SE465039B - MADE TO MAKE SUBSTANCES WITH HIGH SULFIDITY BEFORE SULFAT PREPARATION - Google Patents

Description

465,039 2 hydrogen sulfide. Similarly, the equilibrium (1) expresses a solution with the sulfidity 100%.

Large amounts of sodium sulfide are produced in the sulfate pulp industry. When recycling the cooking chemicals, the so-called black liquor is burned in a recovery boiler, the lower part of which is reductive. In the lower zone of the recovery boiler, the sulfur components of the black liquor are reduced to the sulphide stage and thus change to sodium sulphide.

Sulphate pulp mills usually operate in the sulphidity range 25-40% (white liquor sulphidity). A larger part of the sodium reacts with carbon dioxide when black liquor is burned to form sodium carbonate. The mixture of sodium sulphide and sodium carbonate forms a melt in the bottom of the recovery boiler and this melt is removed and allowed to react with water to form so-called green liquor. A typical green liquor has the composition Sodium carbonate 90-105 gll Sodium sulphide 20-50 gll Sodium hydroxide 15-25 gll (all substances counted as sodium hydroxide) Chemical recovery according to the "sulphate method" (power) leads to a relatively large amount of sulfur when the oxidative zone is expelled from the recovery boiler mainly as sodium sulphate (electric filter ash) and sulfur dioxide. If the white liquor sulphide exceeds 35X, problems begin to arise, among other things, through high emissions of sulfur dioxide from the recovery boiler. Gas washing with alkaline medium is therefore often used to eliminate or greatly reduce sulfur dioxide emissions. The resulting green liquor is converted to white liquor according to a known causticization procedure. A white liquor composition may vary from factory to factory but approximate concentration values can be given as sodium hydroxide, NaOH 80-120 g / l sodium sulphide, Na = S 20-50 g / l sodium carbonate, Na = CO = 10-30 g / l Z sodium sulphate, Na = SO 10 g / l (all substances counted as sodium hydroxide) If Na = S is calculated to be completely hydrolyzed according to Na = S + H10 -> NaOH + NaSH 465 039 this means that the amount of sodium, bound as carbonate, often constitutes more than% of the sodium present as hydroxide.

It is known that the presence of sodium hydrogen sulphide is advantageous in pulp cooking according to the sulphate process. The presence of sodium hydrogen sulfide increases the selectivity of the boil towards higher lignin release. The effect can also be expressed as an increased hydrogen sulphide content giving the opportunity to reach a lower kappa number at the same viscosity, when the conditions are otherwise comparable.

The kappa number is a measure of the lignin content and the viscosity is considered to be a measure of the strength of the cellulose fiber.

It is of interest to be able to boil the mass to as low a kappa number as possible.

This is especially true if it is to be bleached to high (90 ISO) brightness. For this purpose, bleaching with chlorine-containing bleaching chemicals is required, which gives rise to synthetic chlorine-carbon bonds (TOCl = total organically bound chlorine), which constitute a large environmental impact. To reduce the proportion of lignin, which is bleached away with chlorine-containing bleaching chemicals, bleaching with oxygen has also been developed. This technology is being developed in Sweden and Japan, among other places.

In order to reduce the bleaching with chlorine chemicals, it is known that by so-called modified cooking, the selectivity of the cooking can be further improved with even lower kappa numbers as a result. Modified cooking according to current technology is based on the following process conditions: 1. The alkali concentration must be as even as possible during the cooking process. The hydrogen sulphide concentration must be as high as possible, especially at the beginning of the bulk delignification phase. The hydrogen sulphide concentration may be low in the final phase of the cooking. The concentrations of released lignin and sodium ions should be as low as possible, especially in the last part of the cooking. 4. The temperature should be low, especially at the beginning and end of cooking. Of the above points, point 2 is of particular interest with respect to the present invention. So far, the concentration of hydrogen sulphide ions given by the 40% sulphide in white liquor has been sufficient.

By, among others, S. Norden et al, Tappi, Vol 62, No. 7, July 1979, page 49; B. Johansson et al, Svensk Papperstidning no. 10, 87 (1984) page 30 and D. Tormund et al, Tappi, Vol 72, no. 5, May 1989, page 205, it appears that a further increased content of hydrogen sulphide ions in addition to 40% sulphidity is very beneficial in the initial stage of cooking.

In the case of modified cooking, the cooking liquor is added in two or more places.

Extra high sulphidity in the cooking liquor that is added at the beginning of the cooking makes the greatest benefit, while the sulphidity in cooking liquor that is added in the final stage of the cooking can be low.

We have now surprisingly found a way of producing a cooking sulphide with a high sulphidity, which is particularly suitable for modified cooking according to the sulphate method, in which a cooking sulphide with a high sulphidity is added in such a way that when cooking according to the prior art be produced. More particularly, the invention relates to a process for producing boiling liquids with high sulphide for reducing sulphate pulp boiling under reducing conditions, wherein the black liquor formed during the boiling process after evaporation is wholly or partly fed to a reactor operating at elevated temperature, which is produced by energy from a external heat source and / or release of energy from the black liquor, whereby a melt consisting mainly of sodium sulphide is formed and deducted for further conversion to boiling liquor.

The method according to the invention is characterized in that the reactor is also fed wholly or partly sulfur-containing and / or sulfur- and sodium-containing materials present in the pulp mill, including sulfur-containing and / or sodium- and sulfur-containing substitute chemicals used for the total chemical balance of the pulp mill in such a way that the sodium ratio sulfur in the total mixture fed to the reactor falls in the range 1.5 to 4.

Preferably, the molar ratio of sodium to sulfur is sought in the total mixture fed to the reactor in the range 2-3 and in particular in the range 2-2.8. It is also preferred to feed to the reactor up to about 30% of the black liquor flow generated in the pulp mill.

The sodium sulphide melt obtained in the process according to the invention can be dissolved in water in a known manner and further converted into boiling liquors. According to a preferred embodiment, a solution of the melt is fed directly to the digester for optimal utilization of its high sulfidity in modified boiling.

In an alternative procedure, a solution of the melt is mixed with a portion of the white liquor, prepared in the usual manner.

In order to cause the reduction reactions in the reactor to take place quickly and thereby obtain short residence times and then reactor volumes, additional energy, in addition to the energy released from the black liquor during parillal oxidation, can be supplied to the reactor mixing zone through a hot gas, whose heat content and oxidation potential are The heat energy can be supplied, for example, by a gas heated by a plasma generator. The very hot gas or gas mixture can also be produced directly or indirectly with an oxy-fuel burner. Air, recycled process gas, hydrogen gas can be used as gas or gas mixture. , natural gas, carbon monoxide, etc. When using oxy-fuel burners, the gas or gas mixture is obtained by burning, for example, acetylene or LPG with oxygen-enriched air or pure oxygen.

A preferred method of the invention is to introduce the hot gas into the reactor in close proximity to the feed material, which in turn must be atomized, which can be done by various types of atomization techniques known to those skilled in the art. The design of the reactor must be large enough for the reaction to take place, ie the reactor volume must ensure a certain minimum residence time.

The reactor is preferably a closed reaction vessel and the temperature in the reactor should be at least the temperature at which the sodium sulphide is formed from other prevailing conditions. The person skilled in the art can determine this from case to case, for example by routine experimentation. The temperature is preferably not below 700 ° C.

The pressure in the reactor is preferably atmospheric pressure. However, the process can be operated at elevated pressure in order to, for example, reduce the reactor volume. 465 059 6 Swedish patent SE 8501465-2 describes a method of relieving the recovery boiler by means of plasma gasification of a partial stream of black liquor. This makes it possible to increase pulp production in a factory with too little recovery boiler capacity or, for example, to introduce oxygen bleaching and / or modified cooking in a recovery boiler-limited factory without losing production capacity.

Of essential importance for the process according to the invention is that the molar ratio of sodium to sulfur in the total mixture fed into the reactor is less than about 4 and falls in the range 1.5 to 4, preferably 2 to 3. This adjustment of the sodium to sulfur ratio takes place by sulfur-containing and / or sulfur- and sodium-containing materials present in the pulp mill, including sulfur-containing and / or sodium- and sulfur-containing substitutes used for the total chemical balance of the pulp mill.

The replacement chemicals used to adjust the proper molar ratio of sodium to sulfur can be sulfur, sulfur dioxide, sulfuric acid, sodium hydrogen sulphate, sodium sulphate, sodium sulphite, sodium hydrogen sulphite and sodium thiosulphate.

Among the sulfur-containing and / or sulfur- and sodium-containing materials present in the pulp mill, the following can be mentioned: a. Residual acid from chlorine dioxide production. A mixture of sulfuric acid and sodium sulphate with a Na / S ratio of $ 1 is obtained from so-called Mathieson plants. In other common processes of the so-called R-8 type, sodium sesquisulphate (Na = H (SOa) => with a Na / S ratio of 1.5 The deposition of the residual acid is generally a problem, it is most often dumped, so-called electric filter ash which mainly consists of sodium sulphate. 60-125 kg of electric filter ash is normally formed per tonne of pulp, which is recycled to the combustion zone of the recovery boiler The Na / S ratio is š 2. c. Sulphite-containing solutions from soda ash scrubbers The Na / S ratio is approx. d. EP 87B50238.4 describes a process in which a partial flow of the white liquor's sodium hydrogen sulphide is reacted with copper oxide, whereby sodium hydroxide and 465,039 7 copper sulphide are formed.The copper phosphide is roasted here to form sulfur dioxide and copper oxide.The sulfur dioxide formed is a sodium-free sulfur source.

Otherwise, elemental sulfur can be used or any other sulfur-containing chemical with a Na / S ratio, which is equal to or less than about 4.

By suitable combinations of all or part of the black liquor flow and one or more of the above products, a not insignificant amount of cooking liquor with high sulfidity can be produced.

The invention is further illustrated by the following working examples.

E-xí-lakvel 1 For, spirit streams of material were fed continuously per hour to a reactor operating at atmospheric pressure - 620 kg black liquor (652 TS content) containing 129 kg sodium (Na) and kg sulfur (S) per tonne of black liquor - a residual mixture from chlorine dioxide production according to the Mathieson process containing 80 kg H = SO. and 62 kg Na 2 SO 4. - 800 kg Na; SO. in the form of an electric filter ash The material streams above were mixed with an oxygen-containing gas and introduced into a reactant space. The oxygen-containing gas was heated to about 750 ° C in a plasma generator.

Through partial oxidation of the black liquor, energy was released, whereby the temperature in the reactor was maintained at about 950 ° C.

The process gas evolved during the partial oxidation was cooled. After possible final oxidation, heat recovery and gas purification, it can be taken to the atmosphere.

Alternatively, a larger portion of the energy content of the liquor can be released by partial oxidation, whereby the oxygen-containing gas does not need to be preheated in a plasma generator. 465,039 In the reaction space, incoming sulfur compounds were reduced to mainly sodium sulfide (Na = S), which formed a melt phase, which was subtracted from the system.

Due to the high partial pressure of sulfur in the reaction space and the higher affinity of sulfur for sodium compared to carbon dioxide at the current reaction conditions, the formation of sodium carbonate in the inorganic melt phase was displaced.

From the prepared melt, a 4.0 molar solution of sodium was prepared, containing 1.85 moles of NaOH, 1.85 moles of NaSH and 0.15 moles of Na = CO = per liter.

Prior to experiments with two-stage modified pulp cooking, in which 702 of the cooking chemicals were charged in step 1 and the remaining 30% in step 2, the following cooking liquors were prepared.

One (1) part of the lye prepared above according to the invention was mixed with 4.63 parts of ordinary cooking liquor (white lye) containing 2.8 moles of NaOH and 0.7 moles of NaSH per liter of solution. The thus prepared cooking liquor with a sulphidity amounting to 51% was charged in the first boiling step, while the normal white liquor with 40% sulphidity was charged in step 2.

Example 2 The following material streams were fed continuously per hour to a reactor operating at atmospheric pressure 566 kg black liquor (652 TS content) containing 129 kg sodium (Na) per ton and 35 kg sulfur (S) per ton - 48 kg sulfur dioxide 80 kg Na = SO ¿In the form of electric filter ash - 25 kg Na = SO. as make-up It was seduced in exactly the same manner as in Example 1 and a melt phase was obtained, which was removed from the system. 4A 46509 p 9 The supplied sulfur dioxide had been generated via roasting according to the process for producing sulphide-free liquor described in EP 87850238.4.

From the prepared melt, a 4.0 molar solution was prepared with respect to sodium containing 1.75 moles of NaOH, 1.75 moles of NaSH and 0.25 moles of Na 2 CO 3.

Prior to experiments with two-stage modified pulp cooking, in which 70% of the cooking chemicals were charged in step 1 and the remaining 30% in step 2, the following KOKIUtBI 'was prepared.

Following the same batch procedure as in Example 1, a mixture of 68% sulfide was obtained by mixing 1.0 part of the above-prepared liquor with 1.14 parts of ordinary cooking liquor (white liquor) (402 sulfidity). This lye was charged in the first cooking step.

For the second step, the sulphide-free lye prepared according to EP 87850238.4 was used.

Claims (6)

465 059 10 PATENT REQUIREMENTS A method of producing, under reducing conditions, boiling sulphides with a high sulphidity for sulphate pulp boiling, in which the black liquor formed during the boiling process after evaporation is wholly or partly fed to a reactor operating at elevated temperature, which is achieved by energy supply from an external heat source and / or release of energy from the black liquor, whereby a melt consisting mainly of sodium sulphide is formed and subtracted for further conversion to boiling liquor, characterized in that the reactor is also fed wholly or partly sulfur-containing and / or sulfur- and sodium-containing materials used for the pulp mill's total chemical use. sulfur-containing and / or sodium- and sulfur-containing substitute chemicals in such a way that the molar ratio of sodium to sulfur in the total mixture fed to the reactor falls within the range of 1.5 to 4. 2. A process according to claim 1, characterized in that the molar ratio of sodium to sulfur in the total mixture fed to the reactor falls in the range 2 to 3, preferably 2-2.8. 3. A method according to claim 1, characterized in that the sulfur-containing and / or sulfur- and sodium-containing materials fed to the reactor fed to the reactor consist wholly or partly of one, several or all of the electric filter ash, residual product from chlorine dioxide production, sodium hydrogen sulphite-containing solutions from gas scrubbing of sulfur dioxide, effluents from CTMP, NSSC or other sulphite pulp production, sulfur dioxide from roasting of copper sulphide and hydrogen sulphide-containing condensates or air streams. 4. A method according to claim 1, characterized in that the sulfur-containing and / or sulfur- and sodium-containing replacement chemicals consist of one or more of sulfur, sulfur dioxide, sulfuric acid, sodium sulphite, sodium hydrogen sulphate, sodium thiosulphate or sodium sulphate. 46% 059 11 5. SI. A method according to claim 1, characterized in that the sodium sulphide melt or an aqueous solution thereof is mixed with the white liquor, whereby a white liquor with increased sulphide is obtained. 6. A method according to claim 1, characterized in that an aqueous solution of the sodium sulphide melt is used in so-called modified sulphate boiling.