WO1992008816A1

WO1992008816A1 - Titanium dioxide production from upgraded anatase ore concentrates using fluid sulphuric acid sulphation

Info

Publication number: WO1992008816A1
Application number: PCT/BR1991/000025
Authority: WO
Inventors: Noé CHAVES
Original assignee: Curuá Mineração Ltda
Priority date: 1990-11-20
Filing date: 1991-11-19
Publication date: 1992-05-29
Also published as: AU8916091A; ZA919187B; BR9005862A; PT99561A; CN1063665A

Abstract

Process of producing titanium dioxide of pigment quality, by chemically processing previously enriched and purified anatase ore, herein denominated Upgraded Anatase Concentrate, through digestion with sulphuric acid, maintenance of the reaction always in the form of a fluid suspension, utilization of high acid concentrations, as well as high tempertures, under boiling conditions, so as to separate from the liquid medium, after the digestion is completed, a crystallized cake of titanium sulphates, containing also the crystallized sulphates of most of the impurities still present in the ore, which, by means of a subsequent and easy dissolution with water or recycled black liquor, produces, in an effective way, the so-called 'black liquor', which is treated for producing the titanium dioxide by the usual other steps of the Sulphate Process, the whole process requiring a minimum consumption of acid and ensuring a minimum emission of polluting effluents.

Description

TITANIUM DIOXIDE PRODUCTION FROM UPGRADED ANATASE ORE CONCENTRATES USING FLUID SULPHURIC ACID SULPHATION.

FIELD OF THE INVENTION

This invention relates to a method of chemically processing anatase ore concentrates that have been previously submitted to a purifying and upgrading treatment, which method promotes the chemical digestion of nearly the totality of the concentrate, under a industrially feasible condition, thereby leading, in the most complete character of the invention, to a full process of producing titanium dioxide from the refered new raw material, herein called "Upgraded Anatase Concentrate".

By an unprecedented method it is possible to have a very low polluting and economic industrial process using this anatase ore, which ore has showed itself as being refractory to technical attempts towards its commercial usefulness and has not for the past twenty years , been the object of an effective industrial use.

The invention, therefore, is related to the chemical industry field of titanium dioxide and titanium pigment production. BACKGROUND OF THE INVENTION

Titanium dioxide production have great economic meaning and utilizes tradicional technologies, based on the so called "Sulphate" and "Chlorine" processes. The Sulphate Process, the older of them, utilizes ilmenite ore concentrates as raw material and/or a metallurgical product obtained from ilmenite and known as Titania Slag, both raw materials being soluble in sulphuric acid under favorable conditions, permitting its easy digestion and subsequent processing until the obtention of the final titanium dioxide product. The high iron content of these raw materials results in a very polluting process, as is well known. The Chlorine Process, more recent, utilizes as raw material rutile ore concentrates due to the convenience of its high TiO₂ content and high purity, conditions that are technically and economically indispensable to the cited process.Besides this, the Chlorine Process utilizes other raw material called "Synthetic Rutile", also produced from ilmenite ore, and, more recently, a richer type of "Titania Slag" known as Chlorine Slag or Richard's Bay Slag. Note that rutile is not adequate to the Sulphate Process due to its refractoriness to sulphuric acid digestion and, even if this was not a problem, due to its high cost compared to that of ilmenites. In their turn, ilmenites are not significantly used in the Chlorine Process due to its low TiO₂ grade and high iron content. The Chlorine Process also have some polluting problems, of a somewhat smaller impact than that of the Sulphate Process

The Anatase is a less common titanium mineral, has its chemical formula TiO₂ and is, therefore, very similar to rutile, reaching, in practice, high TiO₂ contents and in the particular case of the method that is the object of this Patent, presents a less refractory chemical digesting property. Significant deposits of this mineral only occur, as is known, in Brazil (Mineral Facts and Problems -USBM-1985 -pg. 859 and 864) and, even though discovered 20 years ago, has not been put to any effective industrial use to this moment. Studies carried out on typical Anatase ore physically beneficiated concentrates in laboratory , pilot plant and even in a semi-industrial plant, indicate the great potentiality of this raw material, despite certain apparent technical incompatibilities of some of its intrinsic characteristics. From the most promising point of view, there should be mentioned the quite high TiO₂ contents presented by the said concentrates, ranging from 65 to 85% TiO₂ (Brasillian Patents Application PI 007564,PI 7603083 and PI 7608748), which is rather significant for a simple physically beneficiated concentrate, therefore presumably produced at a lower cost if compared to that of slags, which have an equivalent assay range but are produced at a high cost. Moreover, it is a quite favourable aspect the fact that, as the other chemical components of Anatase concentrates are so many, those few that constitute a technical-economic restriction (impurities) are present in a relatively minute proportion, as is the case of iron. This assures to Anatase a quite advantageous position in relation to the ilmenites and even the slags. Furthermore, Anatase concentrates present extremely low chromium and vanadium contents, impurities that definitely pose serious problems to the industrialization of an raw material and that are found at higher levels in ilmenites, slags and even rutiles. For a better understanding of the aforegoing, below are the results of typical chemical analyses of the said raw material concentrates.

A S S A Y S (%)

AUSTRALIAN ANATASE UPGRADED CANADIAN RICHARD'S ELEMENTS ILMETITE PHYSICAL CONC. ANATASE SLAG BAY SLAG

1 2 CONC.

TiO₂ 54.0 74.4 82.20 92.6 72.1 85.0

Fe_Total 28.7 7.3 3.01 1.1 8.2 7.7

Al₂O₃ 0.35 1.40 0.79 0.41 6.50 1.20

SiO₂ 1.0 1.43 2.80 1.11 5.80 2.00

P₂O₅ 0.15 3.13 1.06 0.15 0.01 0.01

MnO 1.40 0.65 0.30 0.17 0.26 2.50

Nb₂O₅ 0.24 0.65 0.72 0.85 0.01 0.10

ZrO₂ - - - 0.92 - -

CaO - 0.56 0.31 0.09 - -

MgO 0.35 0.01 0.19 0.01 5.90 1.30

CeO₂ - 1.08 2.01 0.09 - -

La₂O₅ - 0.95 1.87 0.08 - -

Na₂O - - - 0.17 - -

Cr₂O₃ 0.100 0.027 0.03 0.009 0.240 0.300

V₂O₅ 0.420 0.083 0.07 0.005 0.540 0.600

U₃O₈ - 0.009 0.0172 0.010 - -

ThO₂ - 0.065 0.032 0.008 - -

I.L. - 3.20 1.20 - - - With respect to the above-mentioned technical incompatibilities of the Anatase physical concentrates, it may be verified from the above analysis results that only the contents of P₂O₅ (1.02-3.13%) and Nb₂O₅ (0.65-0.72%) are high and would constitute a serious problem for the Sulphate Process technology. The other elements will not represent any difficulty and, better still, it should be noted the highly favourable assays of chromium and vanadium. The restrictions regarding niobium are not entirely conditioning and may be tolerated in some cases or accommodated in others, as will be described next.

In order to previously remove P₂O₅ from the physical concentrates, which is perfectly conciliative with the advantage of eliminating other elements which are merely diluents, and also to raise as much as possible the TiO₂ content of physical concentrates, a process of assured industrial application has been developed, in a most economical manner, as per Brazilliam Patent Application PI 9005841, by the same authors and applicants hereof, and which transforms the above physical concentrates into the product above indicated under the name of Upgraded Anatase Concentrate , Such product, containing the desirable and specified P₂O₅ content for the Sulphate Process and which presents an extremely high TiO₂ content, is exactly the raw material to be used in the process object of this Patent.

A first and natural alternative for the utilization of the Upgraded Anatase Concentrate in the production of titanium dioxide would be its mere and simple application in the traditional Sulphate Process. In this case, the essential point is that Anatase be efficiently chemically digested in the sulphuric digestion typical of the Sulphate

Process, characterized by solid mass digestion, with the completion of the reaction in the form of a characteristic solid cake (sulphated cake) and by an excess of free acid, not in excess of 50 to 60%, after the end of reaction, so as to ensure the future thermal hydrolysis step of the titanila sulphate black liquor . This idea, however, has led to much frustration and does not work in practice, as the anatase is chemically more difficult to be digested.

The Japanese Patent registered in Brazil -PI 7501384-recognizes the inefficacy of the typical sulphation of the Sulphate Process as applied to Anatase and suggests, for the utilization of this ore, that it be used exactly for the production, in an electric furnace and at a high temperature, of the intermediate product already referred to herein as "Titania Slag". Recognizing this difficulty, the Brazilian Patent PI 8406777 also suggests that the Sulphate Process be used for the utilization of Anatase, prescribing , however, that the process begin with a concentrated fluoridric acid digestion for the obtention of the fluotitanic acid with, further, a dislocation of the fluor in a sulphuric acid solution, which requires the use of expensive reagents of considerable danger to operators health when handled, besides representing, in the whole, a rather industrial extensive and complicated process with recognized economic limitations. This Patent's author himself has previously suggested, under Brazilliam Patent Application PI 7605001, that the conventional Sulphation Process, the so-called "solid cake process", be applied to the Anatase, including, as part of the digestion, a long period of cure of the reaction cake in an oven-furnace until its thorough solidification. However, aspects related to the dryness of this cake leads to real diffilculties of premature hydrolysis during the following dissolution, besides the fact that there are limitations of an industrial order in the external heat transfer to the solid cake, which limitations always restrict the process extraction efficiency.

As Anatase is a raw material occurring nowhere else but in Brazil, no other technical references have so far been found to its utilization in other countries. In Brazil, however, many other applications have been made for the registration of patents for the utilization of Anatase. All of these applications, however, by recognizing beforehand the difficulties in the application of the Sulphate Process, always lead to the submission of Anatase ore to an upgrading treatment with the only purpose of subsequently using it directly in the Chlorine Process, as is the case in the applications Patents PI 7507645, PI 7604532, PI 7604610, PI 8401823 and PI 8805053. DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

It was, thus, within this not yet defined framework that, during more recent experimental laboratory works carried out with the specific purpose of obtaining a good Anatase raw material for the Sulphate Process, that it was found out, by an entirely new method, that Anatase ore or its upgraded concentrates presenting variable rates of Anatase to Rutile christaline forms, as a consequence of upgrading process, are thoroughly solubilized by sulphuric acid, under normal ambient pressure conditions, provided that particle size, temperature, acid concentration and molar relation (relating to the molar presence of TiO2 in the reaction) are kept adequate and that, once TiO2 is in solution (digestion), it can immediately and in the same environment, if mantained in the fluid form, be crystallized into a form of a mixture of titanium sulphates (sometimes hydrated and/or anhydrous crystals), easily separable from the fluid medium through filtration or other solid-liquid separation method.

This leads, on filtration, to a sulphated cake somewhat similar to that obtained from the normal sulphate process, easy to be subsequently dissolved in water or diluted acid, or preferably in recicled solution of the "black liquor" itself, so as to produce the well known aqueous solution of this liquor. Moreover, the filtrate from the above-mentioned solid/liquid separation showed to be a clear sulphuric acid, appropriate for being recycled to the initial process stage of fluid digestion, with a minimun purge. It was also verified that the fluid diigestion should be conducted under acid boiling conditions to provie constant acid concentration. This procedure, which has been called Fluid Digestion (as opposed to the usual Solid Cake Digestion), has also indicated the indispensable possibility of an easily adjustment of the cake and dissolved solution "free acidity", whenever required, to maximum values ranging from 50 to 60% , as necessary for an adequate hydrolysis stage, typical of the Sulphate Process. Thus, all these facts, essential in themselves, create, in a safe way, the possibility of industrializing "Upgraded Anatase Concentrates" as, once they already have P₂O₅ content within specifications, it is possible, by starting from the crystallized cake (Sulphated cake) obtained, to acquire, with no other restrictions, the final product, the titanium dioxide and its pigments, through stages which are basically the same as those of the already accredited Sulphated Process.

For a better understanding of the essence of the present invention, an explanatory partial flow diagram is shown in Figure 1, which can be explained as follows: The ore to be used, already in the form of an Upgraded Anatase Concentrate, therefore enriched, with a low P₂O₅ content and represented by the number 1, is introduced into the Digestor (or digestors in series), reference number 3. In the same Digestor there is introduced new sulphuric acid, with a commercial concentration of approximately 94-98%, as per reference number 2. A recycling sulphuric acid flow, reference number 4, obtained by mixing recycles numbers 7, 8 and 11, is also introduced into the Digestor. Inside the digestor, a mixture of acid, ore and crystallised titanium sulphates is mantained in suspension, and in the FLUID form, the digestion occuring in a molar ratio acid-titanium, with a large excess of acid, from 2:1 to 10:1. The ore is promptly digested at times varying, depending on other factors, from 15 to 120 minutes. By keeping the suspension under boiling conditions it is possible to adjust the equilibrium of the system, under constant acid concentration, (reactions that produce H₂O and its evaporation), causing the solubilized titanium sulphates to crystallize, thereby forming crystallized solids, similar to a fine sand, which is kept in suspension until it overflows from the Digestor at the end of the digestion process, all operations being continous. This outgoing flow is numbered 5. Similarly, from the Digestor there go out acid-containing vapors, flow 9 -, which, following heat-exchange and condensation, eventually produces the waehing-acid recycle number 10. After digestion, the suspension proceeds on to the solid-liquid separation 6, where a sulphated cake (incoherent granular, crystallized, damp solid cake) numbered 14 is produced and goes on to dissolution and subsequent Sulphate Process stages. The clear filtered acid recycles, via flows 7 and 4, back into the Digestor. The acid recycles numbered 8 and 11, are mixed and proceed into the Digestor via the decanter-mixer number 12, from which there also goes out, as an essential condition, the adjustable flow of purge 13, which represents a minimum liquid effluent containing the impurity-containing sulphates, which may be neutralized or reserved for a different utilisation outside the system, including, eventually, the previous Anatase ore upgrading treatment. The acid concentration inside the Digestor may vary from 75 to 95% as may the respective reaction and boiling temperatures from 180 to 290 degrees Centigrade. The ore may be introduced grounded to sizes varying from minus 325 mesh to that which is the natural particle size of the anatase concentrate. Depending on each individual case of acid concentration in the digestion, the indispensable adjustment of the sulphated cake free acidity, to not exceed 50 to 60%, may be effected by ensuring adequate crystallization conditions as well, if found necessary, as by the washing operation during the solid-liquid separation 6, by means of the washing flow 10. In dissolving the sulphated cake to produce TiO₂ in solution, and depending on its crystalline forms, addition of a minimum quantity of scrap-iron, water and eventually recycled Black Licuor come to be necessary.

The method described constitutes, in the interrelation of its whole, as shown in Figure 1 and applied to the Anatase ore concentrates, the essential novelty of this invention. A partial method, somewhat similar to the fluid digestion and subsequent crystallization operation, was already mentioned in the German Patent 54.431 (1956) registered in Brazil, which is, however, for application to ilmenite ore and at low temperatures (below 160 degrees Centigrade) and contains no reference to the recycling and purging of the acid used. More recently, another reference to fluid digestion with crystallization was presented and inserted into Patent US 4.562.049 (1985), which is specific for perovskite ore (calcium titanates), directed towards the separation of Calcium sulphate (gypsum) formed in the reaction, which is not the case of Anatase, and which, moreover, does not focus the needed control of the sulphated cake free acidity in the first stage of separation, as is the case now, nor envisages the required purging of the acid for the purpose of eliminating minor circulating ore impurities. The actually new and innovative character of this Patent is thus explained, as indicated, for application specifically to Anatase ore concentrates. The industrial applicability of this Patent is made evident by considering the state-of-the-art feature described above. As a matter of fact, the "sulphate process" for producing TiO₂ and its pigments is recognised as of wide industrial application and, to date, prevails in terms of production tonnage if compared to the "chlorine process". The invention object hereof implies the utilisation of a new raw material, through a special technique whereby a first phase of this same sulphate process is replaced and an adjustment and simplification included for the betterment of the process as a whole. This will, moreover, represent a great economic and social advantage in the production of titanium pigments as, starting from a high TiO₂ content raw material with a rather lower presumed cost per unit of in-ore TiO₂, the process yield will be increased whilst significantly decreasing the consumption of sulphuric acid, provided that it be advantageously recycled and, further, drastically reducing the quantity and cost of the disposal of polluting effluents, typical of the sulphate process using ilmenites, besides securing the public's favourable opinion at large given the dramatic reduction of the environmental problem caused by the Titanium Dioxide industry.

The presence of niobium in the Anatase to be used as raw material will imply a significant content of this metal dissolved in the "black liquor" produced and its inconvenient incorporation into the pigment. However, as per the final treatment technique indicated in U.S. Patent 4.183.768 (1980), this effect may be voided without any significant disturbances or costs. In the case of Anatase with higher contents of niobium, it will also be possible to remove the metal from the "black liquor" through the utilisation of well-known extraction techniques, so as to ensure the obtention of a good-quality pigment and a valuable by-product as pure Nb₂O₅.

Examples of two types may clarify the application of this invention. In the first type, complete flow diagrams illustrating the full process of the invention show its application in different contexts towards an industrial objective. These are the example below, denominated Serie A examples. The second type of examples are related to products, specially intermediary products obtained experimentally from Upgraded Anatase concentrates. These are denominated Series B examples.

Example A-1. Figure no.2 illustrates the method as applied in the production of titanium dioxide pigment in a preferred but not limited integration of the "Sulphate Process", by using the Upgraded Anatase Concentrate. For those familiar with the subject, the figure is self-explanatory, if figure 1 was understood.

Example B-1. An 81-gram sample of Upgraded Anatase Concentrate, the complete analysis of which is indicated at the beginning of this report, has been sulphated, keeping its original particle size of minus 200 mesh, through a reaction with 608 ml of 90 % sulphuric acid, for a period of 25 minutes, at the constant temperature of 250 degrees Centigrade. After the digestion, the suspension was left to cool to 80 degrees Centigrade and then filtered, thereby producing a damp sulphated cake and a clear acid with a concentration of approximately 90%. The sulphated cake was easily dissolved in water pre-heated to 70 degrees Centigrade and kept under agitation. A small amount of scrap iron was then added for reducing Fe₊₃ to Fe₊₂, until the formation of a minimum quantity of Ti₊₃, resulting in a black liquor solution of excellent characteristics, low level of impurities, except for Nb₂O₅, and at approximately 220 g/litre of TiO₂. It was also verified that the total extraction of TiO₂ from the ore was approximately 97.0%, therefore with excellent extractibility. As Nb₂O₅ may in some cases come to affect the quality of the pigment, it was verified, in a subsequent test, that approximately 80% of the said metal was selectively removed from the black liquor by a known extraction method using solvent, which, therefore, evidenced, in any of the cases, the guaranteed quality of the black liquor and, therefore, of the pigment produced from Anatase ore. The present example does not limit but rather illustrates the application of the invention, given that the parameters temperature, acid concentration, molar relation, particle size and digestion time may vary within the limits mentioned above.

Example B-2. An 81-gram sample of upgraded Anatase concentrate, obtained from an ore deposit different from that used for obtaining the sample in Example B-l, was used in the sulphation test. The assays of this sample were: TiO₂=92.2%, CaO=0.06%, SiO₂=6.68%, Al₂O₃=0.15%, P₂O₅=0.13%, MgO=0.01%, Na₂O=0.13%,Nb₂O₅=0.68%, ZrO=0.47%, Fe₂O₃=1.61%, MnO=0.16%, CeO₂=0.05% and La₂O₃= 0.05%. After sulphating in a laboratory reactor heated in an oil bath at the constant temperature of 215 degrees Centigrade, for 30 minutes, and having the ore particle size below 325 mesh, a sulphated cake was produced which, after having been filtered and washed, resulted in a free acidity of approximately 52%, with the production of an excellent quality black liquor having the following composition: 200g/l of TiO₂, 5.0g/l of ferrous sulphate expressed in Fe₂O₃, 0.25 g/l of P₂O₅, 1.4 g/l of Nb₂O₅ and traces of the other impurities, including chromium and vanadium. The TiO₂ solubilization efficiency was 95.8%.

Example B-3 - An 81 gram sample of Upgraded Anatase Concentrate, obtained from an ore deposit different from that used in the previous examples, of especial quality, but with greater proportion of rutile crystalline form , was used in the sulphation test in its natural particle size of 5% - 200 mesh. The assays of this sample were: Ti0₂=94.8%, CaO=0.06%, SiO₂=1.40%, Al₂O₃=0.23%, P₂0₅=0.23%, MgO=0.10%, Na₂O=0.14, Nb₂O₅=0.75%, ZrO=0.28%, Fe₂O₃=1.40%, MnO=0.18%, CeO₂=0.08%, La₂O₃=0.07%. After sulphating in a laboratory reactor, heated in an industrial oil bath at the constant temperature of 275 °C, for 60 minutes, a sulphated cake was produced which, after have been filtered and washed, resulted in a free acid of only 35%. The dissolution using 200 ml of previously obtained black-liquor, was efficiently completed in 30 minutes, and resulted in a solution of excellent quality and extractibility efficiency of 96%.

Claims

C L A I M S

What is claimed is:

1- A Process for Production of Titanium Dioxide of basic pigment qualities, using, with some especific and characteristic differences and simplifications, the well known Sulphate Process and utilising a new raw material, the anatase ore, as a beneficiated concentrate, called Upgraded Anatase Concentrate, presenting an unusual high TiO₂ content and singular properties in sulphuric acid digestion.

2- The process, as defined in claim 1, characterised by using, for the production of the water solution of titanium, known as "black liquor", the following steps and conditions:

a) Digesting the raw material, an anatase ore beneficiated concentrate called "Upgraded Anatase Concentrate", no matter its ratio of anatase / rutile, in its somewhat modified final crystalline structure.

b) Digesting the anatase concentrate with sulphuric acid, in a large excess acid, in the acid/ore range of 2:1 to 10:1 , mantained in a fluid suspension environment inside the reactor, where the titanium solubilised is subsequently completely crystallised as titanium sulphates or mixture of them, toghether with other impurity sulphates, in a form of sand like material, easy to be separated from the liquid phase.

c) Digesting the anatase concentrate at high temperature, within the range from 180 to 290 degrees centigrade and acid concentration between 75 and 95% by weight, using external heat as needed, and mantaining the suspension under boiling condition, to assure constant concentration of the acid, as reactions form water, and under normal atmospheric pressure or light vacuum, so as to facilitate vapor removal.

d) Digesting the anatase concentrate for a period (residense time) sufficient not only for the solubilization of the ore, but also for the complete recrystallization of the solubilized titanium and impurity sulphates, which occurs in a relatively short time from 15 minutes up to 2 hours, depending on other factors, like particle sise, final modified crystalline form of the anatase concentrate in use, concentration of acid, molar ratio and temperature. e) Solid-liquid separation of the digested suspension,after a rapid codling to 50 - 130 °C, which leads to the easy separation of a wet sulphated cake from a clear filtrate constituted by essentially the acid itself, in a concentration aproximately equal to the original one, which acid is returned in a first recycling operation to the initial digestion, together with new make-up acid.

f) Introduction, into the solid-liquid separation operation described above, if needed, depending on the nature of the crystallisation occurred, which is based on the acid concentration selected for the digestion stage, of a step where washing is carried out by means of diluted recycling acid, for adjusting the final free acidity of the sulphated cake produced to a remaiming range of 50 to 60% or below these values, as may be required for the stage of hydrolysis, typical of the Suphate Process.

g) Leaching the sulphated cake with water, to produce the solution of titanium, at temperatures in the range of 50 to 130 degrees centigrade, for the necessary period of time, using a small and calculated addition of scrap-iron, to produce the appropriate (g/l of titaneous) black liquor or preferably using more voluminous black liquor recycle, depending on the sulphated cake crystalisation nature, and also an eventual addition of recycled hydrolisis mother-acid liquor.

3- The process, as defined in claims 1 and 2, wherein the aqueous solution containining the titanium values , is clarified and subjected to hydrothermal precipitation to recover the titanium dioxide, by final filtraction and calcination, dispensing the step of cooperas crystallisation and evaporation, typical of the usual "Sulphate Process".

4- The process, as defined in claims 1 to 3 above, characterised by the fact that it is possible to remove, from the black liquor produced, the Nb₂O₅ present in the solution, originated from the ores with a high content of the said metal or due to more strict specifications as to this metal, concerning the production of certain types of pigments, which removing step is carried out in the solution state by well-known extraction means, using solvent for example, with no disturbance of the whole of the process and producing a valuable by-product as pure Nb₂O₅ or otherwise, through the annulment of its effect on the pigment, by using appropriate additives in the roasting step of the TiO₂ pigment production process.

5- The process, as defined in claims 1, 2 and 3, characterized by the total recycling of the hydrolisis liberated acid, in this particular case an acid containing a lot less impurities, part of said acid being directlyused in the dissolution of the sulphated cake, to adjust the free acidity, and the other part to be recycled to the digestor, after a step of evaporation, less expensive due to the high quality of the acid, includind the fact that the produced vapors can be condensed and recycled to an acid sulphuric plant, eventually existing nearby.