FI116074B3 - Sulfur recycling in the sulphate pulp process - Google Patents

Description

Sulfur recycling in the sulphate pulp process

The invention relates to a process for recycling sulfur in the sulphate pulp process and further relates to a process for the production of tall oil and magnesium sulphate utilizing sulfur in the sulphate pulping process.

Traditional pulp production methods for raw wood still in use in the 21st century include mechanical pulping processes and chemical pulping processes. Chemical defibration methods include: the soda process, the sulfate process, and the sulfate process. These methods have been constantly refined and further developed. In recent years, particular attention has been paid to the energy efficiency and environmental friendliness of processes. For example, the use of additives has been increased in the production of chemical pulp and efforts have been made to minimize emissions of both malodorous sulfur compounds and sulfur dioxide. Emission limit values set by air protection are constantly tightening, and today efforts are being made to recover sulfur dioxide from the combustion of odorous sulfur compounds and other process steps. Sulfur dioxide can be removed from the flue gases, for example in gas scrubbers where it is known to convert sulfur dioxide into sodium bisulfite with sodium hydroxide.

The sulphate process is currently the most widely used chemical pulping process. In it, the alkaline cooking liquor or white liquor contains approximately three parts sodium hydroxide, NaOH, and one part sodium sulfide, Na 2 S. An essential part of the process is the recovery of cooking chemicals and certain by-products. After cooking, the pulp is washed and the fatty acids and resin acids from the wood extractives are removed from the wash liquor, the black liquor. In addition, recovery of lighter organic components such as turpentine is part of the recovery. In cooking, most of the wood's lignin is soluble in the cooking liquor, which is burned in a recovery boiler, where the sulfur and sodium in the cooking liquor are also recovered. A large portion of the black liquor is evaporated from the water and then the residue is burned in a recovery boiler. In the recovery boiler, the organic matter is removed and the inorganic portion of the black liquor which at this stage consists mainly of sodium carbonate, Na 2 CO 3, and sodium sulfide, Na 2 S, remains. From the boiler, this salt melt is dissolved in water to give a so-called. green liquor. The green liquor is led to causticization, in which sodium carbonate reacts with calcium hydroxide in aqueous solution:

(1)

The resulting calcium carbonate is further regenerated in the lime kiln by burning it into calcium oxide:

(2)

The white liquor obtained from the causticisation contains mainly sodium hydroxide and sodium sulphide. The composition of this white liquor is of considerable importance for the properties of the pulp to be cooked. As the process is also essentially closed in this respect, i.e. the cooking chemicals are not consumed in the process, it is essential that the sulfur-sodium ratio in the chemical cycle is kept in equilibrium. At present, the main problem in managing the equilibrium is, for example, the excess sulfur entering the process. Pulp mills have a number of different sulfur-producing sub-processes, the most important of which is the tall oil production process. In addition, sulfur comes from the wood itself, water, lime kiln fuel oil and magnesium sulphate used in oxygen bleaching.

In sulphate pulp cooking, from wood additives, pitch fatty acids and resin acids are saponified to sodium salts, or soaps, and also unsaponifiable organic components remain, depending on the quality of the raw wood used. These soaps are still micellarized and some unsaponifiable matter is dissolved in the micelles. The pulp soup is usually followed by a washing step, in which the soaking micelles in the leaching solution, the black liquor, rise to the surface of the brine solution. The soap formed is decanted from the black liquor, after which it can be broken down into tall oil with sulfuric acid and heat. Thus, in the plating, the soup separated from the surface of the black liquor is boiled with sulfuric acid, whereupon the water-soluble sodium salts of fatty acids and resin acids produce the corresponding acids, which are referred to as crude tall oil. The reaction in plotting is of the form:

(3)

The fatty acids, in their acid form, are insoluble in water and are thus separated from the leachate. Sulfuric acid is used in the plating mainly to convert the juice into tall oil, but also to neutralize the black liquor present in the juice and to adjust the pH to a low enough level to convert the fatty acids to the acid form. In practice, sulfuric acid should be added in such a way that the pH drops below 4. In plating, tall oil is separated from the mother liquor phase, which is returned to the mill's chemical cycle.

For example, the waste acid from a chlorine dioxide reactor can be used and often used as a sequestering acid. Because the aqueous phase produced during the plating is recycled, the sulfur in its dissolved form also goes into the chemical cycle. However, the waste acid entails a considerable amount of sulfur in the process because the waste acid contains, in addition to sulfuric acid, a considerable amount of sodium sulphate or sodium sesquisulphate. Depending on the wood species and the site where it is grown, tall oil extraction can bring sulfur into the process at 2-4 kg / tonne pulp. The sulfur losses of a modern pulp mill, that is sulfur that does not go into recycling, but is removed from the chemical cycle, can be replaced by sulfur-containing chemicals from bleaching chemicals that even exceed the sulfur requirements of the pulp mill. In particular, the tall oil plantation adds significantly more sulfur to the process. Thus, excess sulfur must be removed from the process.

If excess sulfur is removed from the process as sodium sulfate, fresh sodium hydroxide has to be replaced, which increases the cost of desulfurization. When controlling the chemical balance of a sulphate pulp mill, it is important to ensure that the sulfur to sodium ratio or sulfide is controlled. One of the major causes of sulphide imbalance in the sulphate pulp process is the tall oil column.

If excess sulfur is removed, for example in the form of fly ash, it will also remove sodium, which must be replaced by fresh liquor introduced into the process.

One solution to this problem would be to use sulfur free acid. Hydrochloric acid is not suitable because of the problems caused by chloride. Organic acids such as formic acid could at least partially replace sulfuric acid. However, they are probably too weak acids and also relatively expensive. Nitric acid is also unsuitable for the process as it can produce nitrogen oxides when burned in a recovery boiler.

For the recycling and reuse of sulfur in the production of tall oil, WO 9411571 discloses acidification of a salt at a high overdose relative to the amount of sodium bisulfite solution. The acidification may be carried out in one or, preferably, several stages. The bisulfite solution used pre-acidifies the salt at elevated temperature 90-150 ° C and at atmospheric pressure or mild overpressure. However, the soap must be post-acidified with sulfuric acid to complete the cooking. The bisulfite solution as such is not very acidic, with a pH slightly below 5. The pH of the paraffinic acid should be less than 4.

Another step in introducing additional sulfur into the process is the soluble magnesium compound used in oxygen bleaching. Typically, magnesium sulfate is introduced externally into the sulfate pulp process. Instead of magnesium sulphate, for example, acetates or formates could be used to reduce the sulfur load through the choice of chemicals, but in practice they are too expensive. Magnesium hydroxide and magnesium carbonate would be good alternatives in composition but have the problem of insufficient solubility.

It is an object of the present invention to simply and efficiently reuse recycled sulfur from the pulp mill's own chemical cycle without significantly adding additional sulfur to the process, whereby the sulphide content of the process does not become a problem.

Now, we have surprisingly discovered that the cooking of crude tall oil can be accomplished using recycled sulfur from the pulp mill's own chemical cycle. The process of the invention is based on oxidizing the recycled sulfur to a bisulfate solution by oxidation. The resulting bisulphate solution is significantly more acidic than, for example, a bisulphite solution, whereby its acidity is, for example, sufficient to produce a desalting crude tall oil in one step, similar to using fresh sulfuric acid externally introduced into the process balance. In addition, the bisulfate solution can be used to prepare sulfate compounds needed in the process, such as magnesium sulfate for bleaching.

In the process of the invention, the bisulfite obtained from the process cycle can be oxidized to the bisulfate, for example, with hydrogen peroxide acting as an oxidant. In this case, only water remains as an additional reaction product. At the same time, the pH of the solution drops substantially. The acid constants of sulfuric acid (H2SO3) are pKal = 1.8 and pKa2 = 6.8, while sulfuric acid has pKal = -3 and pKa2 = 1.8, respectively. Sodium bisulfite can be oxidized to sodium bisulfate, for example, by hydrogen peroxide according to equation (4):

(4)

Thus, the process according to the invention can convert the process sulfur sulfur from the sodium bisulfite form to a sufficiently acidic bisulfate form to be well suited, for example, to tall oil milling. When bisulfite is used as such, tall oil can only be partially columned and an external acid, such as sulfuric acid, is needed for the final column. The sulfuric acid is not necessarily required in the process of the present invention.

Hydrogen peroxide, ozone or, for example, organic peracids such as peracetic acid or permuric acid may be used as the oxidizing agent in the process of the invention. Sulfite can also be oxidized by oxygen, but it usually cannot be done directly without the use of a catalyst, which complicates the required process equipment. The most preferred oxidant of the invention is hydrogen peroxide, which is also the easiest to use. Alternatively, a peroxide solution from another process step, such as a residual peroxide filtrate from pulp bleaching, may also be used for oxidation.

It is known that tall oil can be paraffined with an imported acid such as sulfuric acid or waste acid from the chlorine dioxide process to provide a sufficiently low pH. The pH of bisulfite obtained directly from the process is not sufficient as such. By oxidizing the bisulfite solution according to the invention, it is possible to reach this pH range. It is important to lower the pH sufficiently so that fatty acids that are weak acids can be acidified and separated. A bisulfate solution having a pH below 2 would provide most of the fatty acid to the acid form. When the pH is about 2 units lower than the pKa of the fatty acid, most of the fatty acid is in the acid form and not in the form of a salt.

In the process according to the invention, hydrogen peroxide is consumed for the stoichiometric oxidation of bisulfite in a weight ratio of 1: 3 H 2 O 2: NaHSO 3, i.e. peroxide consumption is in the order of 0.33 kg H 2 O 2 / kg NaHSO 3. Preferably, however, the bisulfite need not be completely oxidized to achieve a sufficiently low pH, but partial oxidation is sufficient.

The drop in pH resulting from the oxidation reaction of the invention is shown in Figure 1. As shown in the figure, the pH of a 10 g / l bisulfite solution rapidly drops from its initial value to just over four when hydrogen peroxide is added. The pH of the bisulfite solution decreases significantly even with a small amount of peroxide. The pH of the resulting bisulfate solution can be adjusted to a suitable level for cooking tall oil, in the range of pH <2, preferably pH <1.5, wherein the amount of hydrogen peroxide is greater than 0.1, preferably greater than 0.2 times the stoichiometric amount required for bisulfite oxidation. Thus, the bisulfate solution is more acidic than the bisulfite solution, so it is much better suited for use in, for example, tall oil extraction. In this case, the amount of sulfuric acid externally introduced into the process can be substantially reduced, for example, for this purpose. Salting the filter entirely with sulfuric acid introduced into the process would produce about 2-4 kg / Adt of sulfur into the process, so that at least in this respect, the amount of sulfur would be significantly reduced using the process of the invention.

When operating according to the invention, the equipment required for the oxidation reaction is simple, as shown, for example, in Figure 2. Essentially, the apparatus comprises a mixing tank 1 in which the reaction between the sulfur-containing bisulfite entering the sulfate process, S, and the oxidized reaction takes place. In addition, the apparatus may include, for example, a dosing device 2 for the oxidant, and an intermediate storage tank 3 for the reaction product bisulfate from which the bisulfate solution may be recycled to a desired process point such as tall oil milling or magnesium sulfate.

Alternatively, the bisulfate solution provided in accordance with the invention may be prepared already in the bisulfite scrubber itself, whereby sulfuric acid and bisulfate are also formed in the bisulfite solution. However, materials resistant to oxidation-induced corrosion and oxidation reaction SO2 -> SO3 should be used in the process equipment. However, this embodiment becomes more expensive than the above. The resulting bisulfate solution also contains some sulfuric acid.

The sulfur circulation, recovery and reuse of the sulfur pulp process according to the invention is exemplified in Figure 3.

The bisulfate solution is prepared according to the invention by oxidizing in the mixing tank 1 the sodium bisulfite from bisulfite scrubber 4 to the mixing tank 1. The sodium bisulfate prepared in the mixing tank is introduced into the from sodium bisulfate. The magnesium sulfate thus produced is used in oxygen bleaching 7. The oxygen bleaching filtrate is further fed to evaporation plant 8 and therewith to a recovery boiler 9. In the production of crude tall oil, pulp from the pulp 10, washing plant I1 and evaporator 8 is fed to the digestion reactor 5. The tall oil boiling reactor 5 discharges crude tall oil 12 from the process and the residual sulfur-containing mother water is fed through the evaporator 8 to the recovery boiler 9 for burning organic compounds and sulfur. In the boiler 9, the sulfur compounds are burned to sodium sulfide. An aqueous solution of sodium carbonate and sodium sulfide, green liquor, is passed from the soda boiler 9 to the causticization 13, whereby the sodium carbonate is converted into calcium carbonate and the resulting sodium hydroxide-sodium sulfide solution, white liquor, is recycled to the pulverized calcium carbonate 10. The main source of sulfur dioxide is the combustion of odorous gases 15, whereby the reduced sulfur compounds, mainly H2S, methylmercaptan, dimethylmercaptan and dimethyldimethylcaptan, are incinerated as sulfur dioxide and further recycled to the bisulphite scrubber 4. In the bisulfite scrubber 4, the sulfur dioxide is absorbed into the NaOH solution to form sodium bisulfite. The sodium bisulfite from the bisulfite scrubber is further introduced into the mixing tank 1 for oxidation according to the invention.

The process according to the invention avoids, for example, the use of waste acid from the manufacture of chlorine dioxide 16 either partially or completely and thus reduces the external sulfur entering the process.

In one embodiment of the invention, the plating is carried out in one step using a stoichiometric amount of the sodium bisulfate solution prepared according to the invention as the plating acid. In some cases, it may be advantageous to use excess oxidant. Alternatively, the bisulfate may be only partially oxidized. It is also possible to use a mixture in which the recycled bisulphate according to the invention is in one part, for example a mixture of sulfuric acid and bisulphate. Also, this sulfuric acid can be made from recycled sulfur dioxide by hydrogen peroxide by first oxidizing sulfur dioxide to sulfur trioxide and then to sulfuric acid. It is known to produce sulfuric acid by direct oxidation of sulfur dioxide to sulfur trioxide. However, the procedure of the invention avoids investing in a separate sulfuric acid plant.

In one embodiment of the invention, the columning of tall oil is carried out in two or more stages. In the two-step process, a bisulfate solution prepared according to the invention can be used in the first step and another acid such as fresh sulfuric acid or a waste acid or a mixture of acids in the chlorine dioxide reactor in the latter. Alternatively, in the first step, the sodium bisulfite solution or carbon dioxide directly obtained from the process may be used, and only in the latter step may the sodium bisulfate solution according to the invention be used. In this case less bisulfate and thus peroxide is needed than the one-step process. Alternatively, the bisulphate solution characteristic of the invention is used in both stages of tall oil cooking.

If the amount of sulfur entering the plating is 2-4 kg S / Adt, the amount of stoichiometric peroxide needed would be approximately the same, i.e. 2-4 kg. However, the amount need not be quite stoichiometric, as shown in Figure 1. Preferably already about half of it, more than 0.15 g of H 2 O 2 / g of NaHSO 3 is sufficient. Most preferably, the amount of hydrogen peroxide to be added is greater than 0.20 g

The magnesium compound used in oxygen bleaching can be prepared by utilizing sulfur recovered in the sulfate process process cycle, eliminating the need to use exogenous sulfur loading magnesium sulfate. Recycled sulfur can be utilized as sulfur dioxide, bisulfite or sulfite which is reacted by oxidation to magnesium sulfate.

As the sulfur-free raw materials for the magnesium compound, for example, magnesium hydroxide, magnesium oxide or magnesium carbonate may be used. Preferably, the magnesium source is technical MgO.

According to one embodiment of the invention, the magnesium compound is dissolved in the bisulfate solution obtained from the sulfate process according to reaction equation (5) to give magnesium sulfate.

(5)

One alternative way of making magnesium sulfate is to carry out the reaction already in a scrubber, where the SO2-containing gas is washed directly with water containing an oxidant, for example hydrogen peroxide, and Mg (OH) 2. The peroxide consumption is then between 0.28 and 0.33 kg / kg MgSO 4.

A further alternative way of preparing magnesium sulfate is to first dissolve the magnesium compound in the bisulfite solution and oxidize the resulting magnesium sulfite to magnesium sulfate, for example with hydrogen peroxide. Magnesium sulfite as such is a reducing compound which is not useful in the bleaching process because it unnecessarily consumes other chemicals.

Since MgSO 4 contains 26.6% sulfur, the process according to the invention reduces the amount of sulfur by 0.26-0.8 kg / tonne if the amount of MgSO 4 to be bleached is 1-3 kg / tonne pulp.

The following illustrates, by way of example, certain features of the invention, without, however, limiting the invention thereto:

Example 1.

Consider the sulphate pulp process from which excess sulfur is removed in the form of fly ash. In this case, the sodium leaving the sulfur must be replaced by fresh liquor introduced into the process. The broth requires 2.5 kg NaOH / kg sulfur to be removed. If the amount of sulfur to be removed is 4 kg / tonne mass, the corresponding need for NaOH is 10 kg / tonne mass.

This is compared to the sulfate pulp process of the invention. If the excess sulfur is reduced by recycling the sulfur contained in the bisulfite through a tall oil column, a little more than 4 kg of hydrogen peroxide is needed to oxidize that amount of sulfur. At current chemical prices, the price of hydrogen peroxide will be € 2.1. If the sulfur is removed as fly ash, the cost of the required sodium is € 3.2, which means that the cost effect is 1.5 times higher than the process according to the invention.

Example 2.

The sodium bisulfite solution of the sulfate pulp process, NaHSO 3, at a concentration of 43 g / l and pH 4, was oxidized by adding 50 wt% hydrogen peroxide solution. Figure 4 shows the change in pH of the solution as a function of peroxide addition. The bisulfite solution supplemented with 11 g / l H 2 O easily dissolved in 12 g / l Mg (OH) 2, resulting in a solution containing 24.9 g / l MgSO 4. The solution also contained sodium sulfate.

In the non-oxidized bisulfite solution, magnesium hydroxide appeared to dissolve slightly slower. However, the oxidation of the magnesium bisulfite thus obtained by the addition of 11 g / l hydrogen peroxide gave the same result as the oxidation of the bisulfite before dissolution.

The prepared magnesium sulfate solutions work in oxygen bleaching and peroxide bleaching in exactly the same way as exogenous, pure magnesium sulfate.

No undesirable effects were observed with the included sodium sulfate.

Example 3.

Comparison of the sulfur-sodium balance of one sulfate cellulose process in two different cases, A and B. In the first case, A, the sulfur was not recycled, the entire required amount of sulfuric acid was introduced and sodium bisulfite was removed. In another case of the invention described above, B, the bisulfite was oxidized with hydrogen peroxide to the bisulfate and the required amount of exogenous sulfuric acid was used.

Figure 5 shows the two cases A and B and their sulfur-sodium balances at different stages. Step 1 describes the amount of sulfur present with sulfuric acid, Step 2 describes the required Na substitution (sodium hydroxide), Step 3 describes the sulfur in the process with oil, wood and MgSO 4, Step 4 illustrates e.g. removal of ash salt, fly ash, sulfur dioxide and soda ash, step 5 depicts washing losses and step 6 is other removal.

In case B, bisulfite was recovered using recycled white liquor, whereby no new sodium was introduced into the system with Na bisulfate. The sulfur that came with the bisulfate went into circulation normally, so it doesn't appear on the balance sheet.

The procedure of Case B resulted in the reduction of sulfuric acid entering the system by replacing 2kg sulfur, or 6.1kg sulfuric acid, with a bisulfate solution prepared by oxidation.

Figure 5 shows how the need for NaOH substitution will be significantly reduced. When hydrogen peroxide was used as the oxidant, the peroxide consumption depended on the degree of oxidation of the bisulfite. If 100% of the bisulfite is oxidized, then 2.1 kg of H 2 O 2 and 1.1 kg of H 2 O 2 respectively are required at an oxidation rate of 50%.

Example 4.

The sodium bisulfite solution of the sulfate pulp process, NaHSO3, at 250 g / L was taken up in 4 L and placed in an 8 L autoclave. The autoclave was equipped with a gas recirculating mixer using oxygen as the gas. The temperature of the autoclave was adjusted to 80 ° C and pure oxygen was introduced into it so that an overpressure of 6 bar was created in the autoclave. The solution was sampled at initial t = 0 and several other samples as the oxidation proceeded. Samples were measured for pH and analyzed for sulphite and sulphate ions. Figure 6 shows the evolution of pH over time as a function of oxidation, and Figure 7 shows the concentrations of sulfate and sulfite ions, respectively.

The pH of the bisulfite solution rapidly drops to below three during the initial oxidation and continues to decrease as the oxidation progresses. The increase in acidity is clearly due to the almost complete oxidation of bisulphite to bisulphate. The experiment shows that by using oxygen as an oxidant, it is also possible to reach a suitable pH range, which makes the recyclable solution suitable for reuse in tall oil oil plating.

Example 5.

A 300 ml sample was taken from a sodium bisulfite solution of sulfate pulp process, NaHSO 3, at a concentration of 44 g / l, which was placed in an open flask with a la-sintered bottom. Through a glass sinter, oxygen was introduced into the bisulfate solution at a flow rate of 1 1 / min 02. Rotation is illustrated in Figure 8 as the pH of the solution changes over time as the oxidation progresses.

The pH of the bisulfite solution decreases rapidly at the beginning of the oxygen supply and equates to slightly less than three.

Claims (19)

A process for recycling sulfur in a sulphate pulp process, characterized in that the sulfur recovered in the process cycle is converted, by means of an oxidant, into bisulphate, which is further used as recycling acid at various stages of the process. Process according to Claim 1, characterized in that the sulfur recovered in the process cycle is partially or completely converted into bisulfate. Process according to Claim 1, characterized in that hydrogen peroxide, ozone, organic peracids, oxygen, a peroxide-containing solution from another process step or mixtures thereof, preferably hydrogen peroxide, are used as the oxidizing agent. Process according to claim 3, characterized in that, when the oxidant used is hydrogen peroxide, the amount thereof is more than 0.1, preferably more than 0.2 times the stoichiometric amount required for the oxidation of bisulfite. Process according to Claim 1, characterized in that the sulfur recovered in the process cycle is converted into bisulfate by an oxidant after it has first been converted into the bisulfite form in a bisulfite scrubber. Process according to Claim 1, characterized in that the sulfur recovered in the process cycle is converted into bisulfate by means of an oxidizing agent in a bisulfite scrubber. Process according to one of Claims 1 to 6, characterized in that the process further comprises the step of columning the tall oil filter, wherein the bisulfate obtained according to one of Claims 1 to 6 is used at least in part as acid. Process according to claim 7, characterized in that the plating is carried out in one or more steps, at least one of which uses the bisulfate in question, either alone or in combination with other acids. Process according to Claim 7 or 8, characterized in that, in addition to the bisulfate according to Claim 1, another acid or carbon dioxide is used as the acid. Process according to Claim 7 or 8, characterized in that sulfuric acid or waste acid or bisulphite from the chlorine dioxide process is used as the acid. Process according to one of Claims 7 to 10, characterized in that the resulting bisulfate is prepared according to Claim 1 by using hydrogen peroxide, ozone, organic peracids, oxygen, a peroxide-containing solution from another process step or mixtures thereof, preferably hydrogen peroxide. Process according to claim 11, characterized in that the amount of hydrogen peroxide is greater than 0.15, preferably more than 0.20 g / g of bisulfite. Process according to one of Claims 1 to 6, characterized in that the process further comprises the step of preparing magnesium sulfate for bleaching sulphate pulp, wherein the sulphate raw material is a bisulfate obtained according to any one of Claims 1 to 6 which is reacted with an exogenous magnesium compound. Process according to claim 13, characterized in that the magnesium compound is a sulfur-free magnesium compound, preferably magnesium hydroxide, magnesium oxide or magnesium carbonate, most preferably technical magnesium oxide. Process according to Claim 13, characterized in that the resulting bisulfate is prepared according to Claim 1 by using hydrogen peroxide, ozone, organic peracids, oxygen, a peroxide solution or a mixture thereof, preferably hydrogen peroxide, from another process step. Process according to one of Claims 13 to 15, characterized in that the magnesium compound is dissolved in a bisulfate solution. The process according to claim 13, characterized in that the magnesium compound and the oxidizing agent are added to the bisulfate washer. Use of the bisulfate obtained according to any one of claims 1 to 6 as a scavenging acid in the process of plating the pine stove. Use of the bisulfate obtained according to any one of claims 1 to 6 as a sulfate feedstock for the production of magnesium sulfate for process bleaching of sulfate pulp. claim