The invention relates to a method of separating impurities in connection with the chemical recovery system of a pulp mill, in particular from the calcium-containing substances used therein, which are mainly lime sludge and lime, and from its liquor circulation.

The term impurities is used in the present patent application to denote various detrimental substances such as silicon, phosphor, vanadium, sulphur, etc., which may also be called non-process elements, building up in the chemical circulation of a pulp manufacture process and originating from various raw material sources. Sources of impurities are mainly wood raw material (phosphor) , make-up lime (silicon) and combustion oil

(vanadium, sulphur) if oil is used as fuel in the lime sludge rebuming kiln. Some raw materials used in the manufacture of cellulose pulp also may contain in their cells lots of silica

(Si0 ₂ ) . The most usual of these are annual plants such as bamboo, sugar cane, rice and wheat. It has also been discovered that some tropical wood species contain such amounts of silicon that are detrimental for pulp production processes.

When pulp is manufactured using the sulphate method the cooking liquor used, or black liquor, is evaporated, combusted and the residue obtained, the so-called soda smelt, is dissolved in water. The green liquor thus formed iε causticized to produce cooking liquor. During the causticizing process, the sodium carbonate contained in the green liquor is allowed to react with burnt lime (CaO) as follows. First, lime slaking takes place:

CaO + H ₂ 0 -> Ca(OH) ₂ (1)

Subsequently, the causticizing reaction itself takes place:

Ca(OH) ₂ + Na ₂ C0 ₃ -> 2NaOH + CaC0 ₃ (2)

The sodium hydroxide thus produces which contains white liquor and calcium carbonate (lime sludge) is separated and the white liquor is recirculated to pulp digeεtion. White liquor and lime sludge may be separated either by settling or filtering. White liquor filters are usually conventional candle filters or drum or disc filters operating with pressure or vacuum. Lime sludge iε washed in order to remove alkali therefrom. Lime sludge is thickened in a lime εludge filter for combustion in a calcinating device such aε lime εludge rebuming kiln in which the lime sludge iε regenerated to calcium oxide to be returned to the cauεticizing. Thiε circulation process of lime is called the lime cycle.

In the chemical recovery system of a pulp mill the material flow deεcribed above iε partly cleaned aε part of the soda smelt does not disεolve in the εoda dissolving stage and the so-called green liquor sludge formed is removed. Undesirable substanceε may thus be removed from the process in this way.

The problem in the process is that certain impurities are not easily removed because they are not sparingly soluble enough and thus to a large extent remain in the solution. For example phosphor, silicon and vanadium compoundε aε well aε many other compound are this kind of substances. These compoundε tend to precipitate during causticizing in the lime εludge and thus build up in the lime cycle. For example εilicateε precipitate aε calcium hydroεilicateε to the lime uεed in the causticizing procesε. In thiε way, impuritieε are accumulated with time in the lime εludge which means that the lime sludge is contaminated. Phosphor tends to concentrate in the fine dust of the lime sludge rebuming kiln and it has been deεirable to try to remove it in this form. Fine dust has sometimes been used as phosphor-containing soil conditioner.

Contamination of lime sludge results in various problems, for example deposit rings and balls accumulate in the lime sludge rebuming kiln, filtration of white liquor becomes difficult and burnt lime passivates, i.e. it iε bound in non-reactive compounds. For this reason, at least a part of the lime sludge must be replaced from time to time, i.e. a part of the lime sludge muεt be removed continuouεly and the lime cycle must thus be kept open to some extent.

A further problem is that the lime sludge discharged must either be transported to a landfill area or used to neutralize acid effluents from a pulp bleaching procesε whereby phosphor and other impurities end up in watercourses.

Further, as the tendency today iε to close the water circulation of a pulp mill and to return the bleaching effluents to the process the impurity load of the chemical circulation system increases and drawbacks become more severe.

Instead of destroying the fouled lime it has been suggested to separate the silica already from the black liquor by lowering the pH of the liquor with carbon dioxide to a range of about 9.1 - 10.2. The solubility of the silica dissolved in the black liquor in ion form, mostly as HSi0 ₄ ^3" and Si0 ₃ ^3~ , decreases and it precipitates as colloidal silica gel. It has been reported that even 90 % of the silicate may be separated from weak black liquor (6 g Si0 ₂ /1) by a method developed by The United Nations Development Organization (UNIDO) and the Swedish International Development Authority (SIDA) . In that case carbon dioxide iε bubbled in a bubbling reactor into black liquor and εubεequently the precipitated εilica iε separated by filtering. CPPRI and Lurgi have developed similar approaches.

Silica may be εeparated from green liquor with carbon dioxide in the εame way as from black liquor. Green liquor may contain

about 10 - 20 g Si0 ₂ /1 of silicate, sometimes even more, depending on the raw material used, which is remarkably more that in weak black liquor, and therefore better yield may be expected. Separation of silica precipitate from green liquor is not as problematic as from black liquor since organic matter is not present in green liquor. A drawback of this method is that odorous sulphur-containing compounds such as hydrogen sulphide are released when sulphate liquor is treated with carbon dioxide.

Another known method of removing εilicon from green liquor is to add lime (CaO or Ca(OH) ₂ ) to the green liquor whereby εilicon precipitateε with the lime and thiε lime rich in silicon may be removed. It has been suggeεted to divide the cauεticizing process of green liquor in two stageε and to add part of the total volume of lime required in the causticizing process to the first stage. Then silicon precipitates as calcium silicate in the lime εludge produced and the silicon- rich lime sludge may be discharged. The remaining portion of the lime is added to the second stage and this lime iε circulated in the process. A limitation of this method iε that a high silicon removal rate is achieved only if the silicon content of the green liquor is high, for example more than 12 g/kg H ₂ 0.

The object of the present invention is to eliminate the drawbacks mentioned above.

In particular the object of the invention is to provide a new kind of method of separating detrimental impurities from the chemical circulation of a pulp mill, particularly from lime cycle.

A further object of the invention is to prolong the operating life of lime sludge in a pulp procesε.

A further object of the invention iε to provide a method of recovering more efficiently and reusing the useful chemicals of a pulp procesε. Particular attention iε paid to a feature of separating the impurities discusεed above in a readily separable form and from solutions which contain low concentrations of these impurities.

Further, an object of the invention iε to provide a method by which the chemical circulation of a pulp proceεε may be closed as far as possible.

The characteristic feature of the invention are defined in the appended patent claims.

The invention is based on the obεervation made in teεtε performed that by εoaking lime-containing material εuch aε lime sludge in a solution of carbonate or hydrocarbonate, preferably having a high concentration of carbonate or hydrocarbonate, anions (C0 ₃ ^2~ , HC0 ₃ ^" ) which are difficult to separate are dissolved and may thuε be εeparated from the lime sludge or lime. Thus, the invention makes use of the solubility properties and products in the conditions mentioned in a new way.

In the context of the present invention the term lime means material containing calcium oxide (CaO) or calcium hydroxide

(Ca(OH) ₂ ) used in the mill for carrying out for example causticizing processes. The term lime sludge means material which contains calcium carbonate (CaC0 ₃ ) .

The solubility of sparingly soluble substances is often expresεed as solubility product as follows:

LMemXn = ^e] ^m [X] ⁿ (3)

in which Me is a metal ion; X is an anion forming a sparingly soluble salt with the metal ion in question; m and n are integral numbers; parentheses [ ] express concentration; and L is solubility product which with certain conditions is constant.

The solubility product is in fact constant only with dilute solutionε in which the ionic strength or the value expreεsing the εummed concentration of all ions iε low. If there are large amounts of dissolved salts in the solution which means that the ionic strength is high the equation (3) is valid only if the concentrations are replaced by activities. The connection between the activity and the concentration is :

a = F[A] (4)

in which a is the activity of the substance; [A] is its concentration and f is the activity coefficient. The solubility product is then:

^L MemXn ^{~ a} Me ^a x ^ ⁵ ^

Information on the activity coefficients are often not available for concentrated solutions having complicated compositionε. However, it is not necessary to know them in order to recognise the efficiency of the method of the invention.

If anion X is a carbonate, rising its activity reεultε in a decrease in the activity of all metal ionε forming sparingly soluble carbonates while the solubility product remains constant. This presupposes that their concentration in the liquid is sufficiently high to be in balance with solid carbonate.

Me may be any metal ion meeting these requirements. The carbonate activity may be raised by a readily soluble salt such as sodium carbonate. A decrease in the activity of the metal ions results, according to equation (5) , in dissolving of other anions X than carbonate, which also form sparingly soluble compounds with the same metals and are present in the form of precipitate in the lime sludge; i.e. the activities of anions in the solution increase.

Metals which form poorly soluble carbonates in the conditions discussed and the activity of which in the solution may be decreased, are primarily calcium, barium, iron, manganese, magnesium, etc. Anions the activity of which may be raised by this method are primarily P0 ₄ ^3~ , Si0 ₃ ^2~ , V0 ₄ ^3~ , S0 ₄ ^2" , etc.

The exact form of the anion, for example S0 ₄ ^2" vs. S0 ₃ ^2" or V0 ₄ ^2" vs. V ₂ 0 ₅ ^2- , need not necessarily be known, neither the metal with which it has precipitated. The only prerequiεite of the method of the invention is that anions and cations are in balance with the solid phase according to equation (5) .

Sparingly soluble compounds the anionε of which may be dissolved when the activity of the metal ion decreases are for example the following: L ^L _r C _a a3-, ₍ (pP _Ω 0 _i 4 ₎ ) ₇ 2 = ⁼ a ^a _r C _a a3,. ^• a ^a _p P ₀ 0 ₄ 4 ₂ 2 calcium phosphate

^L CaSi03 ^{= a} Ca - ^a Sio3 calcium metasilicate

^L CaS04 ^{= a} Ca ^{■ a} S04 calcium sulphate

^L Ca3 (V04 ) 2 ^{= a} Ca3 ^{• a} V042 calcium vanadate

^L CaHV04 ^{= a} Ca3 ^{• a} HV04 calcium hydrogen vanadate LpF _e e33 ((PPθ04)) 22 = ^{= aa} FF _e e ₃ 3 - ^• a ^a _P Po04422 ^{i ron} phosphate

^L BaS04 ^{= a} Ba ' ^{• a} S04 barium sulphate

According to the method of the invention, lime εludge is εoaked preferably in a εtrong εolution of carbonate or hydrocarbonate, for example in a εolution of potassium, sodium or ammonium carbonate or hydrocarbonate. An increased carbonate or

hydrocarbonate content decreases the calcium concentration of the solution in a way required by the solubility product. The same happens with other cations forming sparingly soluble carbonates or hydrocarbonates . When the concentrations of calcium and other cations decrease the solubility of anions, such as phosphate, silicate, vanadate, sulphate, sulphite, etc., which form sparingly soluble salts with them, increase correspondingly in a way required by the solubility product of the salts . These anions precipitated in the lime sludge are thus diεsolved. After the soaking, the lime sludge and the solution, i.e. the dissolved impurities are separated from each other by any method known per se.

When for example a solution of sodium carbonate iε uεed as the lime sludge soaking solution, the sodium is purified and reused or it is recycled to cooking chemical preparation. Separating the sodium carbonate by crystallization is advantageouε as the sodium carbonate concentration remaining in the impurity concentrate is then as small aε poεεible (see Table l. Solubility of sodium carbonate in water) .

The volume of the impurities concentrate to be removed may be adjusted by choosing for the soaking a suitable carbonate concentration, a suitable crystallizing temperature and by proceeding the crystallization suitably far. A suitable carbonate concentration when using for example sodium carbonate is about 5 g/1 - saturated solution, preferably about 200 - 400 g/1. Sodium carbonate containing 7 to 10 crystal water, binds a large volume of water and the impurities may be concentrated in a small volume of liquid. The limit is set by the solubility of the impurities. If the limit is exceeded the impuritieε precipitate with the εodium carbonate and the purifying efficiency of the cryεtallization decreases . If necessary the carbonates may be dissolved in water and recrystallized, thus reducing the amount of impurities entrained with the carbonate back to the soaking.

Table l

solid g mole/ Na ₂ C0 ₃ / density steam 1000 g H ₂ 0 100 (g/g) pressu H ₂ 0 Sat.Sol Hg

-2.10 Na ₂ C0 ₃ - 10H ₂ O + 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

32.00 Na ₂ C0 ₃ - 10H ₂ O + 4.28 45.4 31.2 - 29.0 Na ₂ C0 ₃ - 7H ₂ 0 m 32.96 Na ₂ C0 ₃ - 10H ₂ O + 4.71 49.9 33.3 29.5 Na ₂ C0 ₃ H ₂ 0 ^m 30 Na ₂ C0 ₃ -H ₂ 0 4.78 50 .7 33 .6 35.37 Na ₂ C0 ₃ - 7H ₂ 0 + 4.67 49 .5 33 .1 34.0 Na ₂ C0 ₃ -H ₂ 0

40 Na ₂ C0 ₃ ^• H 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 Na ₂ C0 ₃ -H ₂ 0 4.20 44. 5 30. .8 1.15 (Atm)

110 Na ₂ C0 ³ 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 "

To solid gg mmoollee// NNaa ₂₂ CC00 ₃₃ // density steam 1000 g H ₂ 0 100 (g/g) pressure H ₂ 0 Sat.Sol Hg

220 Na ₂ C0 ³ 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

The temperature at which the calcium-containing substance such as lime sludge is soaked affects the soaking efficiency. A high temperature is advantageous for the soaking efficiency but in view of the economy of the process it is favourable to use the waste heat from pulp digestion or other energy of little value, i.e. condensates of less than about 85°C. Thus the more valuable primary energy in the form of steam need not be used. A suitable soaking temperature according to an embodiment of the method of the invention is 20°C - the boiling point of the solution preferably 80 - 110°C. The boiling point depends on the concentration and pressure of the carbonate-containing solution used. If desired, the lime sludge may be soaked at normal pressure or at a desired superatmospheric or subatmospheric pressure.

The solution/lime sludge ratio (weight of solution/weight of lime sludge dry solids) in the lime sludge εoaking may be of the order of 2 - 20, preferably 6 - 15.

According to the method of the invention the lime sludge and the carbonate εolution, preferably εodium carbonate εolution, used in the dissolving are separated from each other for

example by filtering, centrifuging, sedimenting or by other known methods. The lime sludge may also be washed with water to intensify the cleaning.

The carbonate may advantageously be crystallized from the carbonate solution containing impurities. The carbonate and the solution containing impurities may preferably be separated from each other.

Crystallizing sodium carbonate at a temperature of 5 - 20°C produces pure decahydrate crystals and at a temperature of 35 -

90°C produces pure monohydrate crystals. The crystallizing may be practices for example by cooling crystallization which produces decahydrate crystals or by evaporation which produces monohydrate crystals. In conventional evaporation heat is brought to the process which evaporated water and thus raises the carbonate content of the solution over the limit required by crystallization. The carbonate solution may be evaporated in order to crystallize the carbonate in a way known per se by one-stage or multi-stage evaporator. The evaporation may be carried out in a tube, lamella or flash apparatus.

In an embodiment, the suitable crystallization temperature range for the carbonate is -2 - 30°C. At the end stage a temperature of the solution of almost -2°C iε required to accompliεh as complete crystallization as possible. Outer air may be used for the cooling the temperature of which is low enough for the most part of the year at least in the Nordic countries.

If desired the carbonate may advantageously be disεolved in water; water, preferably lime sludge wash water, may be added to it and the liquid produced may be used in the soaking of the solution.

A suitable temperature for the dissolving of the sodium carbonate crystals is for example about 50°C. The condensate used for heating the soaking reactor may be used for raising the temperature to the desired level; in this way the heat may be reclaimed economically.

If desired the separated carbonate may be returned to the chemical circulation.

The solution containing impurities, the εo-called concentrate may be discharged from the process. The impurities concentrate may be subjected to one or several recrystallization/s in order to recover useful chemicals, mostly carbonate crystals.

The above description refers mostly to the use of carbonate in the cleaning of lime εludge. However, alεo hydrocarbonate may be used in the solutions used for soaking the lime sludge in addition to the carbonate or to instead of it (0 - 100 %) . Hydrocarbonate may be recovered in a similar way as carbonate.

The water volume used in washing the lime sludge and the volume of the discharged solution containing impuritieε may preferably be balanced so as to keep the total volume of water in the process constant.

An interesting embodiment of the invention is removal of silicon and phosphor from lime sludge and lime containing high concentrations of these substances by dissolving them from the lime sludge or lime into a carbonate solution. A solution of this kind containing soluble carbonate forms with calcium sparingly εoluble calcium compoundε εuch aε calcium carbonate and preεerveε the εolubility of calcium low, thus no εilicate precipitates. Preferably the solution of this kind is green liquor or some other solution containing carbonate, particu- larly potasεium carbonate. If the carbonate solution such as green liquor contains also silicon the invention is preferably

practised at a silicon concentration of about 0.8 - 12 g Si0 ₂ /kg H ₂ 0, preferably 1 - 6 g Si0 ₂ /kg H ₂ 0 . When silicon has been removed from the lime sludge or lime to the solution the solution may be cleaned from theεe substances. For this purpose, such an amount of lime is added to liquor which is sufficient to precipitate the silicon from the liquor. Lime obtained by combusting lime sludge which has been purified earlier, or more reactive lime obtained for example by combusting porous limestone may be used for the precipitation. The advantages of the invention are obvious here as the volume of lime required for precipitating the silicon is substantially smaller than the volume of purified lime or lime sludge and thus the volume of silicon-rich lime sludge, which muεt be wasted, is small.

Disεoluble silicon reacts with lime and forms calcium hydrosilicates when the amount of εilicon in the green liquor coming to the causticizing process is larger than in the white liquor produced. These hydrosilicates react in the lime sludge rebuming kiln with lime and form calcium silicateε in which the CaO/Si0 ₂ ratio is higher than in the original calcium hydrosilicate. The calcium silicate most likely formed is β- dicalcium silicate. The resultε of thorough thermodynamic analyses of the compounds formed by silicon and lime, experimental εtudies and computer simulations based on these show that in the causticizing the solution is not superεaturated with calcium silicate hydrate which iε in metaεtable balance with β-dicalcium εilicate but the β- dicalcium silicate coming in with the lime tends to dissolve from the lime to the solution until the amount of the dissolved silicon in the solution and the amount of added lime have reached a certain level . When the calcination advances the solution becomes superεaturated with calcium silicate hydrate and the silicon in the solution begins to precipitate as calcium silicate hydrate. Compounds of this kind are for

example hydrosilicates like tobermorite (4CaSi0 ₃ *Ca (OH) ₂ ) and jennite (4CaSi0 ₃ *5Ca (OH) ₂ ) .

The invention is particularly advantageously applied in a causticizing process in a pulp mill . Most preferably the causticizing iε practised in two εtageε and 40 - 90 %, preferably 50 - 70 % of the total lime volume required is added in the first stage in the flow direction of the green liquor (as generally known, the total lime volume required in the causticizing is the volume of lime, the amounts exceeding which cauεe calcium hydroxide to remain in the lime sludge to a harmful extent) . When adding in the first stage the amount of lime mentioned silicon does not precipitate but on the contrary the silicon possibly contained in the lime disεolveε from the lime to the liquor. Thus, from the first causticizing stage lime sludge iε obtained which iε clean aε far aε εilicon is concerned, i.e. the silicon content of the lime sludge iε substantially lower than that of the lime fed into the causticizing process. The rest of the lime required for the causticizing, i.e. 10 - 60 %, preferably 30 - 50 %, is added in the second stage and the silicon contained in the liquor is concentrated in the lime sludge. A part of this lime sludge may be discharged from the procesε in order to reduce the silicon load of the lime circulation and the rest may be mixed with the silicon-poor lime sludge obtained from the first stage. The lime sludge mixture is regenerated to lime in a lime sludge combustion apparatus εuch as lime εludge rebu ing kiln. The silicon and/or phosphor concentrationε in the chemical circulation of a pulp mill may be controlled in this way.

Also the alkali content of the green liquor may have an effect when practising the invention. As known, green liquor is produced by dissolving the chemical smelt obtained from the combustion of black liquor in water or weak white liquor which is produced in washing of lime εludge after εeparation of white

liquor. One of the main purposes of washing lime sludge is to remove alkali (NaOH) from the lime sludge as completely as possible. If separation of white liquor and lime sludge is not efficient, much alkali remains in the lime sludge and further in the weak white liquor. If weak liquor rich in alkali is used in the dissolving of smelt the alkali content of the green liquor will correspondingly be higher which in turn reduces the share of sodium carbonate in the green liquor. Presεurized disc filter, the use of which is presently becoming more common, separates more efficiently white liquor and lime sludge so that in the washing of lime sludge the content of NaOH in the weak white liquor is only about 5 g/1.

When the alkali content of the green liquor is kept aε low as possible by dissolving the smelt in a solution containing aε little NaOH as posεible, the εhare of εoluble εodium carbonate in the green liquor increases . Then the lime contained in the liquor reacts to a larger extent according to the reaction equation (2) and does not precipitate soluble silicon. Thus the amount of lime added in the causticizing stage may be increased and more silicon is diεεolved in thiε cauεticizing εtage and aε a result also more cleaned lime sludge is obtained. Then the amount of lime in the final causticizing stage in which silicon accumulates iε εmaller.

The εame kind of results are obtained when the silicon content of the green liquor is reduced before causticizing. This may be brought about, as previously described, by pretreatment of green liquor or black liquor with carbon dioxide-containing gas. An advantageouε method of reducing the silicon content in green liquor iε disclosed in PCT patent application no. PCT/FI95/00556 according to the method of which sodium carbonate-containing smelt obtained from combustion of black liquor iε pretreated εo that the εodium carbonate iε recovered in εolid form whereaε εilicon and/or phoεphor are separated as

a solution containing dissolved sodium silicates/sodium phosphates.

Causticizing may be practised also by a countercurrent method, in which liquor and lime flow in opposite directions. The process preferably comprises one step or several steps. In this case all the lime required in the causticizing is added in the last step relative to the liquor flow. Between the steps, lime/lime sludge and liquor are separated from each other. The liquor is transported to the next step and the separated lime/lime sludge is guided to the previous step relative to the liquor flow. The most impure lime sludge iε the one separated from the last step, and a small portion (for example 1/5) of it is discharged from the procesε and the rest of it is guided countercurrent to the liquor flow and discharged from the first step.

The invention described above may be applied also elsewhere in a pulp mill and not only in the causticizing. Prior to concentration of the lime sludge, a part of the lime sludge flow may be treated with green liquor or a corresponding solution for example in a mixing tank to dissolve silicon. After this the lime sludge is separated, washed and transported to a lime sludge filter for thickening.

According to another embodiment lime sludge iε treated for example outεide the mill building in a similar way as low-grade oreε are dreεεed with the so-called heap leaching method. In this case the leaching medium is green liquor or other carbonate solution with which the lime sludge heap is treated to remove silicon and corresponding impurities therefrom. This embodiment may be employable in some circumstanceε, for example in the treatment of old εilicon-rich lime εludge supplies and subsequently the purified lime sludge of thiε kind may be reuεed. Silicon iε removed from lime εludge treatment solutions by precipitating with lime whereby the εolution may

be circulated but silicon has been concentrated from the original to a remarkably smaller lime sludge volume which is discharged.

The life of lime sludge may be prolonged substantially by employing the invention, i.e. by cleaning the lime sludge or other calcium-containing material according to the invention. Further, the invention provides cost savings due to the longer life of lime sludge and saving of other chemicals; the demand of purchase lime dropε and the landfill coεtε and costs for transport to a landfill area are reduced. By employing the invention, for example silicon may be removed from liquors which contain harmful amounts of silicon but, however, less than could be removed economically by known methods.

Further, due to the invention, lesε detrimental compounds from pulp production end up in the environment .

Further, the invention reduces the need of opening the lime circulation, i.e. it facilitates closing the chemical circulation.

The method of the invention is described below by means of exemplary embodiments with reference to the accompanying drawing figures of which

Fig. 1 illustrates the flow sheet of an embodiment of the method of the invention;

Fig. 2 illustrateε the impurities concentration as a function of the solution/lime εludge ratio,-

Figs. 3 - 8 illustrate εubstance concentrations in the solution when Na ₂ C0 ₃ has been removed from it by crystallizing;

Fig. 9 illuεtrates a εimulation teεt to preεent the principle of the invention; Fig. 10 illustrates an embodiment of the present invention in connection with causticizing; and

Figs, ila, lib and lie illustrate the flow sheet of a test utilizing the method of the invention.

In Figure 1, lime sludge 1 is carried to a soaking vessel containing strong carbonate-containing solution, for example sodium carbonate. After the soaking, lime εludge and the soaking solution are separated from each other e.g. by filtering and subεequently the separated carbonate solution 2 containing impurities is conveyed to a crystallizing vessel and the separated lime sludge 3 is take to a wash. The carbonate crystals 7 obtained iε dissolved and the carbonate solution 8 obtained in this way is recycled to the soaking vessel. The cleaned lime sludge 10 is returned to the pulp proceεε. Subεequent to the crystallizing, the εeparated impurities- containing solution is transported to further crystallizing and the crystals obtained therefrom are dissolved and the dissolved carbonated cryεtalε 6 are returned to the original cryεtallizing vessel . The impurities-containing solution ll obtained from the further cryεtallization iε diεcharged from the process.

Example 1

The effect of the volume of the soaking liquid on the impurities content

A lime sludge sample was soaked in a sodium carbonate solution having a solution/lime sludge ratio (weight of solution/weight of dry εolidε of lime εludge) of 3.9 - 19.6. The solution had been produced by dissolving 400 g of Na ₂ C0 ₃ in 1000 ml of water. The temperature was 90°C and the soaking time 6 hours. The mixture waε mixed during the teεt. After the εoaking, the solution and the lime sludge were separated from each other when warm by suction filtration. The lime εludge waε washed with water. The phosphor content of the lime sludge and the carbonate solution were determined. The results are presented in Figure 2. The analysis results are presented in Table 2.

Table 2

solution/ lime sludge ka P g/kg P residue %

1 Untreated lime sludge 0 1,83 100,0

2 Treated lime sludge 3,91 0, 744 40,7

3 - II . 7,82 0,328 17,9

4 - n . 15,65 0,109 6,0

5 - ιι - 19,56 0, 095 5,2

Example 2

Removal of impurities

A lime sludge sample was soaked in a sodium carbonate solution like in test 1 with a solution/lime εludge ratio of 9.8. Subsequent to the soaking, the lime sludge was washed twice in water with a lime sludge/water ratio of 2.8. After the treatment, contents of different substances in the lime sludge were determined. The results are presented in Table 3.

Table 3

Concentrations of different substances in lime sludge before and after treatment

Original Treated lime sludge lime sludge concentration concentration Removal g/kg g/kg %

Phosphor P 1,83 0,343 81,3

Silicon Si 0,643 0,273 57,5

Aluminium Al 0,754 0, 710 5,8

Sulphur S 5,30 0,56 89,4

Vanadium V 39, 5xl0 ^"3 9,4xl0 ^~3 76,2

The results indicate that phosphor, sulphur and vanadium may be efficiently removed at a solution/lime sludge ratio of 9.8. Silicon may be removed fairly efficiently while aluminium is poorly removed.

Examples 1 and 2 indicate that phosphor may be removed very efficiently by soaking in carbonate solution. A very high solution/lime εludge ratio results in that large amounts of carbonate must be crystallized to clean a small amount of lime sludge. Operating the procesε economically presupposed optimizing of the solutions/lime εludge ratio whereby attention εhould be paid to the fact that reducing the εolution/lime εludge ratio impairs the cleaning efficiency.

Example 3

Effect of soaking conditions

The effect of different conditions on the removal of different impurities (phosphor, silicon, and aluminium) was tested by using different solution/lime sludge ratios, different soaking times, different temperatures and carbonate εoaking solutions of different concentrations. The resultε are presented in Tables 4, 5 and 6.

Table 4

Removal of phosphor in different conditions

Solution/ Soaking Time T Distribution of phosphor lime sludge solution Solution Lime sludge Removal ratio h °c 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

5 g/1 Na ₂ C0 ₃

10 water + 70 102 1240 7 , 6 5 g/1 Na ₂ C0 ₃

10 water + 70 628 745 45 , 7 50 g/1 Na ₂ C0 ₃

16 water 90 1234 23 , 5 98 , 1 1000 ml +

Na ₂ C0 ₃ 400 g**

* mg phosphor transferred to the solution per kg lime sludge

** nearly saturated solution

*** wash with water/lime sludge ratio 30

The resultε show that the sodium carbonate solution has a decisive influence on the disεolving of phosphor . Mere water disεolveε phoεphor very little but increasing the carbonate concentration improves the dissolving decisively . If the sodium carbonate solution is nearly saturated, the solution/lime sludge ratio is high enough and if lime sludge is adequately washed with water, a phosphor removal of as high as 98 % or even more may be achieved . Operating the procesε in this way may, however, not be economical .

Table 5

Removal of silicon

Solution/ Soaking Time Distribution of silicon lime sludge solution Solution Lime sludge Removal ratio h g/kg g/kg %

16 water 6 90 6,24 0,40 94,0

1000 ml * +

Na ₂ C0 ₃ 400 g

* wash with water/lime sludge ratio 30

Table 6 .

Removal of aluminium

Solution/ Soaking _T ime T Distribution of aluminium lime εludge solution Solution Lime sludge Removal ratio h °C g/kg g/kg %

16 water 90 0 , 135 0 , 245 35 , 5 1000 ml * +

Na ₂ C0 ₃ 400 g

_* wash with water/lime εludge ratio 30

94 % of silicon was removed with the same conditions as with phosphor . Only about one third of the aluminium waε removed regardleεs of the ef f icient conditions .

Example 4

Cleaning of sodium carbonate solution used in soaking

Subsequent to the soaking, the sodium carbonate solution must be cleaned for reuse. Economical use of the method presupposes that the amount of recycled impurities is small and that the loss of the soaking solution, usually sodium carbonate, is as small as possible, i.e. the amount of material, for example Na ₂ C0 ₃ , diεcharged with the impuritieε concentrate iε small.

Cleaning and recovery of the sodium carbonate used in the soaking was tested by crystallizing the sodium carbonate from the impurities-containing dissolving solution. Also calcium oxide and magneεium εulphate were uεed in the test to precipitate impurities. The results are presented in figureε 3 - 8. The figureε illustrate the concentrations of the impurities remaining in the impuritieε solution as a function of crystallization. The analysis results used as the basiε for the curves are presented in Table 7.

Table 7

Impurities in a solution portion

Solution Crys¬ P P Si V NaCa Not volume g tal. % 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 *

4 633,5 54,7 213,4 175,5 3,96 69 6,09 **

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

* MgS04 precipitation ** CaO precipitation

The reεults indicate among other things that when the solution from the soaking was crystallized so far that the crystal portion constituted about 54 % of the weight of the εolution and the crystals were washed after suction filtration with light spraying, low impuritieε concentrationε were obtained.

The moεt important results of εodium carbonate cleaning are preεented in Table 8.

Table 8

_C leaning of sodium carbonate by crystallizing and spraying

Impurity Solution from Carbonate crystals soaking after cleaning mg/kg mg/kg %*

P ₁₅ 9 14,8 2,7

_S i ₁₄₁ 21,6 10,4

_V 2, ₈₄ 0,19 2,6

* amount remaining in crystals, percentage of total amount contained in lime sludge

When the εolution is crystallized further cleaning is not as efficient but more impurities remain in the crystal portion. Then, recrystallization is needed. Test reεults indicate that recrystallization εhould be uεed when about 55 - 90 % of the solution has been crystallized. When about 90 % haε been cryεtallized no cleaning takeε place but aε much impuritieε remain in the crystal portion as in the solution.

Crystallization is brought about by cooling and the solubility of the sodium carbonate decreaseε with the temperature decrease. When about 90 % of the solution has been crystallized and the temperature is about 0 - -2°C the solubility is only 5 - 7 % (see Table 1) . In the test, only 1.6 % of the sodium supplied to the process remained when about 90 % of the solution had been crystallized. Thus it may be stated that the amount of εodium removed with the impurities concentrate is small .

The test alεo εhow that if it iε desirable to precipitate impuritieε with calcium oxide or magnesium sulphate this

should be done only at the end stage of the process where the carbonate concentration is low. If precipitation is carried out at the beginning stage where the carbonate concentration is high, calcium and magnesium precipitate at once as carbonates and have no effect on the phosphor, silicon and other concentrations . Only when the carbonate content has decreased adequately the calcium and magnesium concentrations may rise to a level to cause precipitation of the substances mentioned.

Further, the teεt show that it may be more advantageous to use the lime sludge wash water in the disεolving of the recryεtallized crystal portion than in the disεolving of the first crystal portion. In this way the share of the returned impurities may be minimized.

Figures 3 - 8 also indicate that the calcium content riseε only when the carbonate content iε low.

Deεpite its small volume the impurities concentrate contains a certain amount of sodium which because of the economy and environmental reasons should be returned to the process. The estimated amount is 20 - 45 kg sodium per ton of lime sludge. Because only a portion of the lime sludge flow is treated according to this method to bring the phosphor balance of the mill into equilibrium, the sodium amount per pulp ton is however smaller, the eεtimate is about 3 kg.

There are various ways available for returning the sodium to the procesε. For example the impurities concentrate may be treated with sulphur acid to neutralize it, the imparities may be precipitated with calcium oxide or magnesium sulphate and the sodium may be returned in sulphate form to the chemical circulation of the pulp mill.

A simulation test which monitored proceeding of the causticizing process in green liquor by adding small amounts of lime in the solution, has been presented in Figure 9. The composition of the initial solution (synthetic green liquor) was the following: 60 g Na ₂ C0 ₃ /kg H ₂ 0, 30 g NaOH/kg H ₂ 0, 35 g Na ₂ S/kg H ₂ 0, and 2 g Si0 ₂ /kg H ₂ 0. The lime supplied contained 92 weight per cent of CaO and 8 weight per cent of Si0 ₂ . Figure 9A indicates that at firεt the silicon content of the solutionε increases when εilicon dissolves from lime and reaches its maximum concentration 4 g Si0 ₂ /kg H ₂ 0 when about 26 g of lime has reacted. After this the silicon concentration of the solution begins to drop when silicon begins to precipitate as calcium hydrosilicate. Figure 9B indicates that calcium hydrosilicate begins to precipitate when about 26 g lime/kg H ₂ 0 has reacted. The (calculated) hydroxide and carbonate concentration of the εolution iε then 59 and 15 g/kg H ₂ 0 calculated aε NaOH and Na ₂ C0 ₃ . Calcium hydroxide beginε to precipitate when 35 g lime/kg H ₂ 0 haε reacted. The (calculated) hydroxide and carbonate concentration of the εolution iε then 70 and 5 g/kg H ² 0 calculated as NaOH and Na ₂ C0 ₃ .

Figure 10 illustrates applying the invention in two-staged causticizing. Green liquor is brought from the dissolving tank (not illustrated) of a soda recovery boiler in line 21 to the first causticizing stage 22. Preferably the green liquor has been manufactured so that its sodium hydroxide content iε aε low as posεible, preferably less than 10 g/NaOH/kg. Preferably, the green liquor may also be pretreated in order to decreaεe its silicon content. Also lime via line 27 is brought to the causticizing 22 and silicon contained in the lime disεolves to the green liquor during the causticizing. In this way lime sludge is produced the εilicon content of which iε lower than that of the lime εupplied to the causticizing procesε. Lime εludge iε

separated form the precausticized liquor in stage 23 and is brought in line 28 to combuεtion 4 for regeneration to lime.

Liquor is transported to the following causticizing stage 25 and the rest of the lime required for the causticizing process is added thereto via line 29. Then the amount of dissolved silicon in the solution and the amount of the added lime rise so high that silicon begins to precipitate and is concentrated in the lime sludge. Thus the silicon transferred in the first causticizing εtage 22 to the solution precipitates in the lime sludge which is separated from white liquor in stage 26. A portion of the silicon-rich lime sludge may be removed from the process via line 31 to a landfill area or a corresponding place and thus the silicon load of the lime circulation may be controlled. The rest of the lime sludge may be taken via line 30 and further via line 28 with the lime sludge from the stage 23 to combustion 24. The removed lime sludge amount is replaced with make-up lime brought to the first cauεticizing εtage 22. Make-up lime may be added alεo to the second stage 25 aε a more reactive lime than the kiln lime.

The invention is not limited to the embodiments presented here as examples only but many modifications may be made of it within the scope of protection defined by the appended patent claims.