DESCRIPTION Technical field The present invention -relates to a process for the preparation of an aqueous solution of sodium compounds which, together with a hot gas containing carbon monoxide, carbon dioxide and hydro¬ gen gas have been formed at partial combustion of cellulose spent liquors, the gas with its content of sodium compounds be- ing brought into direct contact with one or more solving Li¬ quids in a device consisting of a vessel, partially filled with a solving liquid and a vertically arranged downcoming tube, the upper part of which being connected to a gas generator and the lower part of which debouching below the resting surface of the solving liquid.

Background of the invention

At the manufacture of paper pulp spent liquors are generated which comprise alkalimetal containing and carbonaceous com- pounds having different compositions depending on which tech¬ nique having been used at the delignification. Bleaching of pa¬ per pulp generates spent liquors having different composi-t ions as well.

At the delignification according to the sulphite process dif¬ ferent inorganic alkaline components are used, such as sodium or magnesium compounds. At the delignificat on according to the sulphate process the inorganic alkaline compounds consist of sodium compounds. Spent liquors from the bleaching of sulphate pulp contains a high amount of sodium chloride. The above and other spent liquors from the cellulose industry can be the starting material at the use of the present invention, which be exemplified in detai l below with reference to a sulphate spent liquor without being restricted to this spent liquor.

At the application of the sulphate process the wood is digested using a digestion liquid the active components of which sub-

stantially consist of sodium hydroxide and sodium hydrogen sul¬ phide. Besides cellulose fibres a spent liquor is produced at the digestion which spent liquor is called black liquor con¬ taining recoverable chemicals and combustible carbonaceous com- pounds. For the present being the chemicals are mostly recover- ^' ed by combusting the liquor in a recovery boiler whereby the energy content of the carbonaceous compounds is used for the production of steam.

In the recovery boi ler the liquor is partially combusted in the bottom zone of the boiler whereby the sodium and sulphur con¬ taining chemicals are reduced and form a melt substantially consisting of sodium carbonate and sodium sulphide. The carbon¬ aceous compounds form a combustible gas containing carbon mon- oxide, carbon dioxide, and hydrogen, which gas is finally com¬ busted by adding secondary air in the upper part of the recove¬ ry boiler.

The melt of chemicals form a pool of melt in the bottom part of the recovery boiler from which they are drafted in melt form in order to be dissolved later on in a separate tank, the so call¬ ed melt dissolver. The solution obtained at the dissolution of the above sodium compounds in water or a weak alkaline aqueous solution will mainly contain sodium carbonate, sodium sulphide and sodium hydroxide and is called green liquor. The green li¬ quor is treated with quick lime during the formation of sodium hydroxide and hardly dissolvable calcium carbonate according to the fo llowing : Na ₂ C0, + CaO + H ₂ 0 → 2NaOH ÷CaCO-, The sodium sulphide does not participate in the reaction. The liquid solution which then mainly consists of the active di¬ gesting chemicals sodium hydroxide and sodium hydrogen sul¬ phide, is then named white liquor. The calcium carbonate is normally burned in a roller oven for recovering the calcium ox- ide. When washing precipitated calcium carbonate, the so called lime sludge, a diluted alkaline solution named weak li-quor is obtained. Weak liquor and white liquor have a similar chemicals

content. The weak liquor has, however, a considerably lower concent rat i on .

The green liquor used for the preparation of white liquor is normally composed as follows: sodium hydroxide, NaOH 15 - 25 g/l sodium sulphide, Na^S 20 - 50 g/l sodium carbonate, Na ₂ C0, 90 - 105 g/l sodium sulphate, Na-SO, 5 - 10 g/l (all calculated as sodium hydroxide)

The white liquor, being used as digesting liquor, is normally composed as follows: sodium hydroxide, NaOH 80 - 120 g/l sodium sulphide, a^S 20 - 50 g/l sodium carbonate, Na^CO, 10 - 30 g/L sodium sulphate, Na ₂ S0, 5 - 10 g/l (all calculated as sodium hydroxide)

The content of chemicals of the green liquor is normally be¬ tween 150 and 200 g/l calculated as sodium hydroxide. Higher concentrations are undesirable due to precipitation of salts and a lower concentra ion means that unnecessary large amounts of water being handled in the process.

Partial combustion of cellulose spent liquor can take place in for example a gas generator of the type described in Swedish Patent 8502731-6. According to this process droplets of melted sodium compounds and a combustible gas comprising carbon mon- oxide, carbon dioxide, and hydrogen are formed at the partial combustion. The object of the present invention is to separate sodium compounds of such a mixture of gas and melted droplets and to prepare an aqueous solution suitable for further pro¬ cessing into digestion liquor.

At the partial combustion of cellulose spent liquor at tempera¬ tures from 700 C to 1400 C the sodium compounds being formed be

present in both solid, liquid, and gaseous form. Furthermore a fine smoke of sodium, sodium oxides, and sodium hydroxide is formed. The separation of gas and sodium compounds which are present in different form is complicated partly by the tacki- ness of the melted sodium compounds, partly by the difficulty of separating the fine smoke, which gives raise to depositions on heat exchange tubes and thus requires advanced filtering and washing equipment in order to avoid discharge to the environ¬ ment .

A further complication is the risk for explosions at the con¬ tact between water and collected melt of sodium compounds.

At the partial combustion of black liquor between 700 C and 1400 C a combustible gas comprising carbon monoxide, carbon di¬ oxide, hydrogen, methane, hydrogen sulphide, carbon oxysul- phide, steam, optionally nitrogen and gaseous sodium and sodium compounds, as well as sodium, potassium and sulphur compounds mainly in melted form are obtained. These melted compounds are mainly sodium carbonate, sodium sulphide and sodium hydroxide. A small amount of sodium and sodium compounds is present as a fine smoke, which is difficult to separate of from the gas. In the lower part of this temperature range part of the sodium and sulphur compounds are present in solid form. Even at these lo- wer temperatures, however, some amount of sodium and potassium compounds are present in liquid form. Mixtures of solid, li¬ quid, and gaseous sodium compounds exhibit a large tackiness. As mentioned above there exists a risk for an explosion if lar¬ ger amounts of melt is brought into contact with water, and thus a collection of melt should be avoided.

The problem of tackiness and deposition as well as the risk of explosion makes it desirous to use a wet separation method and to dissolve the sodium compounds in a liquid without being col- lected prior to that in such an amount that a risk of explosion s at hand.

Description of the present invention

It has now been shown possible to solve the above described problems by preparing an aqueous solution of the solid, melted and gaseous sodium compounds which together with the hot gas are being formed at the partial combustion of cellulose spent liquor. The solution of sodium compounds is in turn used for the preparation of digestive liquor whi le the gas being freed from sodium compounds is used for, for example, the production of steam and electricity. The separation between gas and sodium compounds is characterized in that the gas and its contents of sodium compounds first is allowed to pass down into a vessel through a downcoming tube connected to the gas generator, into which tube a dissolving liquid is added, and is then allowed to pass between the downcoming tube and an upcoming tube, which is opened in both ends and upper opening of which debouches above the resting surface of the dissolving liquid, and the lower end of which debouches below the resting surface of the dissolving liquid, and which wholly or partly surrounds the downcoming tube, whereby gas, sodium compounds and dissolving liquid being brought with the gas are brought into intense contact, whereby the sodium compounds are separated off and are dissolved in the dissolving liquid, which is drafted for the preparation of a digestion liquid, and the gas, now substantially freed from so¬ dium compounds are discharged from the vessel.

The present invention prevents tackiness and deposition at the separation of gas and sodium compounds as gas and sodium com¬ pounds already in the downcoming tube are rapidly cooled by in¬ jected dissolving liquid to a temperature below the melting point of the sodium compounds. The dissolving liquid also has the task of wetting the downcoming tube and thereby preventing deposition of sodium compounds. The dissolving liquid being added to the downcoming tube is suitably injected into the up¬ per part of the downcoming tube, so that the inner surface of the tube is being completely protected by liquid. Optionally, further dissolving liquid may be added further down in the tube. The liquid can be added through a slot, several minor or-

ifices or nozzles.

After having passed the downcoming tube, the gas turns and raises through the interstice between the downcoming tube and the upcoming tube. The gas hereby brings with it dissol ing liquid and an intense contact is obtained between the gas, the sodium compounds and the dissolving Liquid. The melt remaining in the gas and solid sodium compounds including optional smoke of sodium are hereby separated from the gas and are dissolved into the liquid. Optionally a smaller amount of larger droplets of melted sodium compounds and soot form a solid residue that must be separated off from the solution.

In order to maintain stable flow conditions and thereby also a pred ctable and good function of separation in the vessel, the gas flow rate through the upcoming tube has to be restricted by selecting the cross-section between downcoming tube and upcom¬ ing tube in such a way that the gas flow rate in the upper part of the upcoming tube is less than 20 meter per second, prefer- ably less than 10 meter per second, more preferably less than 5 meter per second.

In the present invention the previously mentioned sodium and potassium compounds are efficiently separated although they are present in a hardly separable mixture of solid, melted and gas¬ eous salts. Furthermore, a collection of melt is avoided as the compounds mentioned are collected in the dissolving liquid whi le they are small particles and small melted droplets.

In the present invention a solution is obtained which as to its composition remains of that of green liquor. The concentration of sodium compounds in the liquid drafted of the present inven¬ tion is adjusted to between 100 and 200 grammes per litre, for example by adding external liquid to the vessel. The liquid added should be neutral or weakly alkaline. Water and weak li¬ quor are both suitable such liquids.

The susceptibi lity of deposition of green liquor decreases with increasing temperature and thus it is suitable to adjust the temperature in the vessel in such a way that it exceeds 40 C, preferably exceeds 60 C. The temperature of the liquid in the vessel should be at least 5 C lower than the boi ling point of the dissolving liquid in order to avoid local boiling in vessel and connected pumps with worse efficiency as a consequence.

In the present invention the temperature of the liquid in the vessel is ad usted and thus the temperature and the concentra¬ tion of the green liquor drafted by adding dissolving liquid at a lower temperature than desired temperature of the green li¬ quor and/or by circulating dissolving liquid from the vessel through a cooler and back to the vessel.

In those cases the green liquor drafted according to the pre¬ sent invention is a smaller part of of the total green liquor flow of a sulphate pulp industry, only, it is possible to fur¬ ther decrease the concentration of the green liquor without in- creasing the total flow of water to the process by uti lizing in the sulphate pulp industry internally generated weak liquor as a dissolving liquid. In such cases the concentration of so-dium compounds may be adjusted to at least 10 grammes per lit-re, preferably at least 30 grammes per litre.

At the combustion of black liquor using ai r as an oxidation agent the volume part of carbon monoxide wi ll normally be ad¬ justed to between 8 and 20 %, preferably between 10 and 16%. Using pure oxygen as an oxidation agent the volume part of car- bon monoxide wi ll normally be adjusted to between 15 and 40%, preferably between 19 and 32%. The volume part of hydrogen sul¬ phide is normally less than 1%. The volume parts above are given as part of dry gas.

The following chemical equilibria are of importance for the composition of green liquor produced according to the present invention :

Na ₂ C0 ₃ + H ₂ 0 < - > NaHC0 ₃ + Na ₂ S + H ₂ 0 < - > NaHS + 2NaOH + C0 ₂ < - > Na ₂ C0 ₃ + 2NaHS + C0 ₂ < - > Na ₂ C0 ₃ + NaOH + H ₂ S < - > NaHS +

At the contact between gas containing carbon dioxide and hydro¬ gen sulphide and sodium alkaline solutions sodium carbonate and sodium hydrogen sulphide are formed. Sodium carbonate in an aqueous solution forms sodium hydrogen carbonate. The sodium hydrogen carbonate is an unwanted compound in green liquor as it, at the causticising requires the double amount of quick lime per mole of sodium compared with sodium carbonate. The formation of sodium hydrogen carbonate is supported by a high partial pressure of carbon dioxide and a high total pressure in the gas bulk.

A high partial pressure of carbon dioxide will counteract the absorption of hydrogen sulphide as well, and can even contri¬ bute to evaporation of hydrogen sulphide from the solution, which is disadvantageous by the increased need for gas cleaning that will occur.

Selective absorption of hydrogen sulphide rather than carbon dioxide is supported by a short contact time and a large con¬ tact area between liquid and gas. In the present invention the desired ratio is obtained as the gas flow rate through the downcoming tube and the upcoming tube is more than 0.5 meter per second, preferably more than 1 meter per second, more pref¬ erably more than 2 meter per second. In order to further sup¬ press an unwanted formation of hydrogen carbonate it is advan- tageous if the gas pressure of the vessel is below three bars.

The invention will be i llustrated more in detai l by the cross-

section in FIG. 1, wherein 1 denotes the reaction zone of the gas generator, wherein atomized black liquor is partially com¬ busted at 700 to 1400°C.

Gas generated and containing solid particles, melted droplets and aerosol is forced through the venturi 2 and down into the downcoming tube 3. Immediately in connection with the transi¬ tion of the venturi into the downcoming tube an aqueous cooling liquid is injected into the gas flow through nozzles 4 or through an annular slot. The surface of the downcoming tube 3 is thereby coated and the gas is cooled to the saturation tem¬ perature of water steam at the actual pressure.

The cooled gas is forced to turn 180 at the lower end 5 of the downcoming tube, which end is preferably serrated in order to evenly distribute the gas flow around its ci rcumference. The gas then raises through a upcoming tube 6 being concen rically arranged around the downcoming tube, which upcoming tube sur¬ rounds the downcoming tube. The gas then leaves the system through one or more gas outlets 7. The level of the liquid lock 8 is partly monitored by the addition of cooling liquid 9, partly by the draft of product liquid 10, which contains dis¬ solved sodium compounds. The flow of cooling liquid 9 consists of two part streams, the make-up flow 11, and the rec i rcu lati on flow 12 which passes through a heat exchanger 13. The draft of product liquid takes place through the outlet 10.