FI73754C - Process for the preparation of boiling chemicals for cellulose production. - Google Patents

Description

1 73754

The method produces soup chemicals for cellulose production - Förfarande vid beredning av kok-kemikalier för cellulosaframställning.

The present invention relates to a process for washing lime and removing unwanted substances from cellulose mills, for example in the production of sulphate cellulose. The method is particularly suitable for mills that use solid ash-rich fuels to burn lime or where the fibrous raw material contains high mineral contents.

In the production of cellulose according to the so-called sulphate method, wood chips are boiled with cooking liquor, the active ingredients of which are sodium hydroxide (NaOH) and sodium sulphide (Na2S). These are oxidized on cooking to carbonates or sulfates. I.in addition, when cooked, about 50% of the wood dissolves, mainly lignins and hemicellulose, but also the minerals in the wood.

The heat content of the lye is utilized in an evaporator, then concentrated in a recovery boiler where the lye is burned.

The ash melt of the sodium salts runs from the recovery boiler to the melt solvent, where it dissolves in dilute lye. In this case, solid particles (impurities) are separated from the soda liquor formed, usually by clarification in a so-called soda liquor clarifier.

The soda liquor is then used to quench the burnt lime, which is added in excess to the so-called quencher. It also separates the lime particles, which must not be suspended in the soda liquor but settle to the bottom and removed.

Upon quenching, calcium hydroxide (Ca (OH) 2) is formed, which together with sodium carbonate (Nsoda liquor) forms sodium hydroxide (NaOH, cooking liquor) and calcium carbonate (CaCO 2, mesa). This reaction is commonly referred to as causticization.

In order for the reaction between sodium carbonate and quicklime to equilibrate on its own, a certain time is required, which is why the reaction is allowed to proceed in a lime quenching vessel and a caustic soda vessel for about $ 1-1 per hour up to 2 hours.

Alkali and calcium carbonate, mesa are separated after the reaction, which takes place in a white liquor clarifier or by filtration. The lye is stored after the separation of the lime and is therefore pumped into containers from which it is later dispensed for cooking.

The honey goes further to re-causticization, allowing the remaining excess lime to react with the soda liquor. This gives a dilute lye which is used in the aforementioned dissolution of the ash melt.

The honey is dewatered in vacuum filters which increase the dry matter content between 60 and 70% and the lime is washed in the filters by drawing, before being transported to the rear end of the lime kiln (cold).

The honey is dried in an oven and burned into lime, which generates steam and carbon dioxide. For heating, the oil or gas is usually burned immediately in the furnace using a burner.

Reactions in recovery boiler: .. plain. atmosphere .. -Λ

Na2SO4 ^ ___> Na2S + 202

Reaction in fire extinguisher and causticizing vessel:

CaO (s) + «20 (- · ->) Ca (0H) 2 (s)

Ca (OH) 2 (s) + CO32-> CaCO3 (s) + 20H-

Reaction in a lime oven: 100 ° C

CaCl 3 (S) (<= ^) CaO (s) + CO 2 (s) 3 73754

Today, undesired compounds enter the treated lime into the wood (round-sorted wood chips) and process water. In factories that utilize annual plants (grass, reed, straw, bagasse), bamboo, or wood species that contain large amounts of unwanted compounds, cooking chemicals can be difficult to reuse and are therefore discarded.

The burning of solid fuels has become topical in lime kilns, associated with high energy prices and the full utilization of wood. The complete utilization of wood has also led to a deterioration in the quality of the chips, i.e. the number of undesirable compounds has increased. The use of solid fuels and / or wood chips made from whole wood in a lime kiln creates similar problems as the use of annual plants, etc.

Unwanted compounds such as Si, Al, Mn, Fe and P salts are enriched at all stages of the process, depending on the final stage of the system.

Concentrations therefore increase until the system stack is reached. There are usually no disposal sites other than cellulose, soda liquor sludge, and crushed lime.

The enrichment of undesired compounds brings with it several problems: - Above all, the increased tendency of sodium aluminum silicates to scorch on heat transfer surfaces, for example in the evaporator, recovery boiler and cooking boiler. The aluminum content is a controllable factor in the precipitation of sodium aluminum silicates.

- Increased deposits in pipes and equipment at low flow rates, mainly due to poor sediment feeding and filtration properties of the mesa.

- Increased energy consumption, on the one hand because inert substances, such as lime, have to be heated unnecessarily, and on the other hand because the dry matter content of the sludge filtrate has decreased as described above.

4 73754 - Poor plant capacity due to the above effect.

When quenching quicklime, the required amount of carbonate solution (soda liquor) is metered exclusively into the quencher or alternatively in several streams, e.g. for causticization and / or re-causticization.

When turned off, large amounts of energy are released. In order to obtain a rapid reaction, the temperature is required to be between 90 and 100 ° C. With today's technology, extinguishing is difficult to control. Too low a temperature will result in poor shutdown and thus degraded power. Too high a temperature (energy development) causes the liquid in the fire extinguisher to boil over and severe damming problems arise in the dosing of lime. One attempt to overcome the practical problems that an uncontrolled fire extinguisher brings with it has been made by putting the fire extinguisher under pressure and building it into a closed system.

The present invention provides a solution to the above problems.

The invention relates to a process which can be applied in particular in factories where solid fuels such as bark, wood, wood chips, peat and coal are used in a lime kiln and / or in which the fibrous and chemical raw materials contain high concentrations of undesirable substances. Such raw materials may include wood chips from the total use of wood, limestone, water, bagasse, bamboo, reeds, straw and the like. When the above-mentioned fuels are burned in a lime kiln, undesirable substances belonging to the ash, such as Si, Al, Mg, Μη, P y.m.

The process according to the invention is characterized in that the quicklime is quenched in a separate process step in an aqueous solution, whereby a suspension of calcium hydroxide in the aqueous solution is formed. The lime-milk thus formed is then separated from the aqueous solution before the causticization takes place by adding sodium carbonate. The aqueous solution may be pure water, wash tower liquid, purified condensate, external water or other process water. If necessary, the timing can be adjusted with a small addition of e.g. NaOK, silicon liquor, Na 2 CO 3 or soda liquor. For both of the latter, it is suitable that Na 2 CO 2 is reacted with CaO to form sufficient NaOH to bring the pH to the desired level, i.e. 12-15.

In the accompanying drawings, FIG. 1 illustrates a previously known process described above, while FIG. 2 shows a method according to the invention.

Particles that sediment when lime is extinguished, hard burnt lime, stones from kiln masonry and lime treatment are removed from the extinguisher with scrapers or the like. The homogeneous suspension is then passed according to the invention to a separation step, which is preferably formed by a filter, but can also be formed by a sedimentation device or a combination of both. In this case, the excess aqueous solution is separated from the calcium hydroxide, which is then washed with pure water (alternatively with purified theta condensate). Temporary solutions or Na 2 CO 2 solution can also be used as the washing water to adjust the pK value. The lime thus slaked and washed is dosed together with soda liquor for causticization. The strongly temporal aqueous solution is completely or partially recycled back to the extinguisher or discharged to the plant sewer or water treatment. High return recycling requires that the aqueous solution be cleaned of insoluble matter before returning to the extinguisher. Examples of purification processes include lowering the pH and then separating the precipitated material by centrifugation, filtration, sedimentation or separation in cyclones. Preferably, the aqueous solution is passed in excess, especially in the form of wash water, and the removal of dissolved chemicals takes place before the remaining aqueous solution is recycled.

The method according to the invention has several advantages over the previously used technique. Thus, it has been found that several undesirable substances in quicklime. _ ____- - li ..

6,73754 exist in such a form (as such compounds) that they are largely soluble in this proposed quenching method. Particularly valuable compounds with Al and Si can be removed and scaling of NaAl silicates upon black liquor scaling can be avoided. Other trace elements can also be removed, thus avoiding their enrichment in the lye and lime cycle of the plant. Surprisingly, it has also been found that the sedimentation and filtration properties of white liquor / dilute liquor CaCl 2 (mesa) are improved. This results in a more efficient clarification of the white liquor and a better washing and drying of the mesa. The higher dry matter content of the honey results in reduced fuel consumption in the lime kiln. In addition, the control of the extinguishing is facilitated, so that a constant temperature and thus the maximum possible capacity can be maintained without overcooking. Thus, thanks to the better control, the conditions during causticization can also be kept unchanged at optimal levels, which gives the highest possible yield and quality.

Example

In one plant, shell powder was used as fuel in a lime kiln instead of oil. The ash content of the bark powder was about 3%. The ash consisted of several inorganic substances. For example, it had silicon and aluminum concentrations of 14 resp. 12.2 g / kg ash. In addition, treated lime was fed into the lime stream before the lime was burned. The treated lime contained 1.3 g Al / kg and 1.4 g Si / kg.

The previously used quenching method enriched the aluminum, silicon and other unwanted chemicals in the lye and lime cycle. Compared to the combustion of oil in a lime kiln, the aluminum and silicon content of white liquor, for example, increased from 20 resp.

90 to 180 resp. 220. After carrying out the method proposed here (see Fig. 2), the concentrations of 20 mg / l white liquor and 70 mg silicon / l white liquor were measured in white liquor. The honey kiln burned 23.3 tonnes of lime / h (Th = 69%) and 0.04 tonnes of treated lime / h to give 9 tonnes of quicklime.

7 73754

3.6 tons of bark / h were needed for incineration. The quicklime was transported through a mill to a fire extinguisher, where the lime was mixed with the filtrate from the lime milk filter (70 m 2 / h) with vigorous stirring. The lime then quenched, generating heat and the lime grains were ground to a fine slurry. Crushed lime, i.e., grains that were not ground, ("overburned" lime, stones, etc.) settled in the fire extinguisher and was transported away by a cola conveyor. This amount was about 0.030 tons. The residence time in the fire extinguisher was 15 min. From the extinguisher, the lime milk (slaked lime and liquid) at a temperature of about 100 ° C was conveyed to a lime milk filter with a drum filter 2.5 m in diameter and 4 m long, in which the slaked lime was filtered from the liquid and dried in vacuo. The lime was washed on the filter with an aqueous solution (14 m / h) containing 1 part NaOH and 10 parts pure water. Lime with a dry content of 70% was conveyed from the filter by a conveyor belt to a causticizing unit to which soda liquor was added. The filtrate 3 from the drum filter (75 m / h) was pumped through a heat exchanger where the washing liquids were heated to 95 ° C, after which the filtrate was partitioned. One stream was 5 m / h and was allowed to go down the drain, another 3 streams (70 m / h) to the extinguisher.

The following result was obtained at the sampling event: Al Si ash from the shell 14000 mg / kg 12200 mg / kg quicklime 2000 ”2500" liquid part of lime milk 364 mg / 1 320 mg / 1 solid part of lime milk 1240 mg / kg 1574 mg / kg filtrate from the filter 305 mg / 1 264 mg / l _____— - - · Il 8 73754

A comparison between the combustion of oil, bark and bark with the new process section shows the following: bark combustion with the new process section combustion oil fuel consumption GJ / tonne of lime 7.9 8.2 7.7 dry matter content after mesh filter 68-70 60-65 68-70% reactivity of lime in seconds, reaching a maximum temperature of 90 100 180

if a certain amount of all is added to a certain amount of water, the temperature is 85 ° C

amount of inert substances in lime% 1 2,5 1,5 sludge content in clarified white liquor mg / m 80-120 700-1000 150-200

Claims (6)

9 73754 A process for preparing cooking chemicals for the production of cellulose, in which burnt lime from a lime kiln is quenched and then the quenched lime is mixed with soda liquor (sodium carbonate) which is causticized to white liquor, characterized in that the lime is quenched in a separate process step then separated from the aqueous solution before causticization. Process according to Claim 1, characterized in that the pH of the aqueous solution is between 12 and 15. Process according to Claim 1, characterized in that the lime milk separated from the aqueous solution is washed with water before causticizing. Process according to Claim 3, characterized in that the lime milk separated from the aqueous solution is washed with an alkaline aqueous solution before causticizing. Process according to Claims 1 and 2, characterized in that the aqueous solution and the aqueous solution obtained from the lime milk separation step are recycled to the extinguisher. Process according to Claim 5, characterized in that the recycled aqueous solution is purified by adding acid to a lower pH and in that the precipitated substance is separated from the solution. Process according to Claim 1, characterized in that the temperature during quenching is maintained between 60 ° and 120 ° C by cooling or heating the aqueous solution.