SE515833C2 - Electrochemical oxidation of sulphide in green liquor - Google Patents

Abstract

The invention relates to a method of producing pulp comprising a step of forming green liquor containing alkali metal sulfide and alkali metal carbonate. The method further comprises a step of electrochemically treating the green liquor to oxidize at least part of the sulfide therein.

Description

The invention relates to a process for producing pulp, which comprises a step of forming green liquor containing alkali metal sulphide. and alkali metal carbonate. The further step of electrochemically treating the green liquor comprises oxidizing at least a portion of the sulfide therein, preferably by operating an electrochemical cell comprising at least two chambers having the green liquor as the anolyte. The electrochemical treatment further suitably involves the production of alkali metal hydroxide, preferably by operating an electrochemical cell which comprises at least two chambers with water or aqueous solution of alkali metal hydroxide as catholyte.

Green liquor is normally obtained in processes for alkaline boiling of cellulose pulp, for example sulphate pulp or kraft pulp.

The green liquor to be treated can be obtained by heat treatment of sewage effluent, normally in a recovery boiler, in which concentrated black liquor is incinerated in a reducing atmosphere.

However, the green liquor can also be obtained by heat treatment of bleaching effluent, which treatment may involve concentration and combustion, as described in the already mentioned WO 94/12720. The main constituents of green liquor are normally carbonate, sulphide and hydroxide of alkali metals, the concentrations of which may be from 0 to saturation. Green liquor obtained from sewage from boiling typically contains from about 0.2 to about 3 mol / liter, preferably from about 0.5 to about 1.5 mol / liter. 0 to about 2 mol / liter, preferably from 0 to about 1 nml / liter of alkali metals of alkali metal carbonate, sulfide, from 0 to about 2 mol / liter, preferably from 0 to about 1 mol / liter of alkali metal hydroxide, and from 0 to about 0.3 mol / liter of alkali metal chloride. Green liquor obtained from bleaching effluents generally has a corresponding composition, but the sulphide content is normally within the lower range of the potassium indicated range. The alkali metal is normally sodium, or a mixture thereof. In a typical system, from about 90 to about 97% of the alkali metal ions are sodium, while the residue is mainly potassium. However, there may also be systems, which are mainly based on potassium and which contain very small amounts of sodium.

In a preferred embodiment, the green liquor is treated in an electrochemical cell comprising separate anode and cathode true 3 chambers separated by a partially permeable barrier, preferably a cation-selective membrane. The preferred treatment involves the steps of introducing green liquor into the anode chamber, introducing water and preferably alkali metal hydroxide into the cathode chamber, electrochemically oxidizing sulfide into the anode chamber, and conducting alkali metal ions from the anode chamber to the cathode chamber. forming hydroxide ions in the cathode chamber, that the oxidation product consists essentially of polysulfides, the so ", sf, and sf, the anode potential The anode potential is preferably maintained, i.e. Sf", and so on. The exact limits for depend on the magnitude of the overvoltage. Normally, however, the anode potential is suitably kept at -0.6 V, the limit which allows the formation of polysulfide, to the level in a range from about which is the theoretical lower at which oxygen begins to be generated, normally at about + 0.6 V. the anode potential in a range from about -0.6 V to +0.5 V, + 0.4 V, preferably still below the potential for formation especially in a range from about -0.2 V to about By holding the anode potential. slightly higher, but oxygen, gives the oxidation of sulfide substantially sulfate and / or thiosulfate. By keeping the anode potential within the specified limits, it is also possible to substantially avoid the formation of chlorine.

The cathode potential is preferably kept such that, apart from that in -0.9 V to -1.2 V. The hydrogen gas formed can be used as an environmentally friendly from hydroxide ions, mainly. hydrogen is formed, the practice normally takes place in an area from about energy source or as raw material in other chemical processes.

In another mode of operation, the cathode potential is kept in a range from about +0.3 V to about -0.9 V, while at the same time oxygen-containing gas, eg air, is supplied to the cathode in the form of a gas diffusion electrode, which results in reduction of oxygen and formation of hydrogen peroxide and / or hydroxide ions. In the presence of oxygen, it is also possible to let the cell operate as a fuel cell, which results in the generation of electrical energy.

All values indicating electrode potentials, anode as well as cathode potentials, represent potentials measured against a reference electrode of Hg / HgO in 1 M NaOH at 25 ° C. The limits cannot be stated as exact values, since the result at a certain potential depends on the magnitude of the overvoltage in a particular case. each green liquor introduced into the anode chamber can have a degree of recirculation from 0 to almost 100%. The recycled green liquor, if present, may contain about 0.2 - 1.5 moles of sulphide / liter, 1.5 moles of sulphide / liter.

The conversion of 0.5 to 100. The solution introduced into the cathode chamber preferably consists of preferably about 1 to 5 sulphides, for example may be about mainly water and alkali metal hydroxide, especially sodium hydroxide, potassium hydroxide or mixtures thereof.

The concentration of alkali metal hydroxide is not critical, and may be, for example, about 0.1 - 15 mol / liter, preferably about 1 - 10 mol / liter. The upper limit of what is considered suitable is generally determined by the properties of the barrier separating the anode and cathode chambers, since too much leakage of hydroxide ions through the barrier reduces the current yield.

In order to make full use of the electrolysis cell, the process is preferably carried out at a current density exceeding about 0.5 kA / m 2, at excessive current densities the anode wears to an increasing extent, and in particular exceeds about 2 kA / m , increases. If polysulfides are the desired main product, it is also desirable to avoid the formation of sulfate and thiosulfate. Normally, a current density of not more than about 20 kA / m and is preferred. preferably does not exceed about 15 kA / mg. to be kept suitably in a range from about 60 ° C to the boiling point which is usually about 110 - 120 ° C. I The formation of by-products is also reduced by the anode chamber temperature sufficiently high, in practice the upper temperature limit depends on the material of the cell, especially when the barrier is made of a polymer-based membrane, for which reason the especially preferred temperature varies from about 80 ° C to about 100 ° C. For practical reasons, the temperature of the cathode chamber is preferably substantially the same as that of the anode chamber. It has also been found that the amount of by-products decreases if the anolyte flow is sufficiently high.

Preferably, the flow in the anode chamber is turbulent, and suitably the average linear velocity is higher than about 0.5 m / s. The catholyte flow is not critical and is in practice suitably determined by the magnitude of the lifting force of the generated gas. ... un- 10 15 20 25 30 35 515 833 - '5 Otherwise pumps can be used.

It has been found that the precipitation of sulfur on the anode can be avoided by selecting a suitable material. Without being bound by any particular theory, it is believed that the oxidation of sulfide involves an intermediate step in which atomic sulfur binds to the anode surface. If this bond is too strong, the sulfur does not react further, and a part of the sulfur remains on According to the invention, an anode which is made of an anode surface is advantageously used. and there forms a passivating surface layer. support material with high resistance to alkali, such as titanium, or carbon, nickel or nickel alloys, which support material is coated with zirconium, hafnium, niobium, and alloys thereof, on the surface with. one or more oxides of ruthenium, iridium, platinum and palladium. Electrodes which are made of a suitable material and which have a suitable surface coating are commercially available, such as the so-called DSA? Electrodes (dimensionally stable anode). It has been found that DSA9 electrodes designed for oxygen or chlorine gas generation are suitable for use in such as the electrodes marketed under the designations ON 120 and ON 101. has a large surface area and that the sulphide transport to the entire surface works well. The anode used is thus suitably a three-dimensional flow-through electrode, such as a three-dimensional mains electrode, wire balls. It is especially preferred to use a three-dimensional mains electrode, for example connected to each other by spot welding. layers of wire mats, particle beds or metal foam. son: consists of several layers of expanded metal, the cathode material is not critical, and you can use such common alkali-resistant materials as steel, stainless steel, nickel and ruthenium-coated nickel. The cathode may consist of a flat plate, one or more node layers, or a three-dimensional flow-through electrode corresponding to that used as the anode.

If oxygen-containing gas is to be blown into the cathode chamber, an oxygen-reducing cathode should be used, in which case a graphite felt electrode is suitable. Such electrodes are commercially available and are generally used, for example, in fuel cells. The oxygen reducing cathode may be coated with a catalyst, such as platinum, to increase the amount of hydroxide ions formed in the annulus relative to the amount of hydrogen peroxide. By oxygen reduction it is possible to prepare an alkaline hydrogen peroxide solution, which can be used as such for bleaching cellulose pulp. The presence of hydrogen peroxide in the cathode chamber also contributes to the product obtained being perfectly sulphide-free, since any sulphides that leak in from the anode chamber are immediately oxidized by the peroxide to sulphate.

Preferably a cell with two chambers with adjacent anode and cathode chambers is used, but cells with three or more chambers can also be used, in which case the green liquor can be introduced into the anode chamber as well as into one or more chambers located between the anode chamber and the cathode chamber. The barrier which separates the chambers of the cell and which normally exists between the anode chamber and the cathode chamber should release alkali metal cations from the anode chamber through the cathode chamber, but should preferably as far as possible prevent the passage of sulphides and polysulphides and preferably other anions. Hydroxide ions should preferably also be prevented by the barrier, although some may be allowed to pass. Preferably, a cation-selective membrane is used, which is permeable to alkali metal cations, but substantially impermeable to sulfides and polysulfides. If the cell has more than two chambers, different combinations of anion-selective and cation-selective membranes can be used to separate the different chambers of the cell. Furthermore, one or more porous diaphragms can be used as barriers, possibly in combination with one or more ion-selective membranes. Suitable membranes may, for example, be perfluorinated, sulfonated or Teflon-based polymers, or ceramic materials. Polystyrene-based membranes or diaphragms of polymers or ceramic materials can also be used. There are several commercially available membranes suitable for use, such as Nafionf. A variety of electrolytic cells can be arranged in both bipolar and monopolar ways.

The electrochemically treated green liquor can be transferred to a causticization step, in which slaked lime (calcium hydroxide) is added, whereby the carbonate is converted to hydroxide. If the green liquor contains polysulphides, these will be canon.- 10 l 25 20 25 30 35 515 ass in 7 boiling. If the green liquor is substantially free of sulphide and polysulphides, the liquor obtained during the causticization can be used in the bleaching plant. The electrochemically treated green liquor can also be further electrochemically treated to remove carbonate in the form of carbon dioxide and at the same time form alkali metal hydroxide.

In a preferred way of operating an alkaline pulp is treated, boiling is obtained, so that its content of polysulfides is increased before the cess green liquor, which from sewage from is added to the causticization process. The whole amount of green liquor can be treated at a low degree of conversion, for example varying from about 0.5 to about 1%, based on the sulphide present in the green liquor, or a partial flow can be treated at a higher degree of conversion, for example varying from about 10% to 100%, preferably from about 60 to about 95%, based on the sulfide present in the green liquor. The catholyte is conveniently recycled in a special loop, whereby a steady state is maintained by recovering a partial flow as a product which can be used, for example, in the boiling or bleaching process or completely removed from the pulp mill system.

In another way of working, a partial flow is treated by converting a large amount of the sulphides to 70 to 100%, the polysulphides being converted to sulfur or some solid sulfur green liquor, polysulphides, preferably from about compound, for example by cooling and crystallization, and removed from the system. This method is suitable for use in factories, in which excessive amounts of sulfur compounds are added to the process together with the raw material. the way it works.

The catholyte. can. In yet another mode of operation, a partial flow of the green liquor is treated so that the sulphides are oxidized to sulphate and / or The treated sulphide-free liquor in the pH regulator in the bleaching plant. The substantially sulphide-free green liquor thiosulphate. the anolyte can be used as a substantially pulp mill, for example as one can also be causticized with slaked lime to convert the carbonate to hydroxide, and then used in an oxygen delignification step or in alkaline bleaching steps. In another embodiment, the electrochemically treated and: ua-u- 10 15 20 25 30 35 ~ n n -no is now 515 833 y -. 10.0 vøç; .g 8 preferably substantially sulfide-free green liquor electrochemically to remove carbonate in the form of carbon dioxide and at the same time form alkali metal hydroxide. This treatment is carried out by preferably mixing substantially sulphide-free green liquor with an acidic aqueous anolyte, preferably containing sulphate, which results in the formation of carbon dioxide and an acidic anolyte containing less carbonate than the original green liquor, whereby carbon dioxide is removed from the acidic the anolyte, the acidic anolyte is introduced into the anode chamber of an electrochemical cell, water and preferably alkali metal hydroxide is introduced into the cathode chamber of the cell, protons are formed electrochemically in the anolyte, hydroxide ions are formed electrochemically in the catholyte, alkali metal ions pass from the anolyte to the catholyte active membrane, and part of the acid-enriched anolyte is mixed with fresh green liquor. Normally, oxygen and / or chlorine are also formed in the anolyte, thereby enabling the removal of chloride from the green liquor.

The present invention enables the production of alkali on the basis of raw materials available in pulp mills, without the formation of any undesirable by-products and without any need for pretreatment of the green liquor with oxygen or air. required for sulphide oxidation is the alkali production very Thanks to the comparatively low anode potential as energy efficient. If the formed alkali metal hydroxide is not used in the closed part of the pulping process, it is also possible to reduce the risk of increased potassium concentration, thereby avoiding the problems that may arise in the recovery boiler at excessive potassium levels. The yield of wood in pulp production can be increased by increasing the polysulfide content of the white liquor obtained by causticizing the treated green liquor. It is also possible to obtain mainly sulphide-free liquors for use in the bleaching plant. Thus, by the invention, green liquor obtained from sewage from cooking can be treated so that it is useful in the bleaching plant and vice versa, depending on the material balance of the individual pulp mills. Since electrochemical treatment of green liquor normally also involves the removal of alkali metal cations and some water, the load on the causticizing plant will be ~.-.- .. n of which. 10 15 20 25 30 35 515 833 9 be lower than if the corresponding treatment is carried out on the white liquor.

Another advantage of treating green liquor is that calcium added during causticization reduces the life of the cell membrane.

The invention will be described in more detail below with reference to the accompanying drawings, in which Fig. 1 is a schematic view of an electrolytic cell; Fig. 2 is a view of a three-dimensional mains electrode; and Fig. 3 is a schematic flow diagram showing how the invention can be applied to the production of cellulose pulp.

The invention is not limited to the embodiments shown, but is defined by the scope of the appended claims.

The electrolytic cell 1, anode chamber 2, which is provided with a three-dimensional genome as shown in Fig. 1, comprises a flow electrode which serves as an anode 4. A cathode chamber 3, which is provided with a preferably three-dimensional cathode 5, is separated from the anode chamber 2. with a cation-selective membrane 6. The anode 4 and the cathode 5 are connected to a direct current source (not shown). The anode chamber 2 has an inlet 7 and an outlet 8 for anolyte. The cathode chamber 3 has an inlet 9 and an outlet 10 for the catholyte and gaseous products, which extends to a gas separator 13 which has an outlet 12 for gas and an outlet 11 for liquid. When cell 1 is in operation, green liquor is introduced into the anode chamber 12 through the inlet 7. Thereby, sulfides are oxidized to polysulfides and alkali metal cations are transported through the membrane 6 to the cathode chamber 3. Polysulfide concentrated green liquor is discharged through the outlet 8. An aqueous solution of the alkali metal hydroxide 3 is introduced into the cathode chamber 3. 9, and water decomposes to hydrogen and hydroxide ions. The hydrogen gas is discharged through the outlet 10 together with an aqueous solution which is concentrated with respect to alkali metal hydroxide. In the gas separator 13, the hydrogen gas 12 is separated from the alkali metal hydroxide 11.

Figs. 2a and 2b show a three-dimensional mains electrode from above and from the front, respectively. The electrode shown is composed of four networks of expanded metal 40, which by spot welding are connected to a current supply in the form of metal strips 41. u-qv- »- one-_. Fig. 3 shows how an electrolysis cell 1 of the type shown in Fig. 1 can be used for alkaline production of cellulose pulp, such as sulphate pulp. For the sake of clarity, only one cell 1 is shown, but it is obvious to the person skilled in the art that, for example, from two to several hundred, are connected in parallel or in series. any number of cells, A preferred mode of operation can now be described. A boiler 20 is supplied with white liquor 21, as well as with alkali metal hydroxide. Boiling and washing (not shown) results in pulp and other chemicals required, such as 23 and black liquor 24 undergoing various treatment steps to obtain green liquor, which steps are well known to those skilled in the art of pulp production and normally involve evaporation, addition of chemicals, such as sodium sulfate, and combustion in a reducing atmosphere. The treatment of the black liquor gives green liquor 26, which normally contains from 0 to about 0.4 mol / liter of sulfides, from 0.7 to about 1.2 mol / liter of carbonate ions, from 0 to about 0.6 mol / liter of hydroxide ions, and from about 0.7 to about 2.5 mol / liter of alkali metal cations, of which from about 90 to about 97% are normally sodium, the remainder being mainly potassium. A part of the green liquor 27, for example from about 1 to about 30%, is led to a tank 30, which contains polysulfide-containing green liquor. Green liquor, whose polysulfide content increases upon electrolysis, for example in such a way that about 65-95% of the sulfide is converted to polysulfide, circulates between the tank 30 and the anode chamber 2 of the electrolysis cell 1. Polysulfide-rich green liquor 31 is withdrawn from the tank 30 for mixing with the main stream 26, for example about 0.5 so that the desired polysulfide content is reached, 1.5% by weight, after which the resulting mixture is fed to a causticizer 50, in which slaked lime is added and white liquor 21 is formed. The white liquor 21 is transferred to the digester 20 for boiling. An alkali metal hydroxide solution, which for example contains about 2 - 15 moles of alkali metal hydroxide / liter, circulates between the cathode chamber 3 of the cell 1 and a tank 35 via the gas separator 13. A part of the alkali metal hydroxide solution 11 from the gas separator 13 is withdrawn as a product 36 and can be used e.g. the production of pulp or at whole Water 37 is supplied to the tank 35 to thereby keep the volume and concentration substantially constant. other processes.

In a particular mode of operation, sulfur can be expelled from the system by driving the sulfide oxidation to high polysulfide levels in the tank 30, in excess of 70%, based on on the sulphide in the green liquor. preferably a conversion of green liquor from the tank 30 can then be treated so that sulfur precipitates, for example by cooling and crystallization. This can be accomplished by circulating polysulfide-rich green liquor between the tank 30 and a crystallizer (not shown), from which precipitated sulfur is removed, while the mother liquor is recycled to the tank 30.

In another method, the sulfides in the green liquor are largely oxidized to sulfate and / or thiosulfate, which can be accomplished by filling the tank 30 with green liquor, which is then circulated through the anode chamber 2, with no green liquor leaving the circulation system until substantially all of the sulfate or thiosulfate . essentially sulphide-free green liquor, sulphides may have been converted.

The invention will now be further described by the following causticization or further electrochemical examples.

EXAMPLE: consisting of an anode chamber and cathode chamber separated by a Nafion®324 cation-selective membrane in a microfluidic cell. The anode was a mains electrode of type DSAQ ON 201 and the cathode was a flat electrode of stainless steel. The surface area of the membrane and the projected area of the electrodes were 10 cm 2. Synthetic green liquor, which consisted of an aqueous solution of 1.4 M sodium carbonate and 1.275 M sodium sulfide, was used as the anolyte 125 ml of anolyte and 125 ml of catholyte were placed in separate vessels and then circulated through the cell at a flow of 160 ml / min. 1.3 V and the temperature was about 90 ° C. After 64 minutes, the sodium catholyte was 2.8 M and the sulfide concentration in the anolyte was 1.0 M. The current yield of hydroxide and 2.633 M sodium hydroxide as catholyte.

The voltage across the cell was about the hydroxide concentration in the preparation was 60 .mu.s.

Claims (10)

lO 15 20 25 30 35 515 833 12 PATENTKRAV A method of producing pulp, which comprises a step of forming green liquor containing alkali metal sulphide or alkali metal carbonate, the method further comprising a step of electrochemically recognizing the action of the green liquor to oxidize at least a portion of the sulfide. 2. A method according to claim 1, characterized in that the green liquor is treated by operating an electrochemical cell, anolyte. which includes at least two chambers with green liquor which 3. A method according to claim 1 or 2, characterized in that the electrochemical treatment of the green liquor also comprises the production of alkali metal hydroxide. 4. A method according to any one of claims 1-3, characterized in that the green liquor is treated in an electrochemical cell, which comprises separate anode and cathode chambers separated by a partially permeable barrier, the treatment comprising the steps of introducing green liquor into the anode chamber , introducing aqueous alkali metal hydroxide into the cathode chamber, electrochemically oxidizing sulfide in the anode chamber, forming hydroxide ions in the cathode chamber, and conducting alkali metal ions from the anode chamber to the cathode chamber. 5. A method according to any one of claims 1-4, characterized in that the sulphide is oxidized to give mainly polysulphides. 6. A method according to any one of claims 1-4, characterized in that the sulphide is oxidised to give mainly sulphate and / or thiosulphate. 7. A method according to any one of claims 1-6, characterized in that it. electrochemically treated green liquor is further electrochemically treated to remove carbonate in the form of carbon dioxide and at the same time form alkali metal hydroxide. 8. A method according to any one of claims 1-7, characterized in that green liquor obtained by heat treatment of bleaching effluent is electrochemically treated to oxidize sulphide. 9. A method according to any one of claims 1-7, characterized in that green liquor obtained by heat treatment 515 sas - 13 of sewage from boiling is treated electrochemically to oxidize sulphide. 10. A method according to any one of claims 1-9, characterized in that the electrochemically treated green liquor is transferred to a causticizing step. away- w w.ø ~ o ~