FR2514383A1 - Processing chemical prods. in cellulose prodn. - involves slaking quicklime in aq. soln. in separate step and sepg. milk of lime prior to caustifying green liquor to white liquor - Google Patents

Abstract

In a process for treating and converting chemical prods. for cellulose prodn., where quicklime from the limekiln is slaked and the slaked lime is then mixed with the green liquor (NaCO3) caustified to white liquor, the improvement is that the slaking takes place in a separate step and in an aq. soln. and the milk of lime (Ca(OH)2 thus formed is sepd. from the aq. soln. prior to caustification. Process is applicable e.g. for the prodn. of sulphate pulp. Process results in dissolution in the slaking step of most of the substances to be eliminated in the quicklime, partic. Al and Si cpds., so that incrustation of NaAl silicates in the black liquor is avoided. Further the sedimentation and filtration characteristics of the white liquor/extended CaCO3 liquor (lime slurry) are improved, so that the clarification capacity of the white liquor increases and the washing and dehydration of the lime slurry can be improved.

Description

 The present invention relates to a lime washing poeéde and evacuation of non-utllisFes substances obtained in the process of production of cellulose, for example, for the manufacture of sulfate pulp.

This method is particularly applicable to installations where ash-rich solid fuels are used to burn lime again in the case where the fibrous raw material has a high head content.

 In the production of cellulose, according to the process known as the sulphate process, wood chips are transferred into a transformation liquor whose active ingredients are sodium hydroxide (NaOH) and podium mono-sulphide ( Na 2 S). During processing, the hydroxide and sulfide oxidize and form carbonates and sulfates. Also during the processing, about 50% of the wood is dissolved, mainly lignin and lmmd-cellulose, but also the minerals contained in the wood.

 After concentration of the transformation liquor in the evaporators, the enthalpy of the liquor is used in the soda tank where the combustion of the liquor is carried out.

 The melt of the ashes of sodium salt flows from the soda tank to a set, to be dissolved with the aid of extended liquor. The green liquor thus formed is separated from the impurities, as a rule, by the sedimentation process in an apparatus known as a green liquor clarifier.

The green liquor is then used to extinguish the quicklime that is added to a fire extinguisher assembly. In this apparatus, the lime particles which have not been suspended in the green liquor but settle to the bottom are separated and removed.

During the extinction process, calcium hydroxide
Ca (OH) 2 forms with sodium carbonate Na2CO3 (green liquor), sodium hydrate NaOH (digestion liquor) and calcium carbonate CaCO 3 (lime sludge). This reaction is generally known as caustification.

 It takes a certain time to achieve equilibrium in the reaction of sodium carbonate with quicklime and thus, the reaction is allowed to progress in the extinction and caustic containers for periods of time between 1 h and 1 h 30, and even up to 2 h.

 The transformation liquor and the calcium carbonate, i.e. the lime sludge, are separated after the reaction in a white liquor clarifier or by filtration. After separation of the lime sludge, the process liquor is stored and, for this purpose, pumped into tanks from which it can subsequently be withdrawn in metered quantities for use.

 Subsequently, the lime mud is subjected to a recaustification stage where the surplus remaining lime is left in reaction with the green liquor. In this reaction, an extended liquor is obtained which is used to dissolve the ash melt, as mentioned above.

 The lime sludge is dewatered in the vacuum filters, increasing the solids content between 60 and 70%, and is washed on the slurry filters, before being transferred to the (cold) oven rim. lime.

In this furnace, the lime mud is dried and burnt to lime while releasing water vapor and calcium dioxide. The oven is usually heated with oil or gas, using a burner placed in the oven itself.The chemical process is as follows
Reaction in the soda tank

<tb> 2S 4 <SEP> reduced <SEP> atmosphere <SEP> +
<tb><SEP> Na2S <SEP> + <SEP> 2O2
<Tb>
Reaction in fire extinguisher and caustic containers

<tb> CaO <SEP> (s) <SEP> + <SEP> H20 <SEP> t <<SEP> - <SEP><SEP> Ca (OH) 2 (s) <SEP>
<tb> Ca (OH) 2 (s) <SEP> + <SEP> C032 <SEP><SEP> CaCO3 (s) <SEP> + <SEP> 20H
Reaction in the lime kiln

<tb> CaCO3 (s) <SEP>(<SEP> ~~~~~~~ SEP><SEP> CaO (s) <SEP> + <SEP> C02 (s)
<Tb>
In the processes currently used, undesired substances are driven simultaneously with the preparation of lime, such as wood (non-squared wood, carefully graded chips) and process water. In plants using perennial plants (grass, reed, straw, bagasse), bamboo or other kinds of wood containing large amounts of unwanted substances, it is difficult to reuse the transformation chemicals and as a result get rid of it.

 Solid fuel heating is an interesting alternative in lime kilns, as energy prices rise and the use of the whole tree increases. The use of the entire tree has also resulted in a decrease in the quality of the chips as a result of the increase in unwanted substances. When using solid fuels and / or chips from the whole tree in the lime kiln, the same problems occur when perennials are used.

Unwanted elements such as Si, Al, Mn, Mg, Fe and
P are concentrated in all flows of the process, depending on how the circuit is closed Concentrations increase unless equilibrium is reached in the circuit. Usually, there are no other clearing possibilities except with cellulose, green liquor sludge and calcareous gravel.

The concentration of the unwanted substances involves a number of disadvantages, namely - increased inking tendencies of sodium aluminum silicates, especially on heat transfer surfaces, such as in the evaporator, the tank with soda and the digester
The aluminum content is the decisive factor in the precipitatic of sodium aluminum silicates.

- Increased deposits in low flow velocity pipes and slow flow devices, due mainly to reduced sedimentation and reduced lime sludge filtration capacity.

Increased energy consumption, both because the inert materials in the lime are unnecessarily heated and because the dry matter content has been lowered in the lime mud filter, as explained above.

- Lower capacity of the installation, which is the result of the effects outlined above.

 During the extinction of the quicklime, the carbonate solution (green liquor) is burned in quantities dosed with the extinguisher, in the form of several flows, for example at the caustification or recaustification station.

 Considerable amounts of energy are released during the extinguishing process. For fast reactions, temperatures between 90 and 100 are required. Centigrade. Using current technology, it is difficult to control the extinction conditions. Temperatures that are too low result in incomplete extinguishment and, consequently, impaired capacity.

Temperatures that are too high (generating a high energy expenditure) cause the liquid to escape from the fire extinguisher while boiling, and serious dust problems appear during lime supply operations. An attempt to rectify the practical problems caused by the uncontrolled extinction led to the creation of a pressurized fire extinguisher incorporated in a closed circuit.

 According to the present invention, it was desired to obviate the aforementioned drawbacks and to solve the underlined problems.

 The invention relates to a method essentially applicable in installations where solid fuels, such as bark, wood, chips, peat and charcoal, are used in a lime kiln, as well as fibrous and chemical raw materials which have a high content of non-useful substances. Raw materials of this type can be chips from the whole tree, castin, water, bagasse, bamboo, reed, straw and the like. In fuel consumption above in lime kilns, unwanted substances appearing in ashes such as Si, Al, Mg, Mn, Fe, P and are added to the lime.

 The process according to the present invention is characterized in that the quenching of quicklime stops a separate step and in an aqueous solution to produce a suspension of calcium hydroxide. The lime milk thus formed is then separated from the aqueous solution before caustification, which is carried out by addition of sodium carbonate. The aqueous solution could be pure water, wash water, purified condensate, alkaline water or other process water.

The alkalinity may, if necessary, be adjusted by a small addition, for example NaOH, white liquor, Na 2 CO 3, or green liquor.

In the last two cases, Na2CO3 reacts with CaO, forming
NaOH in sufficient quantities to adjust the pH value to the desired level, ie 12-15.

In order to fix the object of the invention, without however limiting it, in the corrugated drawings
Figure 1 illustrates the known process for the production of cellulose;
Figure 2 schematically illustrates the method according to the invention.

According to FIG. 1, the markers (1) to (25) illustrate the components involved in the process, namely
White liquor (1), chips (2), digester (3), liquor (4), wash water (5), evaporator (6), steam (7), sodium hydroxide (8), whitening pulp (9), lime slurry filter (10), extended liquor (11), green liquor (12), lime kiln (13), white liquor (14), washed lime mud dehydrator (15), clarifier white liquor (16), recaustification (17), fuel oil (18), melt mixer (19), green liquor clarifier (20), lime (21), limestone gravel (22), caustification (23), sludge green liquor (24), lime extinguisher (25).

According to FIG. 2, the marks (26) to (42) illustrate the components involved in the process of the invention, to its see
Washing water (26), green liquor (27), heat exchanger (28), white liquor clarifier (29), caustification (30), filter (31), filtrate (32), sewage (33), bark ( 34), quicklime (35), lime kiln (36), lime mud (37), lime mix (38), fire extinguisher (39), lime milk (40), limestone gravel (41), pure water or alkaline solution (42).

In order to make the object of the invention more concrete, it is now described in a preferred form
Particles that have settled during the accident, ie hard lime, brick masonry and lime stone, are removed from the fire extinguisher using scrapers and tools. Similar. According to the process of the invention, the homogeneous suspension is then subjected to a separation step which is preferably carried out by filtration but could likewise be carried out by sedimentation in a sedimentation plant or by a combination of the two means. In this separation step, a surplus of the aqueous solution is separated from the calcium hydroxide which is then washed, preferably with pure water (alternatively purified condensate) .Also the alkaline solutions or the Na 2 CO 3 solution can be used as wash water to adjust the pH value
The resulting slaked and washed lime is brought in measured quantities with the green liquor to the caustification group. The strongly alkaline aqueous solution may either be wholly or partly recycled to the fire extinguisher or vented to the sewer system or other water treatment systems.

When large quantities are recycled, it is necessary to purify the aqueous solution of the dissolved substances contained therein before returning the solution to the fire extinguisher. As examples of purification processes, it is possible to mention the reduction in pH value followed by the separation of the precipitated substances, by means of centrifugation, filtration, sedimentation or cyclone separation. Preferably, an aqueous solution is added in excess, particularly in the form of wash water, and the release of the dissolved chemicals occurs before recycling the remaining aqueous solution.

 The method according to the present invention provides a number of advantages over the technology used and known. For example, it has been found that the majority of the substances to be removed contained in quicklime, will dissolve largely in the extinguishing process carried out according to the invention. In particular, appreciable compounds containing Al and Si can be removed and the encrustation of NaAl silicates in the black liquor can be avoided. Also, other substances that leave traces can be eliminated and in this way their concentration in the liquor digestion and lime cycle of the plant is prevented. Surprisingly, it has also been found that sedimentation and filtration properties of white liquor / extended CaCO3 liquor (lime mud). As a result, the clarifying capacity of the white liquor increases and the washing and dewatering of the lime mud can be improved. As a result of the higher dry matter content of lime sludge, fuel consumption in the lime kiln is reduced.

In addition, the quenching process becomes easier to control, which guarantees a constant temperature, resulting in maximum capacity, without excessive boiling. Also, because of the improved control, the conditions existing during the caustification process can be kept constant at optimum levels, giving white liquor yields and maximum quality.

 As an example of non-limiting application of the process according to the invention, was used for the operation of the installation, bark powder instead of fuel oil, as heating fuel of the lime kiln. The ash content of the bark powder was about 3%. Ashes consist of several inorganic substances. For example, the silicon content and the aluminum content were respectively 14 and 12.2 g / kg of ash. In addition, lime preparation was added to the lime mud flow before the lime mud was burned again. The lime preparation contains 1.3 g Al / kg and 1.4 g Si / kg.

 Using known techniques, aluminum, silicon and other unwanted chemicals in the digestion liquor and lime slurry cycle are concentrated.

Compared with oil heating in the lime kiln, the aluminum and silicon content in the white liquor increased by 20 and 90 to 180 and 220, respectively.

After adoption of the process according to the invention, see FIG. 2, contents of 20 mg of aluminum / l and 70 mg of silicon / l were found in the white liquor. In the lime kiln, 23.3 tonnes of calcium carbonate / h (Th = 69%) and 0.04 tonnes of lime / h were re-burned in 9 tonnes of quicklime. To re-burn, it took 3.6 tons of deco / h. The quicklime was transported to a mixer and then to the fire extinguisher where the lime was mixed by vigorous stirring with the filtrate (70 m3 / h) obtained by a lime milk filter. Thus the lime has been extinguished, while giving off heat, and the nodules of lime have been broken up into fine sludge. The calcareous gravel, that is to say the non-fragmented nodules (inert lime, stones, & c. ..) were deposited in the fire extinguisher and were removed by means of a squeegee transporter. It has been found that this amount is about 0.030 tons / h. The residence time in the fire extinguisher was 15 mm. From the fire extinguisher, the milk lime to about 1000 -C (slaked and liquid lime) was transported to the lime milk filter-which is a filter drum having a diameter of 2.5m and a width of 4 m . In the filter, the slaked lime was filtered from the liquid and was dehydrated under vacuum. The lime was washed in the filter with an aqueous solution (14 m 3 / h) comprising 1 part NaOK and 10 parts pure water. From the filter, the lime now having a solids content of 70%, was directed on a belt conveyor to the caustification assembly where it was added.
Liqueurverte. The filtrate (75 m3 / h) from the filter drum was pumped by a heat exchanger in which the washing liquid was heated to 95 ° C. Subsequently, the filtrate was divided into two streams, one of which was a flow rate of 5 m3 / ha was evacuated to the sewers, and the other, at a flow rate of 70 m3 / ha, was sent to the fire extinguisher.

Following a sample, the following results were obtained
Al Si
Bark ash 14000 mg / kg 12200 mg / kg
Quicklime 2000 mg / kg 2500 mg / kg
Liquid fraction of lime milk 364 mg / l 320 mg / l
Solid fraction of lime milk 1240 mg / kg 1574 mg / kg
Filtrate after filtration 305 mg / l 264 mg / l
A comparison between oil heating and bark heating carried out according to the invention gave the following results:
Heating Heating Heating
bark se - bark with oil
the proci
inventi
tion.

Fuel Consumption GJonne Lime 7.9 8.2 7.7
Dry matter content after lime filter,% 68 - 70 60 - 65 68 - 70
Reactivity of the lime in seconds, to obtain the maximum temperature, provided that a predetermined quantity of water is added at 85 ° C 90 100 180
Inert materials in lime, in X 1 2,5 1,5
Sludge content in clarified white liquor mg / m3 80 - 120 700-1000 150-200
The invention is not limited to that of these modes of application, nor to those of the embodiments of these various parts having more especially indicated; on the contrary, it embraces all variants.

Claims (7)

R E V E N D I C A T IO N S  -1- A process for treating and transforming chemicals for the production of cellulose, wherein the quicklime of the lime kiln is extinguished and said slaked lime is then mixed with the caustic white liquor (sodium carbonate) in white liquor characterized in that the lime is quenched in a separate process and in an aqueous solution and the lime milk (Ca (OH) 2) thus formed is then separated from said aqueous solution before caustification.  -2- Process according to claim 1, characterized in that the pH of the aqueous solution is between 12 and 15.  -3- Method according to claim 1, characterized in that the lime milk which has been separated from the aqueous solution is washed with water before caustification.  -4- Process according to claim 3, characterized in that the lime milk which has been separated from the aqueous solution is washed in an alkaline water solution before caustification.  -5- Process according to claims 1 and 2, characterized in that the recycling is carried out extinguisher, the aqueous solution obtained from the lime milk separation step from the aqueous solution.  -6- Method according to claim 5, characterized in that the purification of the recycled aqueous solution is obtained by adding acids to a lower pH value and separating the precipitated substances from the solution.  -7- Method according to claim 1, characterized in that during the quenching process, the temperature is maintained between 60 and 120 C, cooling or heating the aqueous solution.