratio of 1:1 of sulfuric acid to carbonate may be used. It is to be understood that sulfuric acid is not used when not being needed.

As alternative, the ash may have been pretreated in an ash treatment process step and the reject there from is used. In such an ash treatment process the ash is mixed with and at least partially dissolved in water. Non-desirable process elements for the pulp mill may then be removed as a reject (a bleed stream) and this reject contains dissolved ash in water. This solution may contain the following ions: sodium, sulfates, carbonates, potassium and 537 954 chlorides. In the pretreatment step the desirable process elements, such as NaSO4 and NaOH, may be recycled to the pulp mill process. If needed this pretreated ash mixture reject may be further treated with sulfuric acid in order to decrease and minimize the amount of carbonates in the solution. Sulfuric acid may be added to the pretreated ash mixture reject to react with carbonates present and form sulfate ions, water and carbon dioxide.

Precipitator ash is obtained in the pulp mill process from the black liquor boiler. Precipitator ash from the pulp mill contains mostly sodium, sulfate, and carbonate in various amounts but also small amounts of potassium and chlorides, as well as trace amounts of several metals and elements.

The precipitator ash may vary in chemical content and can roughly be divided into so called low and high carbonate ash depending on the composition.

Below are examples of different precipitator ash contents.

Table 1. Preci itator ash content General range for precipitator ash Low carbonate precipitator ash (general range) Low carbonate precipitator ash (a specific embodiment) High carbonate precipitator ash (general range) High carbonate precipitator ash (a specific embodiment) Na 25-35% 25-35% 30% 25-35% 28% SO4 30-70% 40-70% 60% 30-50% 40% HCO3 0-40% 0-15% 2% 20-40% 29% K 0-10% 0-10% 6% 0-10% 2% Cl 0-10% 0-10% 2% 0-10% 1% The carbonate content of the ash mixture is adjusted or eliminated by means of adding sulfuric acid.

Optionally a subsequent step of pH modification using an alkaline compound may be used, e.g. if the above mentioned acid has been added in the process. Preferably KOH and/or NaOH are used as alkaline compounds. The addition of alkaline compound may be used to increase the pH and achieve a correct stoichiometric relation with regards to K2SO4 and NaCl. 537 954 Potassium chloride, KCI, is added to the aqueous mixture comprising the precipitator ash in order to obtain potassium sulfate. The solid phase obtained in the process may comprise a salt called glaserite composed of potassium and sodium sulfate (K3Na(SO4)2). In one embodiment the intermediate product obtained in the present process after the first addition of the potassium chloride is glaserite.

The obtained glaserite salt is removed from the mother liquor, the liquid remaining part of the mixture, and may be further treated with KCI in order to produce K2SO4. The obtained K2SO4 may thereafter be removed.

The reactions are for the production of the intermediate glaserite and the K2SO4 are disclosed below.

Glaserite: 6 KCI + 4 Na2SO4 4 2 K3Na(SO4)2 + 6 NaCI K2SO4: 2 KCI + 2 K3Na(SO4)2 4 4 K2SO4 + 2 NaCI As an alternative processing, the obtained glaserite salt may after removal from the mother liquor be leached in water in order to provide K2SO4.

However, in a further embodiment, the present process may include a combination of both mentioned treatment steps for the glaserite, in any order. Then the obtained glaserite salt may first be treated with KCI and thereafter leached in water in order to produce K2SO4, or the other way around.

The mother liquor remaining after the separation of K2SO4 may be further processed, e.g. via a cooling step in order to precipitate sodium sulfate and improve the yield of sulfates by returning said sulfates to the pulp mill process.

The mother liquor remaining after the separation of K2SO4 may be further processed, e.g. via evaporation in order to precipitate sodium chloride (NaCI) which may be removed as a solid phase. The liquor remaining after such NaCI separation may then be recycled back to the pulp mill process to maximize the yield of potassium sulfate in said process.

The present invention can further be complemented by the use of a membrane cell process which may convert the obtained NaCI into NaOH, H2 and C12. NaOH is a valuable chemical and used by the pulp mill in the cooking of wood chips and can therefore be fed directly back to the mill. The two other 7 537 954 products H2 and Cl2 may be collected and either used by the pulp mill, e.g. the chlorine bleach process, or sold to third party to improve the economy and profitability of the mill.

In this manner more value adding products than the fertilizer produced may be obtained and reused in the pulp mill or other processes or sold.

With reference to Figure 1 it is shown that in step 1 precipitator ash and water are admixed. In one embodiment the precipitator ash and water may be replaced by or combined with the reject from a pretreatment ash processing system. Optionally acid may be added also in step 1, e.g. sulfuric acid.

The ash comprising mixture may optionally be mixed with KOH and/or NaOH in step 2, where the pH of the mixture is raised and the solution may reach the correct stoichiometric relation with regards to K2SO4 and NaCl to be obtained. The alkaline compounds may not be needed, e.g. if no acid has been added in step 1.

Thereafter in step 3, the ash mixture is mixed with KCI in order to obtain K2SO4. The process may create a mixed salt of potassium and sodium sulfate, which is called glaserite. This glaserite salt may then be removed and forwarded to the next step 4 where it is allowed to react in a water solution with additional KCI and may then be further leached in water in step 5 in order to create the end product K2SO4. It is to be noted that any one of steps 4 and 5 may be used alone, or in combination. The K2SO4 in solid phase is separated from the mother liquor which is recycled. Since ash could also contain carbonates in various amounts depending on its origin and added sulfuric acid, carbonates could also be present in the end-product to some extent.

The remaining liquid of the steps 3, 4 and 5 may be recycled to the previous step of the process in counter current flow with the precipitated salts. In step 3 where glaserite may be formed, the mother liquor from the step is forwarded to a cooling step 6 in order to precipitate more sulfate salts which are separated and recirculated back to step 3.

The remaining solution after the cooling step 6, which has a low amount of sodium sulfate and potassium but also comprises sodium and chlorides, is sent to an evaporation step 7 where water is removed in order to increase the salt concentration and to precipitate NaCI as a solid phase and separate salt from the solution. The water driven off in the evaporation step 8 537 954 where NaCI is precipitated and removed from the solution, can be recycled to the process to close the system and be used to dilute reject or dissolve new ash. Therefore the process decreases the requirements for additional water.

Remaining solution after the separation of NaCI is recirculated back to the process in step 2 in order to optimize the use of the chemicals and the yield of potassium sulfate.

Almost all reactions occur at room temperature or slightly above and therefore the process according to the present invention is not very energy demanding, except for the evaporation of water in the NaCI precipitation step 7.

A membrane cell process can additionally be added to the present process in order to provide NaOH for the pulp mill from the generated byproduct, NaCI. 9

Claims

537 954 CLAIMS

1. A process for producing a fertilizer comprising potassium sulfate, K2SO4, from precipitator ash from a pulp mill, wherein precipitator ash from a pulp mill is provided; water is provided; potassium chloride is provided; and a mixture is provided comprising said water, potassium chloride and precipitator ash, and is allowed to react, wherein potassium sulfate is obtained.

2. The process according to claim 1, wherein the water, the potassium chloride and the precipitator ash of the mixture are in any order or simultaneously, preferably the water and precipitator ash is added before the potassium chloride.

3. The process according to claim 1 or 2, wherein acid is admixed to the mixture, preferably before the addition of the potassium chloride.

4. The process according to any one of claims 1-3, wherein the precipitator ash has been pretreated with the water in an ash treatment process and the reject of the ash treatment process is contacted with the potassium chloride.

5. The process according to any one of claims 1-4, wherein sodium hydroxide and/or potassium hydroxide is added to the water, potassium chloride, and precipitator ash mixture.

6. The process according to any one of claims 1-5, wherein glaserite is obtained by the reaction of the water, the potassium chloride and the precipitator ash, said glaserite is removed and admixed with additional potassium chloride and/or is leached with water to provide potassium sulfate.

7. The process according to claim 6, wherein the remaining mixture after removal of potassium sulfate is concentrated, whereafter any sodium chloride present is removed. 537 954

8. The process according to claim 7, wherein the removed sodium chloride is forwarded to a cell membrane process converting it to sodium hydroxide, hydrogen and chlorine.

9. Use of a process according to any one of claims 1-8 for the production of a fertilizer comprising potassium sulfate. 11 537 954 I foljande bilaga finns en oversattning av patentkraven till svenska. Observera att det r patentkravens lydelse pa engelska som gaiter. A Swedish translation of the patent claims is enclosed. Please note that only the English claims have legal effect.