FI106641B - Process for separating foreign matter from lime sludge - Google Patents

Description

1 106641

METHOD FOR THE EXTRACTION OF FOREIGN MATERIALS FROM MAN

The present invention relates to a process for the extraction of foreign matter in connection with a chemical recovery system for a pulp mill, in particular from a calcium-containing substance used mainly therein, namely lime and lime, and from its lye cycle.

In the context of this application, contaminants refer to various hazardous substances, such as silicon, phosphorus, vanadium, sulfur, enriched in the chemical cycle of the pulp making process and from various sources of raw materials. The sources of contaminants are mainly wood raw material (phosphorus), make-up 15 lime (silicon) and fuel oil (vanadium, sulfur), if used as fuel for the lime kiln. Some of the raw materials used to make pulp may also contain high levels of silica (SiO 2) in their cellular tissue. The most common of these are annual plants 20 such as bamboo, sugar cane, rice and wheat. But it has been observed that some tropical wood species also contain silicon to an extent detrimental to the pulp production process.

25 In the manufacture of pulp, the cooking liquor used in the sulphate process, i.e. the black liquor, is evaporated, burned and the resulting residue, so-called. the soda melt is dissolved in water. The resulting green liquor is causticized into a new cooking liquor. In causticization, sodium carbonate 30 in green liquor is reacted with burnt lime (CaO) as follows. First the lime is extinguished:

CaO + H 2 O -> Ca (OH) 2 (1) 35 This is followed by the actual causticization reaction:

Ca (OH) 2 + Na 2 CO 3 -> 2NaOH + CaCO 3 (2) 106641 2 The white liquor containing sodium hydroxide thus obtained and the calcium carbonate (lime juice) are separated and the white liquor is recycled to the pulp. The separation between white liquor and lime mud can be done either in the clarifier or by filtration.

5 White liquor filters are usually candle filters or drum or disk filters which are either pressure or vacuum filters. The honey is washed to remove alkali from it. The honey is thickened in a lime filter for incineration in a calciner, such as a lime kiln, where the lime 10 is regenerated into calcium oxide for return to causticization. This lime cycle is called the lime cycle.

In the pulp mill's chemical recovery system, the material stream described above 15 is partially purified so that the so-called solids are insoluble in leaching the melt. the green bean deposit is removed. In this way, unwanted substances can be removed from the process.

The problem with the process is that certain contaminants are poorly removed as they are not soluble enough and thus remain largely in solution. Such substances include phosphorus, silicon and vanadium and many others. These compounds have a tendency to causticize in lime mud and thus enrich the lime cycle. For example, silicates precipitate as calcium hy drosilicates among the lime used for causticization. Thus, contaminants are enriched in lime over time, i. the man gets dirty. Phosphorus, on the other hand, tends to concentrate in the fly dust of the lime kiln of the enriched-30 countries, and it is this form that is preferred to remove it. Fly dust has in some places been used as a phosphorous soil conditioner.

35 Honey fouling causes various problems, such as the formation of stratification rings and spheres in the lime kiln, the difficulty in filtering white liquor and the passivation of burnt lime 106641 3, ie binding to non-reactive compounds. For this reason, at least some of the lime must be changed from time to time, i.e. part of the lime must be continuously removed and the lime cycle must therefore be kept open to some extent.

A further problem is that the lime sludge to be removed has to be either landfilled or used to neutralize the acidic bleaching waters of the pulp, whereby phosphorus 10 and other impurities are released into the waterways.

In addition, as the pulp mill's water cycle is now being closed and the bleaching waters reintroduced into the process, the contaminant load in the chemical cycle is increasing and the disadvantages are increasing.

Instead of destroying contaminated lime, it has been proposed to reduce silica already from black liquor by reducing. : pH of the lye with carbon dioxide to about 9.1- <* <20 10.2. Black liquor in ionic form, mostly HSi043 'and

In SiO33, the solubility of the dissolved silica decreases and precipitates as a colloidal silica gel. In a method developed by the United Nations Industrial Development Organization1 (UNIDO) and the Swedish International Development Authority1 (SIDA), low black liquor (6 g SiO 2/1) has been reported to yield up to 90% of silicate. The carbon dioxide is then bubbled into the black liquor in a bubbling reactor, after which the precipitated silica is filtered off. Similar solutions have been developed by CPPRI and Lurgi 30.

In the same way as black liquor, silica can be separated from green liquor with the help of carbon dioxide. Green liquor may contain about 10-20 g SiO2 / l silicate, depending on the raw material used - sometimes more - which is significantly more than the corresponding low black liquor, which can be expected to achieve a better yield. Also, separating the silica precipitate from the green liquor is not as problematic as the black liquor due to the lack of organic material. One disadvantage of this process is the release of odorous sulfur compounds, e.g. 5 hydrogen sulfide, when sulfate broth is treated with carbon dioxide.

Another known way of removing silicon from green liquor is to add lime (CaO or Ca (OH) 2) to the green liquor, whereby the silicon is precipitated with the lime and this high silicon lime can be removed. It has been suggested that the causticization of the vi-lime liquor be carried out in two stages, with the first stage adding a portion of the total amount of lime needed for the causticization. The silicon then precipitates with the lime slurry formed as calcium silicate, which silicon-rich batch of lime is discarded. The rest of the lime is added in a second step, which lime is recycled in the process. A limitation of this process is that a high degree of silicon removal is obtained only when the green liquor has a high silicon content, e.g., greater than 12 g / kg H 2 O.

It is an object of the present invention to overcome the above drawbacks.

It is a particular object of the invention to provide a method for separating harmful contaminants from the chemical cycle of a pulp mill, in particular from the lime cycle.

A further object of the invention is to extend the lifetime of lime slurry in the pulp process.

It is a further object of the invention to provide a process by which chemicals useful in the pulp process can be more accurately recovered and reused. In particular, it is noted that the above impurities are readily separable and can be removed with respect to this impurity. dilute solutions.

It is a further object of the invention to provide a method by which the chemical cycle of the pulp process can be made as closed as possible.

As regards the features characterizing the invention, reference is made to the claims.

10

The invention is based on the surprising observation made in the studies performed that by leaching a lime-containing substance such as lime in carbonate or bicarbonate solution, preferably concentrated carbonate or bicarbonate solution, the hardly separable anions are dissolved and thus separated from the lime or lime. The invention thus utilizes Liuosity values and yields in a new way under the circumstances.

In the context of the present invention, lime refers to calcium oxide (CaO) or calcium hydroxide (Ca (OH) 2) containing material which is used at the factory, for example, to carry out causticization reactions. Honey is intended to be a material containing calcium carbonate (CaCO3).

The solubility of sparingly soluble substances is often expressed by the solubility product as follows:

Lm. «» = [Me] "[X] n (3) where Me is a metal ion; X is an anion which forms a sparingly soluble salt with that metal ion; m and n are integers; brackets [] represent the concentration and L is the solubility product which is constant under certain conditions.

The solubility product is, in fact, only constant in dilute solutions, where the ionic strength, that is, the sum expressing the sum of all ions, is low. If the solution contains a large amount of salts in solution, i.e. a high ionic strength, equation (3) applies only if the concentrations are changed to activities. The relationship between activity and concentration is: a = f [A] (4) where a is the activity of the substance; [A] is its concentration and f is the activity factor. The solubility product is then: L »u Zn = 3-Li 3 * (5) 10 Activity coefficients are often not available for concentrated solutions of a complex composition. However, their knowledge is not necessary to determine the functionality of the method according to the invention.

If the anion X is a carbonate, increasing its activity will cause the activity of all the metal ions which form the sparingly soluble carbonates to fall, respectively, while the solubility product remains constant. This requires that they be present in sufficient concentration in solution 20 to be in equilibrium with the solid carbonate.

We can be any metal ion that fulfills these conditions. The carbonate activity may be increased by a readily soluble salt such as sodium carbonate. As the activities of metal ions decrease, it follows from equation (5) that anions X other than carbonate, which also form sparingly soluble compounds with the same metals and which are precipitated in lime, dissolve, i.e., the activities of the anions in solution 30 increase.

The metals which form the sparingly soluble carbonates under these conditions and whose activity in the solution can be reduced are primarily calcium, barium, iron, manganese, magnesium, etc. The anions whose activity can be increased by the solution by this process are primarily P043 ', Si032', V043 ', S042', etc.

106641 7

The exact form of the anion, such as SO * 2 'vs. S032' or VO * 2 'vs V2Os2', may not be known, nor is the metal with which it is precipitated. The process of the invention only requires that the anions 5 and cations are in equilibrium with the solid phase according to equation (5).

Examples of sparingly soluble compounds whose anions can dissolve as the metal ion activity decreases are as follows:

Lc * 3 (after *) 2 = ac.3. aP0 * 2 calcium phosphate l> c «sio3 = ac..aSio3 calcium methate dirt LCaso * = ac« .a8o42 calcium sulfate l <ca3 <vo *) 2 - aca3 · avo42 calcium vanadate 15 Lc «bvo4 = aca3 - AHvo * calcium pa po4) 2 - ar «3.3po42 ferric phosphate LBaS0 * = aBa.aS04 barium sulfate

In the process of the invention, the lime slurry is suitably soaked in a concentrated solution of carbonate or bicarbonate, e.g. potassium, sodium or ammonium carbonate or bicarbonate. The increased concentration of carbonate or hydrogen carbonate depresses the calcium content of the solution as required by the solubility product. Sa-25 also occurs for other cations which form sparingly soluble carbonates or bicarbonates. As the concentrations of calcium and other cations decrease, the solubility of other anions such as phosphate, silicate, vanadate, sulphate, sulfite, etc., which form sparingly soluble salts, increases correspondingly with the solubility yields of the salts. Thus, these anions precipitated in the lime are dissolved. After soaking, the lime and the solution, i.e. solubilized foreign matter is separated from each other in any way known as such.

'106641 When used as a soaking solution of lime mud, for example, a solution of sodium carbonate, the sodium is purified and reused or recycled for the preparation of cooking chemicals. Separation of sodium carbonate by crystallization is therefore advantageous since the concentration of sodium carbonate remaining in the contaminant concentrate is then minimized (see Table 1 solubility of sodium carbonate in water).

By selecting a suitable carbonate concentration for leaching the lime slurry, a suitable crystallization temperature and advancing the crystallization well, the volume of the effluent concentrate to be removed can be adjusted. A suitable concentration of carbonate, e.g. when using sodium carbonate, is about 5 g / l to a saturated solution, preferably about 200-400 g / l. Crystalline liquid sodium carbonate having 7 or 10 crystalline water binds a large amount of water, and foreign matter can thus be concentrated to a small volume of liquid. The limitation is the solubility of the contaminants. If they are exceeded, the foreign matter precipitates with sodium carbonate and the purification efficiency of the crystallization »20 decreases. If necessary, dissolution of the carbonates in water and recrystallization can be used, whereby the amount of impurities transported back to the carbonate for leaching can be reduced.

25 Table 1 ~ T Solid g mol / Na2CO3 / Density Steam - 1000 g HjO 100 (g / g) pressure

HjO Sat.Soi Hg -2.10 Na2C03 * 10H20 + Ice 0.575 6.10 5.75 1.056 0 ".66 7.0 6.54 -. 5" .84 8.90 8.2 10 "1.14 12.1 10.8 15" 1.55 16.4 14.1 1.1515 12.3 20 "2.09 22.2 18.1 1.1941 16.9 25 "2.77 29.4 22.7 1.2416 21.4 30" 3.70 39.2 28.2 1.342 26.8 9 106641 32.00 Na2CO3-10H2O + 4.28 45.4 31.2 - 29.0

Na 2 CO 3 * 7H 2 O '32 .96 Na 2 CO 3 · 10H 2 O + 4.71 49.9 33.3 - 29.5

Na2CO3 * H2O '30 Na2CO3 * H2O 4.78 50.7 33.6 35.37 Na2CO3 · 7H2O + 4.67 49.5 33.1 34.0

Na2C03 * H20 40 Na2C03 · H20 4.60 48.4 32.8 43.6 50 "4.48 47.5 32.2 74.1 60" 4.37 46.3 31.6 121.5 70 "4.30 45.6 31.3 192.7 75" 4.28 45.4 31.2 239.8 80 "4.26 45.2 31.1 296.2 90" 4.24 44.9 31.0 442.4 100 "4.22 44.7 30.9 631.7 104.8 "4.21 44.6 30.8 760.0 109 Na2C03 · HjO + Na2C03 4.20 44.5 3 0.8 1.15 (Atm) 110 Na2C03 4.20 44.5 30.8 1.19" 113 "4.20 44.5 30.8 120" 4.03 42.7 29.9 1.65 "130" 3.86 40.9 29.0 2.25 "140 "3.71 39.3 28.2 3.02" 150 "3.57 37.8 27.4 4.01" 160 "3.44 36.5 26.7 5.27" 180 "3.16 33.5 25.1 8.67" 200 "2.89 30.6 23.4 13.7" 220 "2.56 27.1 21.3 21.0" 240 "2.16 22.9 18.6 30.9" 250 " 1.95 20.7 17.1 37.0 "260" 1.75 18.6 15.7 44.2 "280" 1.32 14.0 12.3 61.7 "300" 0.88 9.3 8.5 83.8 "350" 0.19 2.0 2.0 166 "10 106641 The temperature at which leaching of a calcium-containing substance such as lime is effected affects the leaching efficiency. The high temperature 5 is advantageous for the efficiency of the soaking, but the use of the process is economical s benefit resulting from pulping of waste heat or low-energy values, i.e. less than approx. 85 ° C lauhtei-O. Thereby, there is no need to use the more valuable primary-10 energy in the form of steam. A suitable soaking temperature in one embodiment of the process of the invention is the boiling point of the 20 ° C solution, preferably 80-110 ° C. The boiling point depends on the concentration and pressure of the carbonate-containing solution used. If desired, leaching of lime slurry 15 may be carried out at normal pressure or at desired overpressure or vacuum.

For the soaking of honey, the solution / lime ratio (solution weight / honey dry weight) may be from about 2 to 20, preferably from 20 to 15.

In the process of the invention, the lime slurry and the carbonate solution used for its soaking, preferably sodium carbonate broth, are separated, for example, by filtration, centrifugation, precipitation, or the like by a known method. In addition, the honey may be washed with water to enhance purification.

Preferably, the carbonate can be crystallized from a carbonate solution containing foreign matter. The carbonate solution and the solution containing the contaminants can preferably be separated.

Sodium carbonate, when crystallized at 5-20 ° C, produces 35 pure decahydrate crystals and at 35-90 ° C it produces monohydrate crystals. Crystallization can be done, for example · . cooling crystallization to give decahydrate crystals or evaporation to give the monohydrate crystals. Conventional evaporation provides heat which evaporates the water and thus raises the concentration of sodium carbonate in the solution above the required level of crystallization. The carbonate solution may be evaporated to crystallize the carbonate in a manner known per se in a single or multi-stage evaporator. Evaporation can take place in tubular, lamella or flash devices.

In one embodiment, a suitable crystallization temperature range for carbonate is from -2 to 30 ° C. In the final step, the solution required a temperature of almost -2 ° C to effect crystallization as completely as possible. Cooling can be done with outdoor air that is sufficiently low for most of the 15 years, at least in the Nordic countries.

If desired, the carbonate may preferably be dissolved in water, water, preferably lime sludge washing water may be added thereto, and the resulting liquid may be used for soaking lime sludge.

A suitable temperature for dissolving the sodium carbonate crystals is, for example, about 50 ° C. In order to raise the temperature to the desired level, the condensate used to heat the soaking reactor can be used and thus the heat can be utilized economically.

If desired, the separated carbonate can be returned to the chemical cycle.

30

Foreign matter solution, so. the concentrate can be removed from the process. The foreign matter concentrate may be subjected to one or more recrystallizations to recover useful chemicals, mainly carbonate crystals. 35

The above has mainly described the use of carbonate for the purification of lime. However, in addition to carbonate or instead of 12 106641 (0-100%), hydrogen carbonate in solution in which lime mud is soaked can also be used. Hydrogen carbonate recovery can be carried out in a manner similar to carbonate recovery.

5

The amount of water used for washing the honey and the extraction solution containing foreign matter can advantageously be balanced so that the total amount of water in the process is kept constant.

10

One interesting application of the present invention is specifically the removal of silicon or phosphorus from lime or lime rich in them by dissolving these substances in lime or lime in a carbonate solution. Such a soluble carbonate-containing solution forms a very sparse calcium compound such as calcium carbonate with calcium and keeps the calcium solubility low so that no silicate precipitates. Most preferably, such solutions include sodium carbonate-containing green liquor or other suitable solution containing carbonate, particularly potassium carbonate. If the carbonate solution, such as green liquor, also contains silicon, the invention is preferably applied at a silicon content of about 0.8 to 12 g SiO2 / kg H2O, most preferably 1 to 6 g SiO2 / kg H2O. Once the silicon has been removed from the lime or lime solution, the solution can be further purified from these substances. In this case, an amount of lime is added to the lye so that the silicon is precipitated from the lye. The lime obtained from the burning of previously purified lime or more reactive lime obtained, for example, by burning porous limestone, may be used in the precipitation. The advantages of the invention are apparent here, since the amount of lime needed to precipitate the silicon is substantially less than the amount of purified lime or lime sludge, so that the amount of lime sludge discarded in the circuit and thus discarded is small.

• is 106641

Soluble silicon reacts with lime to form calcium hydrosilicates when the silicon content in the causticizing green liquor is higher than in the resulting white liquor. These hydrosilicates react with lime in the lime kiln 5 to form calcium silicates with a higher CaO / SiO 2 ratio than the original calcium hydrosilicate. The most likely calcium silicate formed is β-dicalcium silicate. As a result of extensive thermodynamic investigations of silicon and lime compounds, experimental studies and computer simulations based thereon, it has been found that, upon causticization, the solution does not become supersaturated with β-dicalcium silicate with lime in the solution until the amount of silicon dissolved in the solution and the amount of lime added have reached a certain value. As the causticization proceeds, the solution becomes supersaturated with the calcium silicate hydrate precipitated from the solution, whereupon 20% of the silicon in the solution begins to precipitate as calcium silicate hydrate. Such compounds include, for example, hydrosilicates such as tobermorite (4CaSiO3 * Ca (0H) 2) and jenite (4Ca-SiO3 * 5Ca (0H) 2).

It is particularly advantageous to apply the invention to causticization at a pulp mill. Most preferably, the causticisation is carried out in two steps, whereby the first step in the flow direction of the green liquor is added 40-90%, preferably 50-70% of the total amount of calcium required for causticization (total causticization required for causticisation ). With the addition of said amount of lime in the first step, the silicon does not precipitate, but on the contrary, any silicon present in the lime 35 dissolves in the lime in its lye. Thus, the first causticization step produces silicon-pure lime slurry having a silicon content o-14 106641 lower than that of the lime fed for causticization. The remainder of the amount of lime required for causticization, i.e. 10-60%, preferably 30-50%, is added to the final stage, whereby the silicon contained in the lye is enriched in the resulting lime. A portion of these 5 lime slurries can be removed from the process to reduce lime cycle silicon load, and the remainder is mixed with the lime lime from the first step. The honey mixture is regenerated into lime in a lime incinerator, such as a lime kiln. In this way, the concentration of silicon and / or phosphorus in the chemical cycle of the pulp mill can be adjusted.

The green liquor time concentration may also have an effect on the practice of the invention. As is known, green liquor is formed by dissolving the chemical melt resulting from black liquor burning in water or lean white liquor, which is formed by washing lime slurry after separation of white liquor. One of the main purposes of honey washing is to remove the alkali (NaOH) from the lime as closely as possible. If the separation between white liquor and lime is not effective, the more alkaline remains in the honey-20, and from there to lean white liquor. If alkali-rich lime liquor is used to dissolve the molten liquor, the green liquor has a correspondingly higher time-content, which in turn reduces the proportion of sodium carbonate in the green liquor. The presently increasing pressurized 25-disc filter provides a more efficient separation of white liquor and lime so that the NaOH content of lean white liquor is only about 5 g / l in lime washing.

By keeping the alkaline content of the green liquor as low as possible by dissolving the melt in a solution containing as little NaOH as possible, the proportion of soluble sodium carbonate in the green liquor increases. In this case, the lime added to the liquor reacts to a greater extent according to the reaction equation (2) and does not precipitate soluble silicon. Therefore, the amount of lime to be added to the causticization step 35 can be increased and more silicon dissolved in this causticization step, and. . thus more purified lime is also obtained.

106641 15

Accordingly, the amount of lime in which the silicon is enriched in the final causticization step is correspondingly smaller.

A similar result is achieved by lowering the silicon content of the vi-5 herl liquor before causticization. This can be done, as previously described, by pretreating green liquor or black liquor with carbon dioxide gas. Another preferred method of reducing the silicon content of green liquor is also described in FI patent application 950692, which processes a sodium carbonate-containing melt obtained from black liquor combustion to recover the sodium carbonate as a solid while separating the silicon and / or phosphorus .

Causticization can also be performed by the countercurrent method, whereby the liquor and lime flow in opposite directions. The process preferably includes two or more steps. Then all the lime needed for causticization is added in relation to the lye flow in the last step. Between the stairs, lime / lime and lye are separated. The liquor is led to the next step and the separated lime / lime slurry is led to the previous step in terms of the lye flow. The impure lime slurry is the lime slurry separated from the last step, from which a small portion (e.g. 1/5) is removed from the process, and the remainder is led against the lye stream and removed from the first stage.

The present invention can be applied in a pulp mill: in locations other than causticization itself. Part of the lime mud stream may be treated with green liquor or the like prior to thickening the lime slurry, for example, in a mixing tank to dissolve silicon. The lime slurry is then separated, washed and passed to a lime slurry filter for thickening.

106641 16th

In another embodiment, the lime slurry is treated outside the factory building, for example, in a manner similar to that of ore-enriched poor ores. heap leaching method. In this case, the leaching medium is 5 green liquors or other carbonate solution which is used to treat the leachate to remove silicon and related impurities. This embodiment may be useful in some circumstances, such as the treatment of old silicon-rich lime stocks, after which such purified lime may be re-used. Silica is removed from the honey treatment solution by liming, allowing the solution to be recycled, but the silicon has been enriched to a much lesser extent than the original, which is discarded.

15

Thanks to the invention, i. by purifying lime or other calcium-containing material according to the invention, the life of the lime can be substantially extended. Further, the invention enables cost savings through longer life of lime sludge 20 and other chemicals: the need for purchased lime is reduced, lime landfill transportation and charges are reduced. For example, the invention makes it possible to remove silicon from liquids containing silicon in an undesirable amount less than could be removed by known methods.

Further, the invention results in less harmful compounds being released into the environment during pulp production. The invention further reduces the need for water to open the lime cycle, i.e. it helps to close the chemical cycle.

* «

The process of the invention will be described in more detail with the aid of the following exemplary embodiments, with reference to the accompanying drawings, in which: Figure 1 shows a flow chart of an embodiment of the method of the invention; 106641 17 Figure 2 shows the contaminant content as a function of solution / haymaking ratio; Figures 3-8 show the concentrations of the substance in solution after removal of Na 2 CO 3 by crystallization; Figure 9 illustrates a simulation experiment to illustrate the principle of the present invention; and Fig. 10 illustrates an embodiment of the present invention in connection with causticization.

Figure 11 shows a flow chart of an experiment using the method of the invention;

In Figure 1, the lime slurry 1 is led to a soaking vessel containing a concentrated carbonate-containing solution, e.g. sodium carbonate. After soaking, the lime slurry and the soaking solution are separated from each other, for example by filtration, after which the separated foreign carbonate solution 2 is introduced into a crystallization vessel and the separated lime slurry 3 is passed to washing. The resulting carbonate crystals 7 are dissolved and the thus obtained carbonate solution 8 is returned to the soaking vessel. The purified 20 lime slurry 10 is recycled to the pulp process. After crystallization, the separated foreign matter solution is further passed to a further crystallization from which the resulting carbonate crystals are dissolved and the dissolved carbonate crystals 6 are recycled to the original crystallization vessel. The foreign matter solution 11 obtained by further crystallization is removed from the process.

EXAMPLE 1. Effect of Amount of Soaking Liquid Used on Contaminant Content.

A sample of honey was soaked in sodium carbonate solution (solution / lambda ratio / solution weight / lambda dry weight)) from 3.9 to 19.6. The solution was prepared by dissolving 400 g of Na 2 CO 3 in 1000 ml of water. The temperature was 90 ° C and the soaking time was 6 h. The mixture was stirred during the experiment. After soaking, the solution and the sludge were separated by warm suction filtration. The man was washed with water. ·· phosphorus. the concentration was determined in lime pulp and in carbonate solution. The results are shown in Figure 2. The analysis results are shown in Table 2.

Table 2 5 ________—-—-; -

Solution / lime also P g / kg P residue% 1 Untreated lime 0 1.83 100.0 2 Treated lime 3.91 0.744 40.7 3 _ «- 7.82 0.328 17.9 10 4 -« - 15.65 0.109 6.0 5 - 19.56 0.095 5.2 EXAMPLE 2. Removal of foreign matter 15 Samples of honey were soaked in sodium carbonate solution as in Experiment 1 at a solution / lambda ratio of 9.8. After soaking, the lime slurry was washed twice with water at a water / lime slurry ratio of 2.8. After treatment, the concentrations of the various substances were determined in lime. The results are shown in Table 20.

Table 3. Concentrations of different substances in the lime pulp before and after treatment.

25 Original Processed blending lime

Concentration Concentration Removal g / kg g / kg%

Phosphorus P 1.83 0.343 81.3 • Si Si 0.643 0.273 57.5

Aluminum Al 0.754 0.710 5.8

Sulfur S 5.30 0.56 89.4

Vanadium V 39.5x10'3 9.4x10'3 76.2 • «19 106641

The results show that phosphorus, sulfur and vanadium can be effectively removed from the lime at a solution / lime ratio of 9.8. Silicon can be removed fairly efficiently 5 while aluminum is poorly removed.

Examples 1 and 2 show that it is possible to remove phosphorus very effectively by carbonate solution leaching. The very high solution / lime ratio results in the need to crystallize large amounts of carbonate to purify a small amount of lime. Running the process economically requires optimization of the solution / lime ratio, which must take into account that lowering the solution / lime ratio reduces the cleaning efficiency.

EXAMPLE 3. Effect of Soaking Conditions The effect of different conditions on the removal of different contaminants (phosphorus, silicon, and aluminum) was tested using different solution-to-mana ratio, different soaking time, different temperatures, and different concentrations of carbonate addition solutions. The results are shown in Tables 4, 5 and 6.

The results show that the sodium carbonate solution plays a crucial role in the dissolution of the phosphorus. Just. ' With water, the solubility of phosphorus is very low, but increasing it significantly increases the carbonate content. If the sodium carbonate solution is nearly saturated, the solution / lime ratio is sufficiently high and if the lime sludge is washed sufficiently with water, up to 98% phosphorus removal can be achieved. However, running the process in this way is probably not economically viable.

Table 4. Phosphorus removal under various conditions.

20 106641

Solution / Soaking Time T Distribution of Phosphorus in Lime Solution Solution Honey Removal ® Ratio h eC mg / kg * mg / kg% 10 water 1.5 0.04 1370 0.003 10 water 96 75 2.14 1330 0.16 10 water + 96 75 143 1200 10.6 10 5 9/1

Na 2 CO 3 10 water + 6 70 102 1240 7.6 5 g / l

Na2CO3 10 water +6 70 628 745 45.7 15 50 9/1

Na 2 CO 3 16 water 6 90 1234 23.5 98.1 1000 mL *** +

Na2CO3 20 400 g ** * mg of phosphorus in solution per kg of lime juice ** Almost saturated solution *** Washing in water / lime ratio 30

Table 5. Silicon removal.

Solution / Soaking Time T Distribution of silicon lime solution Solution Honey Removal ratio h ° C g / kg g / kg%: 16 water 6 90 6.24 0.40 94.0 1000 ml * +

Na2C03 400 g • * Wash with water / male ratio 30

Table 6. Aluminum removal 21 106641

Solution / Soaking Time T Distribution of aluminum in lime solution Solution Honey Removal ^ ratio h ec g / kg g / kg% 16 water 6 90 0.135 0.245 35.5 1000 ml * +

Na2C03 10 400 9 * Wash / water ratio 30

For silicon, 94% removal is achieved under the same conditions as phosphorus. Only about one third of the aluminum was removed, despite the effective conditions.

EXAMPLE 4. Purifying Sodium Carbonate11i Solution for Soaking 20 After soaking, the sodium carbonate solution must be cleaned for reuse. The economical use of the process requires that the re-entrant contaminants are low and that the leaching solution used, most commonly sodium carbonate, has a minimal loss, i.e., the amount of material exuded by the contaminant concentrate, e.g. Na 2 CO 3.

The purification and recovery of the sodium carbonate used for soaking was tested by crystallization of sodium carbonate from 30 foreign soaking solutions. In the experiment you will use-:. calcium oxide and magnesium sulphate were also precipitated. The results are shown in Figures 3-8. The figures show the concentrations of contaminants remaining in the contaminants as a function of crystallization. The analytical results from which the graphs are plotted are shown in Table 7.

Table 7. Foreign impurities in solution part 22 106641

Solution Kitey-P P Si V NaCa Note.

5 volume g tetty% mg / kg mg / kg mg / kg mg / kg mg / kg 1,1397 0.0 159.0 141.4 2.84 144.8 6.46 2,647.2 53.7 234.4 197.6 4.26 95.5 3 637.2 54.4 231.9 176.8 3.93 67.9 3.89 1 4 633.5 54.7 213.4 175.5 3.96 69 6 , 09 ** 10 5 381.7 72.7 257.7 196.7 4.42 49.5 6.41 6 253.8 81.8 331.8 247 5.83 28 7.45 7 203.9 85 , 4,369.8 273.2 5.3 28 8.68 8 145.4 89.6 447.0 328.7 6.26 29.2 11.1 9 64.1 95.4 357.1 314.3 6.23 20.1 13.4 15 - * MgSO 4 precipitation ** CaO precipitation

The test results show e.g. the fact that the soaking solution was crystallized to such an extent that about 54% by weight of the solution remained in the crystal portion and the crystals were washed with light spray after suction filtration, resulted in low levels of impurity.

The main results of sodium carbonate purification are shown in Table 8.

Table 8.

Purification of sodium carbonate by crystallization and spraying

Foreign substance from Soaking Carbonate Crystals * “solution coming after cleaning mg / kg mg / kg% 1 P 159 14.8 2.7

Si 141 21.6 10.4 V 2.84 0.19 2.6

Crystalline fraction of total lime slurry 23 106641

As the solution is further crystallized, the purification is no longer as effective, leaving more impurities in the crystal. then recrystallization is required. According to test results 5, recrystallization should be used in a step where about 55-90% of the solution is crystallized. After about 90% crystallization, purification no longer takes place, leaving as much impurity in the crystal part as in the solution.

Crystallization is accomplished by cooling and as the temperature decreases, the solubility of sodium carbonate decreases. When about 90% of the solution is crystallized and the temperature is 0-2 ° C, the solubility is only 5-7% (see Table 1). In the experiment, only 1.6% of the sodium added to the process remained after about 90% of the solution was crystallized. Thus, it can be stated that the amount of sodium leaving the concentrate remains low.

The test also shows that if foreign substances are to be precipitated with calcium oxide or magnesium sulfate, this must be done only at the end of the process, where the carbonate content is low. If done in the early stages of high carbonate content, calcium and magnesium will immediately precipitate as carbonates and will not affect phosphate, silicate or other contents. It is only when the carbonate content has fallen sufficiently that the calcium and magnesium contents can rise to such levels that the said substances precipitate.

Further, the experiment shows that lime mud washing water may be: more advantageous to use only to dissolve the recrystallized crystal portion than to dissolve the first crystalline portion. Thus, the amount of impurities that can be transported back can be minimized.

35

It can also be deduced from Figures 3-8 that the calcium content ..... does not increase until the carbonate content is low.

24 106641

Despite its small volume, the contaminant concentrate contains a certain amount of sodium which should be returned to the process for economic and environmental reasons. The estimated amount is 20-45 kg of sodium per 5 tons of lime. However, since only part of the leachate stream is treated according to this method to balance the plant's phosphorus balance, the amount of sodium per tonne of pulp is lower, approximately 3 kg.

10 There are different solutions for restoring sodium in the process. For example, the contaminant concentrate may be treated by neutralizing with sulfuric acid, doping the contaminants with calcium oxide or magnesium sulfate, and returning the sodium in sulfate form to the chemical cycle of the pulp mill.

One simulation experiment monitoring the progress of causticization in green liquor by adding lime in small portions to the solution is shown in Figure 9. The composition of the stock solution (synthetic green liquor) is as follows: 60 g Na2CO3 / kg H2O, 30 g NaOH / kg H2O, 35 g Na2S / kg H 2 O, and 2 g SiO 2 / kg H 2 O. The feed lime contained 92 wt% CaO and 8 wt% SiO 2. From Fig. 9A it can be noted that as the lime dissolves in solution, the silicon content of the solution initially increases * 25, reaching a maximum of 4 g SiO 2 / kg H 2 O when about 26 g of lime has reacted. Thereafter, the silicon content of the solution begins to decrease as the silicon begins to precipitate as calcium hydrosilicate. Figure 9B shows that calcium hy drosilicate begins to precipitate when 26 g of lime / kg H 2 O is reacted. The (calculated) hydroxide and carbonate content of the solution is then 59 and 15 g / kg H 2 O in NaOH and

Calculated as Na 2 CO 3. Calcium hydroxide begins to precipitate after reacting with lime at 35 g / kg H 2 O. The (calculated) hydroxide and carbonate content of the solution is then 70 and 5 35 g / kg H2O, calculated as NaOH and Na2CO3.

25 106641

Figure 10 illustrates the application of the invention in the context of a two-phase seasoning. The green liquor is introduced from the recovery tank solvent tank (not shown) along line 21 to the first causticization step 22.

Preferably, the green liquor is prepared so that its sodium hydroxide content is as low as possible, preferably less than 10 g NaOH / kg. The green liquor may also preferably be pretreated to reduce its silicon content. Lime is also added to causticization 22 through line 27, the silicon contained therein being soluble in green liquor during causticization. This results in a lime slurry having a lower silicon content than the lime imported for causticization. The honey is separated from the pre-causticized liquor in step 23 and introduced into regenerable lime for incineration 4 through line 28.

15

The liquor is passed to the next causticization step 25, where the remainder of the lime needed for causticization is added through line 29. Then, the amount of silicon dissolved in the solution and the amount of lime added are so high that the silicon begins to precipitate and is enriched in the lime slurry. Thus, in the first causticization step 22, the silicon transferred to the solution is precipitated in the lime slurry, which is separated from the white liquor in step 26. Some of the silicon rich lime can be removed from the process via line 31 to landfill or the like. The remainder of the lime is passed through line 30 and further through line 28, together with the lime from step 23 to incineration 24. The removed lime volume is replaced by replacement lime, which is introduced into the first causticization step 22. Alternate 30 lime can also be introduced into second stage 25 more reactive lime.

The invention is not limited to the above embodiments only, but many modifications are possible within the scope of the inventive idea defined by the claims.

Claims (27)

106641 A process for separating foreign matter, such as silicon, from lime or lime in a pulp mill chemical recovery system, characterized in that the lime or lime is contacted with a carbonate or hydrogen carbonate solution to dissolve the foreign matter, followed by separating the lime from the solution containing dissolved foreign matter. . 10 Process according to claim 1, characterized in that the lime slurry or lime and the solution are separated by filtration, centrifugation, settling or other similar separation process. 15 Process according to claim 1 or 2, characterized in that after leaching and separation, the lime slurry is washed with water. Process according to claim 1, characterized in that the carbonate is separated from the carbonate solution containing the foreign substances by crystallization so that the foreign substances are enriched in the liquid part. * Process according to claim 4, characterized in that the carbonate is crystallized by cooling the solution. 6. A process according to claim 4, characterized in that the solution is crystallized by boiling to crystallize the carbonate. Process according to Claim 4, characterized in that the separated carbonate is dissolved in water and the resulting carbonate solution is used for leaching the lime. 35 A process according to claim 4, characterized in that the liquid part containing foreign matter is removed from the process. 27 106641 9. A method as claimed in claim 3, characterized in that the amount of water used for washing the lime sludge and the amounts of impurity-containing liquid removed from the process are balanced so that the total amount of water in the process remains constant. A process according to claim 1, characterized in that the carbonate-containing solution contains K 2 CO 3, Na 2 CO 3 and / or (NH 2) 2 CO 3. Process according to claim 1 or 10, characterized in that the concentration of the carbonate solution used is from 5 g / l to a saturated solution, preferably from 10 to 400 g / l. 15 Process according to claim 1, characterized in that the leaching of the lime sludge is carried out at a temperature of 20 ° C to the boiling point of the carbonate solution used, preferably 80 to 110 ° C. 20 Process according to claim 4 or 5, characterized in that the crystallization of the carbonate crystals is carried out at a temperature of -2 to 30 ° C. : 25 Process according to Claim 7, characterized in that the carbonate crystals are dissolved at a temperature of 0 ° C, preferably about 50 + 30 ° C. Process according to claim 1, characterized in that the carbonate solution is a sodium carbonate solution, for example green liquor. Process according to Claim 1, characterized in that the foreign matter dissolved in the solution is removed from the solution by precipitation with lime. k »28 106641 Process according to claim 16, characterized in that the amount of lime used for precipitation is substantially less than the amount of lime mud or lime 5 treated with the solution. 18. A process for the causticization of green liquor containing contaminants such as silicon in two steps, characterized in that, in the first stage, lime containing foreign matter is causticized in such a way that the contaminants contained in the green liquor remain substantially soluble and the foreign matter removing the lye, and in the second step the remaining lime needed for causticization is added to the lye from the first step, whereby foreign matter is precipitated with the lye formed in the second stage, and at least some of the rf 'y foreign matter rich lime is removed from the process. 20 A method according to claim 18, characterized in that 40-90% of the amount of lime needed for the causticization is added to the first causticizing step. . * 25 A method according to claim 19, characterized in that 50-70% of the amount of lime needed for the causticization is added to the first causticizing step. A process according to claim 18, characterized in that the green liquor has a silicon content of 0.8 to 12 g SiO 2 / kg H 2 O. A process according to claim 21, characterized in that the green liquor has a silicon content of 1-6 g SiC> 2 / kg H2O. 1 A process according to claim 18, characterized in that the green liquor is prepared by dissolving the chemicals obtained from the combustion of the black liquor in a liquid which does not contain 106641 essentially no sodium hydroxide in order to obtain a green liquor containing very little sodium hydroxide. The process according to claim 18, characterized in that the green liquor has been pretreated prior to causticization to reduce its silicon content. A process according to claim 24, characterized in that the green liquor is pretreated with carbon dioxide gas. The method of claim 18, wherein the lime slurry is regenerated into lime in the lime slurry burner, characterized in that the lime added to the second stage is regenerated lime from the same mill. The method of claim 18, wherein the lime slurry is generated as lime in the lime slurry burner, characterized in that the lime added to the second stage is more reactive lime than lime regenerated in the same mill. 30 106641