UA79423C2 - Process for causticisation of bayer's solutions and process for stabilization of hydrocalumite particles - Google Patents

Abstract

An improved process for the causticisation of Bayer's liquors in an alumina refinery, the process including the steps of reacting lime with aluminate ions in a Bayer's liquors within a primary reactor under controlled conditions of low to moderate temperature (between 70-80 DEGREE C) and agitation, to form substantially only a hydrocalumite species and hydroxyl ions; and a secondary reactor wherein said hydrocalumite species formed is subjected to heating in contact with a Bayer's liquors under controlled conditions so as to cause the hydrocalumite species to react with the solution to form calcium carbonate, aluminate ions and hydroxyl ions, whereby a causticised Bayer's liquors is obtained and wherein the efficiency of lime utilisation is substantially increased and/or alumina losses are minimised.

Description

Description of the invention

The present invention relates to an improved method and equipment for the causticization of Bayer 2 solutions in the purification of aluminas, and in particular relates, although not exclusively, to a technological process by which a significantly increased C/5 ratio can be achieved and/or significantly improved application efficiency lime and/or reduce alumina loss.

In the Bayer technological process for the production of alumina, a concentrated solution of sodium aluminate is obtained by grinding and boiling bauxite in a solution of caustic soda, usually under conditions of 70 degrees of elevated temperature and. pressure After cleaning the clay suspension, the concentrated solution of sodium aluminate is cooled and etched with gibbsite crystals, causing crystallization from the solution with the help of gibbsite.

The gibbsite is calcined to produce alumina, and the depleted (or "spent") solution is recycled to digest new batches of bauxite.

During boiling, a certain part of the caustic soda takes part in unwanted reactions with the pollutant 72 substances in the bauxite, thereby reducing the productivity of the solution. One, the most significant of such reactions leads to the formation of sodium carbonate, occurring as a result of the dissolution of inorganic carbonates present in the mineral phase, or as a result of thermal and oxidative reactions of the destruction of organic compounds.

Without control, after each circulation of the solution in the technological process, the concentration of sodium carbonate will increase, and accordingly, the ability of the solution to boil gibbsite or boehmite from bauxite will decrease.

The most common way to control the concentration of sodium carbonate in Bayer technological solutions is caustication using quick or slaked lime. This process can take place within the digestion cycle itself (feeding lime together with bauxite) or more often as a side stream treatment.

Adding lime directly with bauxite is not very common, except when lime is needed to control other contaminants (such as titanium or phosphorus), because highly concentrated solutions degrade performance. As long as the temperature is not too high, most of the lime enters into side reactions (3 reactions with aluminate in the solution with the formation of particles of calcium aluminate, in particular tricalcium aluminate (TSA, which is often referred to as SZA in the cement industry).

In the most common sidestream causticization process, a stream of diluted solution (usually taken at the mud washing stage) reacts with a slaked lime suspension, generally at a temperature close to the atmospheric boiling point of the combined solution. Alternatively, the slurry is sometimes added directly to the mud washer. The amount of converted sodium carbonate and the efficiency of lime application depends on many factors, but at most refineries the efficiency of lime is in the range from 5095 to 7095. Ge»!

In alumina production, as a rule, the level of carbonate contamination of Bayer solution 3о is considered from the point of view of the ratio of caustic and soda or C/5. Here, "C" refers to the sum of the concentrations in sodium aluminate and sodium hydroxide, expressed as the equivalent concentration of sodium carbonate. The term "5" concentration refers to the sum of "C" and the actual concentration of sodium carbonate, whereby this sum is again expressed as an equivalent concentration of sodium carbonate. It follows that a fully causticized (carbonate-free) Bayer process solution will have a C/5 ratio of 1.00. As a rule, the C/5 ratio of the concentrated solution flow at most alumina refining plants is between 0.8 and 0.85. Higher C/5 ratios are difficult to obtain, since modern technological processes of causticization are not capable of completely removing all sodium carbonate from the solution streams involved in them.

For example, a solution with a 5 concentration of 135 g/l will causticize to a C/5 ratio of about 0.890 in most cases. This limitation arises because the traditional causticization with slaked lime is conditioned by a number of complex balances, including a competing reaction involving the aluminate ion, in which TSA is formed. (se) In contrast, the reaction of causticization of pure mixed solutions of sodium carbonate and sodium hydroxide with slaked lime is quite simple. The final concentration of hydroxide and carbonate ions is a function of the action of different types of ions in equilibrium with the solid phase of calcium hydroxide and calcium carbonate. The reaction can be described by the following equation. byu» Ssa(on)»nma»soz»Sasozh2maon (I)

As a rule, it was believed that the above reaction is also used when causticization is carried out in Bayer's technological solutions. However, it has been known for some time that calcium hydroxide readily reacts with

Gee! aluminate ion with the formation of TSA. The formation of TSA is generally believed to have one or both of two mechanisms: a simultaneous competitive reaction in which calcium hydroxide reacts directly with the aluminate ion to form TSA (Spariip, MT. SidnE Meiyaev (1971), 47-61) or a reaction "reversion" in which calcium carbonate formed during caustication reacts with aluminate. However, some authors have expressed the opinion that causticization of 60 Bayer's solutions occurs through "an intermediate compound of hydrated tricalcium aluminate" (Moshpa,

K.S. Tsoi Meiyav (1982), 97-117| or "carboaluminate" phase (Esiaga, A; Tgamkhoih ISBOVA, 12(17) (1982) 149-156) and that TSA is formed when this material ages.

In contrast to the proposed mechanism, causticization practiced in technological processes

Bayer, was inefficient in terms of the achieved C/5 ratio and lime application productivity. 65 Moreover, the low efficiency of lime application meant that a fairly significant amount of aluminate ions were consumed in the formation of TSA. This can cause a significant reduction in alumina production.

In recent years, a number of technological processes of causticization aimed at increasing the efficiency of lime have been proposed. However, the value of such technological processes is not limited by the fact that they are limited to the use of washing solutions with a low "5" concentration, which requires the treatment of large streams when it is necessary to convert a significant mass of sodium carbonate to compensate for the consumption of carbonate in the refinery. (patent 5 Mo2,992,893) discloses a technological process in which thickened mud, after the final stage of mud washing, was causticized and then reacted with an additional amount of sodium carbonate to restore some amount of lost alumina during the formation of TSA. The caustic solution is then used in the dirt washing stages. In addition to the concentration limitation of "5", this technological process is also imperfect because a significant part of the caustic solution is lost together with the sediment of red mud.

The improvement of this process is described in (OZ patent Mo3,120,996), in which causticization takes place in the washer of the first stage with subsequent addition of lime to the washer of the third stage. A higher efficiency of lime (approximately 85-9595) was achieved, but only in fairly diluted flows of the washer (80g/l "5"), /5 and the achieved ratio C/5 of the caustic solution was not very high.

A more recent development, described in (OZ patent Mo3,210,155), involves direct quenching of quicklime in an illuminated washing solution, which was heated to 1002C. After the reaction, the suspension was mixed with another washing solution to initiate the reaction of TCA with sodium carbonate, and thus recover the alumina. While high C/5 ratios were claimed in this process, it was limited to wash streams with "5" concentrations of approximately 15 to 40g/L.

Another technological process was developed in Hungary in the 1980s by Vagvka et al. and described in (patent

B Mo4,486,393|. In this technological process, the suspension of red mud from one of the washing stages was heated and fed to the reaction chamber with an excess of lime suspension. In addition to the "normal" causticization that occurred in this chamber, the excess lime reacted with the desilication products of sodalite and cancrinite to form calcium hydrogarnet, releasing sodium hydroxide. The drain from this chamber was then loaded into the second chamber and subjected to further interaction with the sodium carbonate solution. This solution was obtained by salting out sodium carbonate from concentrated solutions anywhere in the plant. The interaction of sodium carbonate with hydrogarnet or "hydrated" calcium aluminate led to the recovery of alumina and caustic, although this step tended to negate the benefits obtained by the formation of hydrogarnet particles. Along with the improvement of the basic principles of causticization, the loss of lime and alumina due to the formation of TCA still remains significant, and the achieved C/5 ratio is limited by the carbonate/hydroxide equilibrium reaction. In addition, the efficiency of c is greatly degraded if streams with low "5" concentration are not disposed of. "I

Summarizing the above, it becomes obvious that the previous methods of causticization had disadvantages both in relation to the degree of causticization of Bayer technological solutions (that is, the maximum C/5 that can be

Reach), as well as regarding the efficiency with which lime is used to carry out such causticization. Due to the low efficiency of lime application, these technological processes lead to the loss of alumina from the solution, thereby reducing the productivity of alumina purification plants. In addition, known methods are limited by the concentration of solutions that can be causticized; they become very ineffective with solutions approaching typical first stage mud wash solutions or excess mud separator solutions. "

This invention was developed in order to provide a technological process and equipment for 8s improved causticization of Bayer solutions, which would have fewer disadvantages inherent in the former. methods given above. "» In accordance with one of the aspects of this invention, an improved technological process of causticization of Bayer solutions has been developed at an alumina refinery, and the process includes the following stages: - the interaction of lime with aluminates in Bayer solution under controlled conditions from low to medium temperature with significant formation only hydrocalumite particles and hydroxyl ions; and, heating said hydrocalumite particles in contact with Bayer's solution under controlled conditions so as to cause the hydrocalumite particles to react with the solution to form calcium carbonate, aluminate ions, and 50 hydroxyl ions, thereby producing a causticized Bayer solution and where the efficiency of lime application is significantly increased, and the loss of alumina is minimized.

GC) As a rule, the first reaction, which involves the formation of a hydrocalumite solution, occurs at a temperature between about 259C and 1002C. Generally, the best performance for most Bayer solutions is achieved when the temperature is maintained between about 70°C and 80°C. Mostly the first reaction

Occurs in Bayer's solution, which is subjected to stirring.

Mostly, the second reaction, which involves the heating of hydrocalumite particles, takes place at a temperature of about 1002 to 1802C. Most preferably, the second reaction takes place under conditions of low shear at approximately where 120°C.

It is better when the technological process further includes a stage in which a suitable inhibitor is added to the Bayer solution, at a suitable moment 60 before heating the hydrocalumite particles, as a result of which the unwanted interaction of the hydrocalumite particles with the formation of TCA is inhibited. Preferably, the above-mentioned inhibitor is a complex-forming agent and/or surfactant capable of adsorbing on active areas of the surface of hydrocalumite particles, limiting the diffusion of active particles on these areas. Examples of natural surfactants include sugars such as sucrose and glucose and polysaccharides such as starch. The most desirable is the use of anionic organic surfactants. Examples of anionic organic surfactants include the following materials, their salts, and derivatives: any anionic homopolymers or copolymers (polyacrylic acid and its copolymers with acrylamide or polymers containing functional hydroxamate groups), hydroxamic acids, humic and tannic acids, lignosulfonates, fatty acids, sulfonated carboxylic acids, carboxylic acids and polyhydroxycarboxylic acids.

It is better that the Bayer solution used in the first reaction, which involves the formation of hydrocalumite particles, would be pre-causticized, thanks to which the C/5 ratio of the pre-causticized solution can be further increased.

Preferably, the first reaction is carried out in a Bayer solution with a moderately high A/C ratio and a low level of free caustic. A suitable solution will, as a rule, have a "5" concentration between 40 and c5Og/l, and a 7/0 bpvvratio A/C between 0.2 and 0.95. It is more preferable when the solution will have a "5" concentration between 120 and 160 g/l, and the A/C ratio is greater than 0.55. As a rule, the time required for the completion of the first reaction is between 5 and 30 minutes, in the presence of a suitable inhibitor.

Preferably, the hydrocalumite solution formed in the first reaction would undergo a solid/liquid separation, and the hydrocalumite solids reacting with the fresh solution would be causticized during the aforementioned /5 HOUSING reaction.

In accordance with another aspect of the present invention, an improved technological process of causticization of Bayer solutions in the purification of alumina has been developed, and the process includes the steps: adding a suitable inhibitor to the Bayer solution to stabilize the formation of hydrocalumite particles during caustication to inhibit the undesirable reaction of hydrocalumite particles with the formation of TCA, in as a result of which the achieved C/5 ratio of the solution can be increased.

It is preferable that the above-mentioned inhibitor is a complex-forming agent and/or surfactant capable of adsorbing on active areas of the surface of hydrocalumite particles, limiting the diffusion of active particles on these areas. Examples of natural surfactants include sugars such as sucrose and glucose and polysaccharides such as starch. The most desirable is the use of anionic organic surfactants. Examples of anionic organic surfactants include the following materials, their salts, and derivatives: any anionic homopolymers or copolymers (polyacrylic acid and its copolymers with acrylamide or polymers containing hydroxamate and) functional groups), hydroxamic acids, humic and tannic acids, lignosulfates, fatty acids, sulfonated carboxylic acids, carboxylic acids and polyhydroxycarboxylic acids.

It is preferable that the technological process further includes steps of heating the solution during causticization to a temperature ranging from 1002C to 18090. It is most desirable when the solution is heated to a temperature of 1202C to 14020.

In accordance with one more aspect of this invention, an improved technological process of causticization of Bayer solutions in the purification of alumina has been developed, and the process includes the following steps: obtaining a pre-caustic Bayer solution, and b interaction of lime with aluminate ions in the above-mentioned pre-caustic Bayer solutions under controlled conditions from low to medium temperature with significant formation of only hydrocalumite particles and hydroxyl ions, thanks to which the C/5 ratio of the pre-causticized solution can be further increased. "

In accordance with another aspect of this invention, equipment for causticization of Bayer's solutions during alumina purification has been developed, the equipment includes: - a standard reactor for causticization of Bayer's solution; and there is a specially adjusted reactor for the reaction of lime with aluminate ions in the causticized Bayer solution under controlled conditions from low to medium temperature with significant formation of only hydrocalumite particles and hydroxyl ions, due to which the C/5 ratio of the causticized solution can be further increased. -I In accordance with another aspect of this invention, equipment for improved causticization has been developed

Bayer's solutions in the purification of alumina, the equipment includes: a primary reactor for the reaction of lime with aluminates in Bayer's solution in controlled conditions from low to medium temperature with significant formation of only hydrocalumite particles and hydroxyl ions; and a secondary reactor wherein said hydrocalumite particles are heated in contact with the Bayer solution under controlled conditions so as to cause the hydrocalumite particles to react with the solution to produce calcium carbonate, aluminate ions and hydroxyl ions, thereby producing a causticized Bayer solution, and wherein the efficiency of lime application is significantly increased, and the loss of alumina is minimized.

As a rule, the primary reactor is a stirred tank reactor in which lime and Bayer solution are adequately mixed to promote the first reaction. o As a rule, the secondary reactor is a stirred tank reactor. Alternatively, a pressurized tubular reactor can be used.

Preferably, the equipment also includes means for separating the solid hydrocalumite particles from the solution from the primary reactor before reacting the hydrocalumite particles in the secondary reactor with fresh solution.

It is most desirable that the solution causticized in the secondary reactor is used as a raw solution for the primary reactor, thanks to which the C/5 ratio of the causticized solution can be significantly increased. 65 In order to facilitate a more detailed understanding of the nature of the invention, several forms of embodiments of the improved causticization process and equipment will be described in detail by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a simplified conceptual curve diagram of a primary application of an improved process in accordance with the invention.

Figure 2 is a simplified conceptual curve diagram illustrating a further application of an improved process in accordance with the invention.

Figure C is a conceptual curve diagram illustrating reinforcement of the process illustrated in Figure 2.

Figure 4 is a conceptual curve diagram illustrating further reinforcement of the technological 7/0 process illustrated in Figure 3.

Figure 5 is a conceptual curve diagram of a preferred embodiment of an improved process of causticization in accordance with the invention.

Figure 6 is a conceptual curve diagram of another embodiment of an improved process of causticization in accordance with the invention; and

Figure 7 is a conceptual curve diagram of yet another embodiment of an improved process of causticization in accordance with the invention.

This invention is based on the discovery that the reaction of lime (slaked or unslaked) in Bayer process solutions occurs through a series of sequential reactions. Unexpectedly, the inventors' studies show that the direct reaction of calcium hydroxide with carbonate ion, which is described in equation (1), does not reach a significant degree. More significantly, it has been found that by appropriate manipulation of solution compositions and temperature and stirring conditions, these reactions can be separated into distinct steps that can be individually optimized. Such optimization can increase the efficiency of lime application to 9595 or more.

The most telling thing is that the inventors found an opportunity to profitably use completely different balances, which are applied at each of these stages, to significantly increase the efficiency of carbonate removal.

The described technological process of causticization can be directed in such a way that it becomes possible to achieve i) a C/5 ratio close to 1.00, even in fairly concentrated Bayer technological solutions. A very surprising finding in this work is that the C/5 ratios that can be achieved will be even higher than those obtained in pure aluminate-free sodium hydroxide/sodium carbonate solutions at an equivalent sodium concentration.

Such a combination of very high C/5 ratios and high lime application efficiency, even with relatively concentrated solutions, has never been possible using known technological processes. Such flexibility makes it possible to expand the scope of application of this technological process in new directions in the purification of alumina, using solution streams that could not be processed using known technological processes. Without going into theory, it is believed that the following sequence of M reactions occurs.

Reaction 1

The inventors discovered that in solutions containing both sodium aluminate and sodium hydroxide, calcium hydroxide first reacts to produce calcium aluminate of lamellar structure, the areas of the intermediate layer of which "

Filled with charged balancing ions and water molecules. Similar fractions obtained under completely different reaction conditions were reported in the literature on the cement industry. (Gizpeg, K. apa Kigei N.)., Setepi apa Sopsgebe Kezeagspe, 12, (1982), 517-526), where they are designated as C4A compounds. The structures are similar to ;" mineral hydrocalumite, which occurs in nature, and therefore, for convenience, this name is used in some works (Regotsa, Au. apa UMiPatve, R., GidnE MeiyiIz (1995), 77-87). The term hydrocalumite will also be used in this description. Such a hydrocalumite particle is formed very quickly in almost any Bayer solution. In -I chemistry of the technological process, no significant changes were found when calcium oxide (quick lime) is used instead of calcium hydroxide, since initially the quenching reaction simply took place. However, the efficiency of reactions using quicklime is lower than with slaked lime, apparently because their reaction products, which are formed, tend to inhibit the diffusion of calcium to the particle surface. The result is that some of the lime does not react. o The main reaction form in Bayer solutions when calcium hydroxide is used is shown in 4) equation (2) below:

ASakson)g2A TSO) oh-nin 20 (San) vIoh2-pnoovaON:- (2) , , , , , ,

Charged balancing anions can be any of all particles, they are designated as X- (Ф) in the above reaction. A number of particles of this general form, which differ only in the type and number of charged g-balancing anions and water in the intermediate layer, were determined on the basis of their CKO forms and by chemical analysis. In the absence of other anions (especially carbonate), the charged balancing particle is, as a rule, an ion in the hydroxyl, which gives the following reaction:

In the future, for convenience, the particles formed in this reaction will be called hydrocalumite 0, or Hso. c Analysis of this reaction shows that since there is no net change "C" in the concentration of the solution, the concentration (A) of alumina will fall due to the consumption of the aluminate ion. For solutions containing at least some amount of carbonate, one of the hydroxyl ions in the above structure is replaced by one half of the carbonate ion, as shown in the following reaction equation,

АСасон)га2АЗОН) 7 1/20032-5595Н20ИСагА (ОН) in» 1/2сО3:оН.51/2nНоОЗОН- (4)

Hereafter, the particle formed in this reaction will be called hydrocalumite 1 (Hc1). Its formation is a moderate caustic reaction. Since two moles of aluminate are consumed for one mole of Ni, three moles of hydroxyl ions are released. Thus, a net increase in "C" concentration of one mole of hydroxide per 70 4 moles of calcium hydroxide will be achieved.

In the literature, there was information about another reaction, which involves the substitution of two of the hydroxyl ions, which is described by the following reaction:

АСа(сон)гя2АцОН) с СО32-5НоО g» (СагАцОН)вИо СО ЗНО Он- (5)

This is a more efficient causticization reaction, releasing 4 moles of hydroxyl ions for every 2 aluminate ions consumed. Net increase in "C" concentration, thus 2 moles of hydroxyl ions per 4 moles of calcium hydroxide. Since the inventors discovered that the causticization of Bayer's solutions involves a compound whose HKO structure exactly corresponds to the above-mentioned particles, the change in concentration in the solution is incompatible with the formula shown in equation (5). Thus, it is unlikely that a significant amount of the material specified in equation (5) is formed during the reaction of lime in aluminate solutions. However, the amount of water in the interlayer of the hydrocalumite structure is highly variable, and this changes the interlayer spacing. A particle with a CKO structure similar to the compound in equation (5) is known and the inventors propose that this species is formed in

in Bayer's solutions by dehydration of He1 according to the following equation: сч 29 ИСагАТСОН)вИ»1/20О8:ОН.51/2Н20. х|СаоА (ОН) в|»1/20сО3 ОН-А1М/2НоОЗИАНгО (6) Ge)

The particles formed in this reaction will be called hydrocalumite 2 (Neg).

As a rule, during the reaction of slaked lime, Nei will be formed first. As the structure ages, often within minutes, a mixture of Hec1 and Hec2 will be formed. Later, as the reaction proceeds and the carbonate in the o30 solution is exhausted, only Neo will be formed and the final product will consist of a mixture of He1, Neg, and c

Neo. Due to the dependence on aging and carbonate concentration, it is very difficult to determine in advance the exact ratio of Nebo, He1 and He2 formed. Other reaction conditions will also to some extent affect the products M of the reaction. However, Hs1! is the dominant product formed under most conditions, and this fraction can be Ge"! apply for stoichiometric calculations.

The reaction of lime with the formation of hydrocalumite is controlled by diffusion, and therefore the temperature does not greatly affect the rate of formation. On the other hand, it seems that the interconversion between Hc1 and Hc2 phases still depends on temperature.

Equations (4) to (6) are the key results of the development of this invention. Many previous studies "supposed the simultaneous formation of calcium carbonate and TCA, which is caustic neutral, that is, releases two C 70 hydroxyl ions for every two aluminate ions consumed. On the contrary, the above equations mean that the formation of hydrocalumite can be used in causticization of Bayer solutions. It is important to note that the caustic effect of hydrocalumite formation is not limited by the carbonate/hydroxide equilibrium. Assuming that the surface diffusion barrier is not an obstacle, the formation of hydrocalumite will continue until the calcium hydroxide or 75 aluminate ions are almost completely consumed. Carbonate, the preferred ion, will thus continue to adsorb into the structure until no more material is formed, or until almost all of the carbonate is removed from the (Ce) solution. At low concentrations of carbonate, other anions can thus be inserted into the structure, causing the process of causticization of other contaminating salts in Bayer solutions. The last aspect is the subject of patent application MORR 9334). ko 20 All these hydrocalumite particles are quite stable at low temperatures, but become unstable when the temperature rises. In addition to the temperature, the rate of decomposition and the particles that are formed also depend on the composition of the solution with which they are in contact. The dominant decomposition reactions include the desired reaction with carbonate ions, which produces calcium carbonate, and the undesired reaction, which produces TCA.

However, if these compounds form while the calcium hydroxide particles are still reacting to form hydrocalumite, they can act as a diffusion barrier and prevent complete conversion. This action is possible

HF) can be prevented by adding a small amount of a suitable inhibitor such as a complexing agent or surfactant (eg sodium gluconate or sucrose). Reaction 2

The conditions under which the above-mentioned particles react with the formation of calcium carbonate can be deduced from the following 60 reaction mechanism.

IsagATZON)vI»1/2сО3:ОН.51/2Н20х31/20032-«34Сасоз2АТСОН)дД i BON-К51/2Н20 (7)

This is the main caustification reaction and in traditional caustification technological processes it will begin almost immediately after the formation of hydrocalumite. Analysis of the above equation shows that in this reaction each mole of Na reacts with 3.5 moles of carbonate to form 4 moles of calcium carbonate and releases 5 hydroxyl ions along with 2 moles of aluminate. Thus, any aluminate consumed during the formation of hydrocalumite (be it Neo, Ne1, or Nesg) is released again in the reaction.

Therefore, in the traditional causticification reaction, it is observed that the concentration of alumina drops rapidly, usually accompanied by a slight increase in C/5, corresponding to the formation of hydrocalumite. After that, there is a slow, but significantly greater growth of C/5, along with an increase in the concentration of alumina, as the reaction described by equation (7) proceeds.

The reaction of hydrocalumite with carbonate ions to form calcium carbonate proceeds better under conditions of high 7/0 Carbonate concentration, low aluminate concentration and low hydroxide concentration. It is important to note that the reaction is under chemical control and is therefore relatively independent of mixing conditions. However, speed is greatly affected by temperature; and the speed increases approximately by an order of magnitude (10 times) for every 6-8 degree increase in temperature. Therefore, the inventors found that at 1032C, the reaction takes approximately 2 hours to complete, while at 1202C, it takes only a few minutes.

It is also important to note that the degree of reaction will depend on the equilibrium between solids and various particles in the solution. Therefore, the maximum achieved C/5 will be a function of the activity of carbonate hydroxide and aluminate ions. Analysis of equation (7) shows that the equilibrium is much more affected by the concentration of hydroxide than the concentration of aluminate; so it would be beneficial if the solution introduced into the process had a high A/C ratio (ie low free caustic). This can be facilitated by ensuring that hydrocalumite does not form in the solution to be causticized, as this reaction will lower the A/C. However, the rate of reaction (7) decreases if the aluminate concentration is too high.

The preferred A/C interval is from 0.5 to 0.7.

An increase in temperature also leads to a shift of the equilibrium reaction in the direction of the products of this reaction, allowing to achieve a higher C/5. The reaction speed also increases significantly. This is especially useful with Ga solutions that have a high A/C. However, if the temperature is too high, the efficiency decreases, because the rate at which TSA is formed, while not very dependent on temperature, nevertheless increases with increasing temperature. Therefore, the best performance will be achieved with a solution with a moderately high A/C ratio, and at a temperature between 1202 and 140260.

Reaction From co

The last reaction to consider is the formation of TCA. TSA (Sa ZAKON) vI") has a chemical formula similar to hydrocalumites, and it would be fair to assume that these particles can react under appropriate conditions with the formation of TSA. | indeed, so it seems and is. Experiments by the inventors show that hydrocalumite reacts with aluminate ions and hydroxyl ions with the formation of TSA. Obviously, it is not so important what proportion of Hs, described earlier (HsOo-Hsg), is involved in the reaction. Therefore, using Hc1 as an example, it is believed that the following F reaction describes the formation of TSA:

ЗИСагАЦОН)вИ»1/2сО8:ОН.51/2Н2052АЦОН)d В ОН-. x4SazIide 1 (OH)vIo3/2сО32-161/2Н20 (8)

This reaction is facilitated by a high concentration of aluminate and hydroxide and a low concentration of carbonate. These "conditions are reached at the end of the traditional technological process of caustication, which can explain the noticeable consumption of alumina on TSA, and over time a stable drop in C/5, if the time of presence in the causticizer is excessive. In addition, the above reaction is apparently diffusion-stirring dependent, and the presence of a large "" surface area of hydrocalumite greatly affects the rate of TCA formation, but is less affected by increasing temperature. Thus, a suitable balance of high temperatures and gentle stirring during of the main causticization reaction (Equation 7) will reduce the formation of TCA (and increase the efficiency) because - and the rate of consumption of hydrocalumite to form calcium carbonate is much higher than the rate of its reaction with the production of TCA. » a process in which desirable steps are optimized and undesirable reactions are minimized Such a process cannot be

In a single reactor vessel (unless the vessel is designed for batch operation and the conditions cannot be changed during the reaction), because each of these steps requires mutually exclusive conditions to operate effectively. In all known technological processes, individual stages of the caustic reaction are not determined or optimized in this way. Therefore, the technological processes that are now used represent an unsatisfactory compromise between suitable losses of lime and alumina and the depth (degree) of causticization.

Development principles

When developing an improved technological process of causticization, the inventors determined the following

F, principles: slaked or slaked lime was first preferentially subjected to interaction with aluminate ions with the formation of only lamellar SAA structure (hydrocalumite) in a reactor with thorough mixing (primary causticization bo reactor). The exact nature of such a reactor is not critical, and any system that provides adequate mixing of the reactants could be used. In order to guarantee that the process proceeds without residual unreacted lime and to prevent the formation of calcium carbonate or TCA, certain conditions must be provided. The reaction should take place at low to moderate temperatures (between 259C and 10022C). The exact upper limit is a function of alumina, carbonate, and free hydroxide concentrations. But the best performance with 65 most solutions is obtained when the temperature is maintained between 70C and 80C. The greatest efficiency is obtained with a solution with a moderately high A/C ratio and low free caustic. If the selected temperature or concentration of free caustic is too high, there is a possibility that the reaction will be hindered by the formation of TCA, which acts as a diffusion barrier. There will be a tendency to prevent the reaction of calcium hydroxide in full, while particles with a core of unreacted lime will be formed, which will reduce the efficiency of the reaction. Carbonate concentration is less important, but the lower the carbonate concentration, the lower the maximum temperature at which this process step could occur. Suitable solutions will have a "Z" concentration between 40 and 200 g/l. (preferably between 120 and 160g/l), and A/C ratio between 0.2 and 0.95 (preferably more than 0.55). The time required to complete the reaction is in most cases between 5 and

REFERENCE However, if the proper composition of the solution and temperature are used, then a longer time will not have 70 undesirable negative effects.

In a typical application of the principles described above, the hydrocalumite in contact with the solution is heated to initiate the reaction described in equation (7). The solution should be heated to the maximum permissible temperature. In continuous systems, after that, it will be necessary to move the solution to the second reactor (secondary causticization reactor). In a simple leaky mixing tank, the solution should /5 be heated to a temperature close to the boiling point of the solution. Mixing methods in such a system should be provided separately. If the agitation is vigorous, the hydrocalumite will react with the aluminate and hydroxyl ions to form TCA and cause a loss of effectiveness. It is preferable to use a low-shear reactor with a flow barrier (such as a reactor tube) that operates at temperatures between 1002 and 1802C, although the greatest efficiency is achieved at 120C. The exact time required for the reaction of the hydrocalumite formed in step 1 depends on many factors, especially the temperature and the presence of surfactants.

However, a typical stirred tank reactor operating at approximately 1032C will require somewhere around 2 hours to complete the reaction, while a reactor tube operating at 1202C will require approximately 15 minutes.

Ideally, the suspension formed in step 1 is filtered or other methods are used to separate solids and liquids. The solid should then be reacted with a fresh solution, which will be causticized.

Ideally, the solution that is causticized in the secondary causticization reactor is used as a feed solution for the primary causticization reactor. This will ensure that the A/C ratio of the solution fed to the primary reactor is high. More importantly, it allows even higher C/5 ratios to be achieved, since the formation of hydrocalumite is a caustic reaction. Since this reaction is not subject to the same equilibrium that occurs in the secondary reactor, it is possible to achieve a C/5 ratio close to 1.00 in this reactor if the appropriate amount of lime is added. "

The effect of Fu supplements

During the development of this technological process, the inventors discovered that when a suitable inhibitor is added, the occurrence of the undesirable reaction of hydrocalumite with the formation of TSA can be significantly reduced, while significantly not affecting the reaction of He with carbonate to form calcium carbonate. This results in higher maximum values of C/5, with higher efficiency and ease of lime application. This is because the reaction of hydrocalumite with aluminate and hydroxyl ions to form TCA depends on diffusion (Equation 8), while the reaction of He with carbonate is independent (Equation 7). Therefore, the compounds that absorb on the active areas of the surface of He will inhibit the diffusion of active particles on these areas, delaying the tc reaction. On the other hand, since the presence of these absorbed molecules can also partially inhibit the reaction of h with carbonate, the effect will be much smaller. Such retardation of the reaction of Hs with carbonate can be avoided » by strengthening any of the factors that improve the caustication reaction, the most effective and simple of which will be an increase in temperature.

You can use almost any type of surfactant, provided that it is absorbed on hydrocalumite structures. For example, you can use sugars, such as sucrose and glucose, and polysaccharides, such as starch. However, the inventors have found that anionic organic surfactants are the most effective.

The open list of this class of compounds includes the following materials, their salts, and derivatives: any - anionic homopolymers or copolymers (polyacrylic acid and its copolymers with acrylamide or polymers containing hydroxamate functional groups), hydroxamic acids, humic and tannic acids, lignosulfonates, fatty acids, sulfonated carboxylic acids, carboxylic acids and polyhydroxycarboxylic acids.

The inhibitor can be added at any time before or during the reaction in the secondary caustic reactor. Thus, the inhibitor can be added with lime or the solution to be causticized in the primary reactor or directly in the secondary reactor. It is also possible to introduce an inhibitor at other stages

Gee! Bayer cleaning, provided that a significant amount of material is fed to the causticizer. The addition of an inhibitor before the primary reactor significantly increases the crystallinity of hydrocalumite and tends to form de hydrocalumite exclusively of the Nei type (in the presence of adequate carbonate). However, although this increased crystallinity has some benefit, the best time to introduce it is in the secondary reactor, where 60 high causticization results are obtained with minimal inhibitor costs.

It turns out that the use of an inhibitor increases the efficiency of traditional (preliminary) causticization cycles. The presence of the inhibitor stabilizes the hydrocalumite as it forms, preventing the typical simultaneous side reaction that leads to the formation of TCA. Thus, a significant increase in the efficiency of the use of lime and the causticity of the solution can be achieved by introducing a suitable inhibitor at any time before the causticization reaction, or directly into the reactor itself. However, the rate of the caustic reaction is also partially reduced, and with this in mind, it is necessary to either increase the residence time in the reactor,

or, more preferably, raise the temperature. Suitable temperature limits applicable to the process disclosed in this patent are approximately between 1002 and 1802C, preferably between 1209 and 14096.

The amount of inhibitor to be introduced depends on its nature and the time at which it is added to the causticization cycle. Thus, the dosage rate of a specific inhibitor must be determined experimentally. Examples of the action of inhibitors and the doses depending on this are given in this document.

The invention is further described and illustrated by the following examples. These examples are provided to illustrate a variety of possible applications and should not be construed as limiting the scope of the present invention. In each of the following examples, the addition of a suitable inhibitor will provide increased efficacy. 70 Example 1

The main application of the advanced causticization process, which is based on the first of the above design principles, is shown in Figure 1. In this system, the traditional causticizer is used as the primary reactor 5, and the secondary causticizer 8 is used to form hydrokapumite particles. This system uses lime slurry dosing to both primary and 75 Secondary causticizers. Due to the fact that the formation of hydrocalumite occurs in the second reactor and is not used further, this configuration does not show a high efficiency of lime use. However, it is a simple way to increase the C/5 ratio of the solution, by effectively adding "ordered" caustic to the traditional caustic.

The highest efficiency is achieved during the first causticization of the solution using a high-temperature tubular autoclave for the primary causticator 5, followed by plate or flash cooling to a temperature between 20 9C and 10092, preferably to 7020-802C. Agitation conditions in the secondary or "ordered" causticator 8 are not critical, although the contents of the tank should be completely suspended. The amount of lime required in this reactor will depend on the level to which C/5 must be raised and can be determined by the stoichiometry described in equation (4). Ha

In the example given here, the Hs formed in the "ordered" causticator 8 is filtered through the filter 9 and mixed with the spent lime products from the primary causticator 5, and then both of them are removed. An alternative is the use of Ne to influence further causticization, with an increase in the efficiency of lime use and the recovery of aluminate ions. This can be achieved by directing the solids to another reaction vessel filled with a fresh stream of solution to be causticized, but a more preferred embodiment of the invention is described in Example 2.

Example 2

A typical application of an advanced causticization process based on the first two "I" principles is shown in Figure 2. In this system, raw lime (preferably in the form of a slaked lime paste or suspension, although quicklime can also be used) is fed together with a solution that should 3z5 to causticize, to the primary reactor vessel 10. The primary reactor 10 can be a linear mixer, a tubular reactor or a mixing tank. The mixing conditions in such a reactor 10 are not critical, although the contents of the reactor must be completely suspended. In this system, there is, as a rule, a reactor mixing tank with a capacity of 10, and the causticizing solution will, as a rule, be the first or second stream of the washer, as defined in Fig. 1, although the technological process is not limited to this. Valid interval "5"

The concentration should be in the range from 40 to 250 g/l, although the best efficiency is achieved with "5" - with a concentration in the range from 80 to 1 bOg/l. Increased efficiency is achieved for solutions with a high A/C ratio. The temperature in this tank should be between 202C and 1002C, although the best efficiency is obtained between 702C and 802C. The residence time in this tank should be about 5 to 20 minutes, but longer residence times of 2 hours or more should not a very large negative effect.

The purpose of this reactor is the reaction of lime with the formation of pure Hs, with little or no unreacted lime, calcium carbonate or TCA. c The suspension is then fed to the heater 12 and heated. If existing causticization equipment is used, the slurry should be heated to a temperature slightly below the atmospheric boiling point of the slurry. t» For most solutions of the washing stream, the temperature will be from 1022С to 1059С. However, preferably, 7 50 when the slurry is heated to higher temperatures, typically between about 1002 and 18023, preferably between 1209200 and 1409Сb. The drain from the heater 12 is fed to the secondary reactor 14, in which the mixing conditions c" are controlled in such a way that the solids are barely suspended. Such a reactor 14 can be a KRPR (caustic reaction stirred tank), but ideally it is a reactor with a flow barrier.

Under the above conditions, a residence time of 2 hours at 1032 and 10 minutes at 12090 is necessary. 29 The products of this tank are cooled (if necessary) and fed to a device 16 for separation (FI of solids from liquid, such as a separator tank, centrifuge or, preferably, filter.

The caustic solution is returned to the technological process. As a rule, this involves returning it to the dirt separator or mixing it in a tank with a clarified solution of the separator stream.

The reacted lime solids can be removed, but since they are almost exclusively calcium carbonate, they can | then wash and filter to recover stuck particles of soda. The solids can then be dried and calcined to regenerate calcium oxide for further use. The wash can be returned to the dirt wash cycle, or applied anywhere on the installation.

This scheme increases the efficiency of lime application by ensuring that very little lime remains unreacted, thanks to the formation of surface materials capable of acting as a diffusion barrier. The loss of alumina due to the formation of TCA is also significantly reduced. However, the maximum C/5 achieved using this system is equivalent to traditional causticization processes unless elevated temperatures and/or an inhibitor are used.

Example C

Increased efficiency can be achieved by applying the third development principle - forming hydrocalumite in another solution, separating the solid product from the liquid in the solid/liquid separation device 18 and using the hydrocalumite cake (filter deposit) as a caustic agent in a secondary tank 14 charged with suitable solution flow. Figure 3c shows a simple conceptual curve diagram illustrating this process. Identical components of the installation in Fig. 2 are marked with the same numbers.

In this configuration, the A/C ratio of the solution supplied to the secondary causticator 14 is maintained at a high level. This is achieved by Her reaction in accordance with equation (7). A high A/C ratio changes the balance in the secondary causticator 14, allowing higher C/5 ratios to be achieved. Of course, a larger amount of lime is needed to obtain such high C/5, /5 ratios, but the efficiency of the lime will be high, and the loss of alumina will be low. Some caustication also occurs in the primary causticator 10, increasing the efficiency of the carbonate removal system.

Example 4

Figure 4 shows a significant improvement of the basic applications described above. In this embodiment, all four principles of the invention are applied. The lime is fed to the reaction vessel 10 (primary caustication tank) operating at a temperature between 402C and 10023C, more preferably at 7020-809C, together with a portion of the causticized stream from the secondary caustication tank 14. Along with this, a stream with elevated A/C and low carbonate concentration is fed into the primary reactor 10. Lime reacts with aluminate ions to form hydrocalumite particles, and further causticizes the solution in accordance with the reaction scheme described by equation (4) (and in some cases, possibly equation (5)). The product thus formed is pure hydrocalumite particles containing varying amounts of carbonate, the degree depending on the initial concentration of carbonate and the amount of lime added. at

Under the conditions described above, the amount of unreacted lime in the discharge from the primary causticator 10 is low. This hydrocalumite material is the raw material for the secondary caustification stage and is separated now from the highly caustified solution produced in the primary caustifier 10. The caustified "U

The solution is then returned to the installation in a suitable area, such as a dirt separator or cleaning filters.

The separation stage can be carried out using a device 18 for separating solid and liquid substances, which includes a gravity separator, cyclone or centrifugation, but the highest efficiency is achieved with filtration. Filtration is very easy to carry out, since the morphology of the solids obtained at this stage facilitates easy separation. ia

The filtered cake is resuspended in a clarified fresh solution, which must be causticized, in mixing tank 20. The temperature of this stream should be between 402C and 1002C, most preferably between 7090 and 8090. The solution can be any process stream from "Z" with a concentration between 40 and 350 g/l as Ma»CO».

However, the greatest efficiency will be achieved with more diluted solutions with "5" concentration between 100 and "160 g/l. The resuspended solution is then heated to a temperature close to the atmospheric point

Boiling of the solution in the heater 12 is directed to the secondary causticization reactor 14, where it is kept for 30 minutes to 4 hours, preferably 2 hours at 103 C, during this time the reaction described by equation (7) takes place. Agitation conditions in the tank should be controlled so that all solids are suspended, but excessive agitation should be avoided to minimize the formation of

TSA It is preferable to use a reactor with a flow stopper, although a stirred reactor vessel would also be suitable. The reacted suspension is then pumped to a device 22 for separating solids and liquids, such as a gravity separator, cyclone, centrifuge or, preferably, a filter. The solids can be discarded, but the very high efficiency of the process (which yields almost pure calcium carbonate), as described in all examples except Example 1, allows the solids to be washed (and the wash returned to the 7 50 mud wash cycle) and calcined. for regeneration of quick lime. Thus, the waste of slaked lime in the treatment plant can be significantly reduced by using this process. part of the clarified solution from the second stage in the caustic reactor 14 is separated and directed to the primary caustic reactor 10 for the formation of hydrocalumite particles. The best caustication efficiency will be achieved by directing the entire flow to the primary causticizer tank 10, but this will require more filtration power. When using this method, the efficiency increases because the reaction products will contain more hydrocalumite 2. The amount of lime added is calculated based on the requirements of the secondary caustic reactor 14. It can be determined by the stoichiometry indicated in equation (7), and also according to the calculated carbonate/hydroxide/aluminate balance for the solution to be causticized. 60 It is clear that, given the relative caustic efficiency of equations (4) and (7), a smaller amount of lime will be required to achieve the calculated C/5 ratio in the secondary reactor than would be required for complete causticization of the solution in the primary reactor. However, if some reduction in lime efficiency and loss of alumina is allowed, then this process can be used to causticize the solution to a very high C/5 ratio by overloading the primary causticizer with lime. 65 Example 5

The productivity of the system can be further increased by carrying out a secondary causticization process at elevated temperatures (between 1002 and 18022). The best performance is achieved at about 12020. A preferred embodiment of this process is shown in Fig.5.

In this system, the suspension from the mixing tank is directed to the heat exchanger 24, where the temperature rises to 12020. The suspension then passes through a secondary reactor under pressure (autoclave), preferably a tubular reactor 26, with a residence time of 5 to 40 minutes, preferably 15 minutes. In the presence of an inhibitor, a longer time will be necessary, which will depend on its nature. Using such a temperature and such a configuration, the efficiency of the reaction of hydrocalumite with the formation of calcium carbonate is significantly increased, while the formation of TCA is significantly inhibited.

Example 6

The efficiency of the process from Example 5 drops if the C/5 solution in the installation approaches or exceeds the carbonate/hydroxide/aluminate equilibrium value in the secondary causticizer. At a certain point, a state of stability is reached, in which the introduction of carbonate into the installation is balanced by the ability of the causticization process to remove it. In most cases this will not be a problem as the C/5 ratio achieved in the plant using this process will be very high. However, if higher C/5 ratios are required, this can be achieved by over-liming the primary causticizer 10 as noted in the previous Example. The problem here is that if the C/5 of the solution fed to the secondary causticizer is too high for reaction (7) to occur, the lime utilization efficiency will be low and alumina will be lost.

This situation can be corrected by applying the improvement shown in Fig. b. In this process, the solution flow in the secondary causticator 14 is replenished with the sodium carbonate-rich flow 28. This can be done in a number of ways, for example through a salt evaporator such as a Tgopa (to supplement the existing caustic feed to the plant) or by connecting the drain from an oxidative organic removal process such as wet oxidation or electrolysis. In this way, the process can effectively causticize Ge! sodium carbonate in these streams, and to recover all the alumina that would otherwise be lost. In this way the lime efficiency can be restored to over 9095. It should be noted that using this process it is possible to increase the C/5 of the plant to nearly 1.00, depending on the amount of carbonate produced in the digestion cycle and the size of the equipment involved.

Experimental results of co

Action of inhibitors

A number of inhibitors representing the classes of compounds previously described were tested in a series of cyclic sc tests for effects on causticization. These tests freely simulate the former technological processes of causticization and serve only to demonstrate the advantages of using inhibitors. The inhibitor that works effectively in these tests shows even greater efficiency when its correct dose is used in the proposed Ф improved technological process of causticization. uh-

The tests were carried out in 500 ml polypropylene bottles, to which 450 ml of the solution of the first flow of the washer, preheated to 972C, was added. The composition of the solution before the addition of lime slurry and additives is shown in Table 1 below. « with s and no

A lime slurry was prepared containing 58 g of hydrated industrial grade lime (90.395 lime -I as Ca(onH)") in 216 ml of deionized water, which was heated to 9720. 30 ml of this slurry was added to each bottle, along with enough of the appropriate additive to bring the concentration of the resulting mixture to 1 g/l. The bottles were sealed and turned upside down and back in a thermostatically controlled "g" water bath at 972. This mixing is much less vigorous and the temperature somewhat lower than in industrial conditions. This, as well as the inhibitory effect of the additives themselves on the rate of the caustic reaction, required a longer residence time. The test results taken after 360 minutes of reaction are shown in Table 2.

These results clearly show the advantages of inhibitors in Bayer's causticification, both with regard to the increased causticity of the solution (C/5) and with regard to the efficiency of lime use. Due to the appropriate optimization of the rate of dosing of the additive, the time of presence in the causticifier and the temperature of the reactor, it is possible to achieve a significant increase in the efficiency of causticization, even if such optimization is applied to known technological processes of causticization. However, these advantages are particularly clear in relation to the improved technological process of causticization described in this document.

Comparison of the improved technological process of causticization with known methods

A series of cyclic caustic tests were conducted in the laboratory to demonstrate the improved performance and benefits of the proposed caustic process. Known technological processes were also simulated for comparison.

An example of a known method

Known technological processes were modeled by a cyclic reaction in a Rait reactor with a volume of 3.75 liters. The solution of the first stream of the washer (1.795 l) was added to the reactor and heated to 1002C. A suspension of hydrated lime 70 industrial grade (32.77) in 20 ml of deionized water was preheated to 959 before entering the reactor. The amount of lime was calculated to give a target C/5 ratio of 0.950 (at 10095 efficiency). After the lime slurry was added, the reactor was closed and the reaction proceeded at 1002 under thermostatic control for three hours. Mixing was carried out using turbine mixers with double inclined blades, which worked at 200 rpm. Samples were taken regularly, 75 of both liquids and solids. Samples of the solution were analyzed for A, C and 5 and total sodium content. The solids were analyzed for their elemental composition by liquid chromatography, and for their CO2 content by acidification and measurement of the gas released.

An example of a known method is the action of an inhibitor

The procedure of the test described above was repeated, with the addition of 0.1 g/l of sodium gluconate to the solution of the first stream of the washer. The residence time was extended to compensate for the effect of the inhibitor on the rate of the causticization reaction.

The effect of the inhibitor in the known process can be seen by referring to Tables C and 4 below. In each case, data are given for samples taken at maximum C/5: for an example of a known method, this was done at about 45 minutes of residence time in the reactor. In the case of the test when the inhibitor was added, a He similar to C/5 was reached at 45 minutes, but continued to increase until it reached a maximum between 260 and 330 minutes. so zo cm «

Ф and - at the maximum С/5 (95 wt. of dry matter) « и С с хз The effectiveness of lime was calculated on the basis of the content of СО», divided by the content of Сас, and was expressed as a molar ratio and corrected for the content of suitable lime and alumina in the initial hydrated lime. -I 45 It can be verified that the addition of the inhibitor led to a drastic increase in the efficiency of lime use, reflected both in a much higher maximum C/5 and in a high concentration of alumina (se) in the caustic solution. This latter aspect is also evident from the solids analysis, which shows that significantly less loss of alumina has occurred. However, such results are achieved at the expense of a longer reaction time, which can be avoided by additionally increasing the volume of the causticizer tank or by increasing the reaction rate by increasing the temperature by 20%. However, higher efficiency is achieved when using the preferred embodiments described in the Examples. c» Improved technological process of causticization, (Example 2)

Application of the improved technological process of causticization in the basic form (as described in

Example 2) was modeled by a cyclic reaction in a 3.75-liter Parr reactor. The simulation consisted 29 of two parts - the formation of hydrocalumite at 802С, followed by rapid heating and reaction at 120260.

GF) Because of the high temperature mass and low heating rate of the Parr reactors, it was necessary to react lime 7 with a reduced volume of solution at 809C and then add the rest of the solution at a much higher temperature to achieve rapid heating of the mixture to 12026. in Solution of the first wash stream (5X000ml) was introduced into the reactor and heated to 80 °C. The second reactor

Parra, with a capacity of two liters, was filled with the solution of the first washing stream (1,500 liters) and heated to 18590. A solution of hydrated lime of industrial grade (38.01 g) in 21 Oml of deionized water, preheated to 802C, was added to 50 ml of the solution in the first reactor. The amount of lime was calculated to achieve the intended C/5, which would be 0.950 (at 10095 efficiency). The reactor was immediately closed and the reaction proceeded under thermostatic control at 80°C for ten minutes. Mixing was carried out using turbine stirrers with double inclined blades operating at 200 rpm. At the end of the ten-minute reaction, during which hydrocalumite was formed, the contents of the second reactor were distilled under pressure to the first reactor. After mixing, the temperature of the solution and suspension in the first reactor was 12020. Then this temperature was maintained by thermostatic control. The mixture, still stirring at 200 rpm, was allowed to react for 90 minutes. Samples of both liquids and solids were taken regularly. Samples of the solution were analyzed for A, C and 5 and total sodium content. The solids were analyzed for their elemental composition by liquid chromatography, and for their CO2 content by acidification and measurement of the gas released.

Improved technological process of causticization (Example 2) - action of the inhibitor. 70 The procedure of the test described above was repeated, with the addition of 0.5 g of sodium gluconate to a 3.75-liter reactor at the end of the ten-minute stage of hydrocalumite formation. This gave a final concentration of approximately 0.25 g/l of the combined solution during the main causticization reaction.

A similar test, but with the use of sucrose as an inhibitor, was also carried out. In this case, sucrose was added at the beginning of the reaction at a concentration of 2.Og/l to the combined solution.

Typical results of each of the above tests are summarized in Tables 5 and 6 below, showing analysis data for liquids and solids, respectively. For reference, the results are compared with a retest of a known process. In each case, the results presented demonstrate the highest C/5 ratio achieved during the reaction. For the known process, this result was achieved after 45 minutes of reaction, while the improved process without the inhibitor required only 2 minutes. A similar ratio of C/5 was achieved after 2 minutes of the improved process and when the inhibitor was added, however, C/5 continued to increase further, eventually reaching a maximum after about 45-60 minutes. and standard solution at the maximum C/5 2 o

Ф at the maximum С/5 (95 wt. of dry matter) зо и

Example? 615 66 open? "

Lime efficiency was calculated based on the content of CO divided by the content of CaO, and was expressed as a t c molar ratio and corrected for the content of suitable lime and alumina in the initial hydrated lime. The solution selected for the process demonstration had a higher "5" concentration than that used for causticization in most Bayer process refineries.

The reason why such a solution could not normally be used in industry is obvious from the lime efficiency results given in Table 6. The lime efficiency shown for a known process method (5495) was obtained on the basis of a sample taken at the maximum C/5 ratio for the reaction, and therefore represents the maximum efficiency. Such optimal conditions are rarely achieved in production, so the efficiency under normal conditions should be significantly lower.

GF 50 In contrast, a higher maximum C/5 ratio and significantly improved lime efficiency were observed in the process of Example 2. The maximum C/5 was reached quickly (2 minutes) and remained so for about a further 8 minutes before the reverse process became significant.

Even more impressive is the result for the technological process from Example 2, when the inhibitor (sodium gluconate) was added. In this case, a much larger maximum of C/5 was achieved, with a 3090 higher efficiency of lime, compared to the known method of the technological process. Moreover, the reverse reaction is extremely slow: during the next 30 minutes at the temperature, C/5 dropped only by 0.002 points.

Similar test results were obtained when sucrose was used as an inhibitor; the same amount) C/5 was achieved, which was equal to 0.924, but with a much lower lime efficiency of 84.095. However, the time required to reach the maximum C/5 with sucrose (5 minutes) is much shorter than with sodium gluconate. because In addition to the pronounced advantages of a significantly increased C/5 solution and high efficiency of lime, this

The example demonstrates the tolerance of the process to high solution concentrations and to changes in residence time.

Taken together, these factors contribute to improved caustication stability at the Bayer refinery. Moreover, the losses of alumina caused by the formation of TCA are significantly reduced, which should contribute to increasing the productivity of the refining plant. 65 Improved technological process of causticization (Example 3).

The application of the improved technological process of causticization in the preferred form of its embodiment (as described in Example 5) was simulated in the laboratory by conducting a series of sequential cyclic reactions in a 3.75 liter Parr reactor. Each series cycle consisted of two stages: the formation of hydrocalumite at 809 in a pre-caustic solution (primary caustic reaction), and the preparation of this pre-caustic

Caustic solution using hydrocalumite formed in this way (secondary caustic reaction).

To start the series of cycles, several liters of the solution of the first stream of the scrubber were first causticized with a solution of hydrated lime of industrial grade (90.3956 available lime with the formula Ca(OH)"), using a conventional (known) process of causticization. After filtration and removal of collected 7/0 solids, a pre-causticized solution remained, from which the initial sample of hydrocalumite could be obtained.

Two liters of this solution were loaded into a 3.75 liter Parr reactor, and the temperature, which was raised to 80°C, was maintained by thermostatic control. Hydrated lime, in the amount required to achieve the target C/5 of 0.950 (at 10095 efficiency), was suspended in hot 75% deionized water in a 500ml polypropylene bottle, equilibrated at 80°C, and then quantitatively transferred to a Parr autoclave for initiation. reactions Mixing was carried out using turbine stirrers with double inclined blades operating at 200 rpm. After 30 minutes required to complete the reaction, the entire contents of the reactor were filtered under vacuum using a Buchner funnel and a filter flask. The solid residue and solution remaining in the Parr reactor were washed into the filter funnel with hot deionized water. The filter cake was subsequently washed with hot deionized water to remove the remaining solution (this procedure, unnecessary in normal use, was necessary for the mass balance calculations).

The solution of the first stream of the washer (500 ml) was loaded into the reactor and heated to 802С. The second reactor

Parra, with a capacity of 2 liters, was filled with the solution of the first flow of the washer (1,500 liters) and heated to 18526. s

The wet hydrocalumite cake obtained at the previous stage was added to 500 ml of the solution in the first reactor. The reactor was immediately closed and the mixer was turned on to disperse the solids. After approximately 2 minutes of solids dispersion and thermal equilibration, the contents of the second reactor were transferred under pressure to the first reactor. After mixing, the combined temperature of the solution and suspension in the first reactor was 1202C. This temperature was then maintained by thermostatic control. The mixture, which was still i) stirred at 200 rpm, was left to react for 2 minutes. with

After the reaction, a sample of the suspension was taken and immediately filtered through a 0.45 μm Supor filter membrane.

The filtrate was analyzed for A, C and 5 and total sodium content. The solids were thoroughly washed with deionized water on the filter. The wet cake was collected for analysis by HKO. The remaining solids were dried under a partial vacuum (104 mm Hg) at 1052C and analyzed for elemental composition with the help of HKE, and for 30% CO content by acidification and measurement of the released gases. in

The contents of the reactor were moved to a pressure filter equipped with a 0.45-μm Supor filter. The filtrate was collected for use in the next process cycle. A certain part of the solution was lost due to the fact that samples were taken and the suspension was transferred. In the next cycle, this was corrected by adjusting the loading of lime in the primary caustic reaction, and by reducing the volume of the solution of the first flow of C 50 washer in the secondary caustic reaction. This procedure was repeated until four complete cycles of the process were completed. from" Improved technological process of causticization (Example 5) - action of the inhibitor

The test procedure described above was repeated with the addition of 1.0 g of sodium gluconate with hydrocalumite to a 3.75 liter reactor at the beginning of the secondary caustication reaction. This gave a final concentration of approx

O,bg/l in the combined solution during the secondary causticization reaction. To compensate for the inhibitory effect of inhibitor 7, 120 minutes of time was allocated for the secondary reaction. (se) Improved causticization process (Example 5) - maximum C/5

To demonstrate the ability of this process to achieve a very high solution causticity (C/5), the above procedure was repeated with a higher dose of lime, calculated to achieve the target ka of 20 c/v, which would be equal to 1.00. The test procedure described above was repeated with the addition of 1.0 g of sodium gluconate with hydrocalumite to a 3.75 liter reactor at the beginning of the secondary caustication reaction. This gave a final concentration of approximately 0.5 g/l in the combined solution during the secondary causticization reaction. In order to compensate for the inhibiting effect of the inhibitor, and to give the reaction enough time to reach a significantly higher C/5, 150 minutes of time was allocated for the secondary reaction. 29 Typical results for each of the tests in Example 5 are summarized in Tables 7 and 8, which show the data

GF) analysis of liquids and solids, respectively. For reference, the results are compared with a test of a known process. ime) " of a typical solution at the maximum C/5 b5

Example Bhingibitor 81.1 (143.5 | 1539 0.5650.932 | 199.9 at maximum C/5 (95 wt. dry matter)

Device 0000618 66 vav? inBa di

As can be seen from the above results, the preferred embodiment of the technological process, which is described in Example 5, has significant advantages over the known method both in terms of the achieved C/5 and 75 in terms of lime utilization efficiency. In the example above, lime efficiency exceeded 9195.

The preferred embodiment also has advantages over the process described in Example 2. Without additives, the lime application efficiency is equivalent to that of Example 2, but a much higher C/5 ratio is achieved (0.910 vs. 0.897). Both increased lime efficiency (91.190 vs. 86.995) and a higher C/5 ratio (0.932 vs. 0.924) are obtained with the inhibitor. However, the most important advantage of the preferred embodiment over that described in Example 2 can be seen by referring to the test results in which the goal was to achieve the maximum C/5. Applying the preferred embodiment of the technological process, it is possible to achieve an extremely high causticity of the solution (C/5 equal to 0.955 or higher), with a markedly increased efficiency of lime (more than 8090) in comparison with the known technological process. with

Based on the preceding examples and the description of several possible applications, it becomes apparent that d) the improved causticization process described herein has many significant advantages over the causticization technology used today.

These include: (a) Lime application efficiency is extremely high (over 9095 can be achieved), even with fairly concentrated solutions. сч (б) The C/5 ratio, which is achieved, is significantly increased (up to 0.955), even with fairly concentrated solutions, which causes a high concentration of caustic in the installation and increased productivity. It is possible to achieve - even higher ratios due to a certain decrease in the effectiveness of lime. Gee! (c) Spent relatively pure calcium carbonate is formed, which creates the potential for repeated 3o use of lime through firing in a small industrial kiln, with further potential reduction - the consumption of lime. (d) The loss of alumina due to the formation of undesirable particles of calcium aluminate is significantly reduced. Improved alumina recovery from bauxite resulting in increased productivity. (e) Basically, the process of causticization is faster, which leads to a reduction in the volume of the tank, and a more compact arrangement. in c (e) Efficiency is stable, despite changes in the composition of the solution and flows. Reduced carbon dioxide emissions due to improved refinery efficiency and reduced lime consumption. (g) Ease of use at virtually all refineries. (3) Reduced residue volume due to minimal lime consumption - potential savings in residue removal and storage costs. w- (3) The potential for the distribution of the caustic reaction into two or more streams of solution at

Ge) refinery. () The potential for the application of multifaceted technological processes of causticization in different areas of the refinery. Now that several embodiments of the invention have been described in detail, it will be apparent to one skilled in the art of chemical engineering that numerous changes and modifications may be suggested without departing from the basic principles of the invention. It is believed that all these changes and modifications are within the scope of this invention, the essence of which should be determined from the above description and the attached claims. In addition, the preceding examples are given to illustrate certain embodiments of the invention and are not intended to limit the scope of the technological process according to the present invention.

F) km

Claims (58)

The formula of the invention is 1. The method of causticization of Bayer solutions at an alumina refinery, which includes the following stages: (i) interaction of lime with aluminate ions in Bayer solution under controlled conditions at a temperature from 2590 to 1009 with the formation of hydrocalumite particles and hydroxyl ions, and ( i) heating said hydrocalumite particles in contact with Bayer solution to a temperature in the range from 10092C to 1802 so that hydrocalumite particles react with the solution with the formation of calcium carbonate, aluminate ions and hydroxyl ions, and obtaining a causticized Bayer solution. 2. The method of causticization of Bayer solutions according to claim 1, where the solution of stage (i) and/or stage (i) has a 5" concentration in the range from 40 g/l to 350 g/l, and the A/C ratio in the range from 0, 2 to 0.95, where A" is the alumina concentration, C" is the total concentration of sodium aluminate and sodium hydroxide, and 5" concentration is the sum of C and the actual concentration of sodium carbonate. C. The method of causticization of Bayer solutions according to claim 71, where the solution of stage (i) and/or stage (its) has a 5" concentration in the range from 120 g/l to 160 g/l, and the A/C ratio in the range from 0, 55 to 0.95. 4. The method of causticization of Bayer solutions according to any one of claims 1 - C, where the time required to complete the 70 reaction stage (i) is from 5 minutes to 30 minutes. 5. The method of causticization of Bayer solutions according to claim 4, where the reaction of stage (i) is carried out at temperatures from 702C to 8026C. 6. The method of causticizing Bayer solutions according to any of claims 1 - 5, where the Bayer solution is stirred at stage (). 7. The method of causticizing Bayer solutions according to any of claims 1 - b, where the hydrocalumite suspension formed in the reaction at stage () is subjected to solid/liquid separation, and the separated solid hydrocalumite is subjected to interaction with the solution, which must be causticized under reaction time of stage (ii). 8. The method of causticizing Bayer solutions according to claim 7, where the causticized Bayer solution obtained in the reaction of stage (ii), which includes heating hydrocalumite particles, is subjected to cooling and solid/liquid separation and where at least part of the clarified solution is returned to the reaction of stage (i ). 9. The method of causticizing Bayer solutions according to claim 8, where the solids separated from the Bayer solution of reaction stage (ii) include calcium carbonate, which is recalcified for lime regeneration. 10. The method of causticizing Bayer solutions according to claim 9, where the regenerated lime is reused at the refinery. Ha 11. The method of causticizing Bayer solutions according to any one of claims 71 - 10, where the time required to complete the reaction of stage (ii) is in the range of 2 minutes to 240 minutes. i9) 12. The method of causticizing Bayer solutions according to any one of claims 71 - 10, where the time required to complete the reaction of stage (ii) is between 2 minutes and 15 minutes. 13. The method of causticizing Bayer solutions according to any one of claims 1 - 12, where the process further includes the step of adding an inhibitor before step (ii) in order to inhibit the undesirable reaction of hydrocalumite particles with the formation of tricalcium aluminate. with 14. The method of causticization of Bayer solutions according to claim 13, where the process further includes the stage of adding an "I" inhibitor before and/or during stage (1). 15. The method of causticization of Bayer solutions according to claim 13 or 14, where the mentioned inhibitor is a complexing agent and/or a surface-active substance that is able to be absorbed on the active areas of the surface of hydrocalumite particles. 16. The method of causticizing Bayer solutions according to claim 15, where the surface-active substance is sugar or polysaccharide. " 17. The method of causticizing Bayer solutions according to claim 13 or 14, where anionic surfactants are used as the mentioned inhibitor. - with 18. The method of causticization of Bayer solutions according to claim 17, where the mentioned anionic surfactants are selected from the group containing the following materials, their salts and derivatives: anionic homopolymers or copolymers, hydroxamic acids, humic and tannic acids, lignosulfonates, fatty acids, sulfonated carboxylic acids, carboxylic acids and polyhydroxycarboxylic acids. 19. The method of causticizing Bayer solutions at an alumina refinery, which includes the following stages: production of pre-causticized Bayer solution; and that the interaction of lime with aluminates in the aforementioned pre-causticized Bayer solution at a temperature in the range from 25 °C to 100 °C with the formation of hydrocalumite particles and hydroxyl ions. 20. The method of causticizing Bayer solutions according to claim 19, where the solution has a 5" concentration in the range from 40 to 350 g/l, and the A/C ratio in the range from 0.2 to 0.95. 21. The method of causticizing Bayer solutions according to claim 19, where the solution has a 5" concentration in the range from 120 to 160 g/l, and the A/C ratio in the range from 0.55 to 0.95. 22. The method of causticization of Bayer solutions according to any of claims 19 - 21, where the time required to complete the reaction is from 5 to 30 minutes. 23. The method of causticizing Bayer solutions according to claim 19, where the temperature of the previously causticized solution is maintained in the range from 702 to 8090. 24. The method of causticizing Bayer solutions according to any of claims 19 - 23, where the Bayer solution is stirred. 60 25. The method of causticizing Bayer solutions according to any one of claims 19 - 24, wherein the process further includes the stage of adding an inhibitor before or during the stage of interaction with lime. 26. The method of causticizing Bayer solutions according to claim 25, wherein the time required for the completion of the reaction is at least 5 minutes. 27. The method of causticization of Bayer solutions according to claim 25 or 26, where the mentioned inhibitor is a b5 complexing agent and/or a surface-active substance that is able to be absorbed on active areas of the surface of hydrocalumite particles. 28. The method of causticizing Bayer solutions according to claim 27, where the surface-active substance is sugar or polysaccharide. 29. The method of causticizing Bayer solutions according to claim 28, where anionic surfactants are used as the mentioned inhibitor. 30. The method of causticizing Bayer solutions according to claim 31, where the mentioned anionic surfactants are selected from the group containing the following materials, their salts and derivatives: anionic homopolymers or copolymers, hydroxamic acids, humic and tannic acids, lignosulfonates, fatty acids, sulfonated carboxylic acids, carboxylic acids and polyhydroxycarboxylic acids. 70 31. The method of causticizing Bayer solutions according to any of claims 19 - 30, where the causticized Bayer solution obtained in the reaction is subjected to solid/liquid separation to remove hydrocalumite particles. 32. A method of stabilizing hydrocalumite particles, which includes the following stages: formation of hydrocalumite particles in Bayer's solution; and the addition of an inhibitor that stabilizes hydrocalumite particles by inhibiting the formation of /5 tricalcium aluminate in Bayer's solution. 33. The method according to claim 32, where the inhibitor is added to the solution before the formation of hydrocalumite particles. 34. The method according to claim 32 or 33, where the inhibitor is added during the formation of hydrocalumite particles. 35. The method according to any one of claims 32 - 34, where the inhibitor is added after the formation of hydrocalumite particles. 36. The method according to any one of claims 32 - C5, where the inhibitor is an anionic or nonionic surfactant. 37. The method according to claim 36, where the anionic surfactant is selected from the group that contains: homopolymers or copolymers, including polyacrylic acid and its copolymers with acrylamide and polymers bearing hydroxamate functional groups, hydroxamic acids, humic and tannic acids, lignosulfonates, fatty acids, sulfonated carboxylic acids, carboxylic acids and polyhydroxycarboxylic acids, combinations of their salts or derivatives, or combinations thereof. with 38. The method according to claim 37, where the anionic surfactant is gluconic acid. 39. The method according to claim 37, where the anionic surfactant is gluconate. at 40. The method according to claim 39, where the gluconate is sodium gluconate. 41. The method according to claim 37, where the anionic surfactant is sugar or polysaccharide. 42. The method according to claim 41, where the sugar is sucrose or glucose. so so 43. The method according to claim 41, where the polysaccharide is starch. 44. The method according to any one of claims 32 - 43, which further includes the stage of heating the Bayer solution. with 45. The method according to claim 44, where the heating stage is carried out before the stage of formation of hydrocalumite particles. "IS 46. The method according to claim 45, which also includes the stage of heating the Bayer solution to a temperature in the range from 252C to 1002C. ia 47. The method according to claim 46, where the Bayer solution is heated to a temperature in the range from 702 to 8020. 48. The method according to claim 44, where the heating stage is carried out after adding the inhibitor. 49. The method according to claim 48, where the Bayer solution is heated to a temperature in the range from 1002C to 18020C. 50. The method according to claim 49, where the Bayer solution is heated to a temperature in the range from 1202C to 14096. 51. The method according to any one of claims 32 - 50, which also includes the step of mixing the Bayer solution. 52. The method according to any one of claims 32 - 51, which also includes the step of removing stabilized hydrocalumite particles. " 53. The method according to claim 52, wherein the step of removing stabilized hydrocalumite particles includes a filtering step. ," 54. The method according to claim 53, where the filtering stage is carried out in a vacuum. 55. The method according to any one of claims 52 - 54, which also includes the step of washing the removed hydrocalumite particles. -| 56. The method according to claim 55, where the washing stage is carried out using deionized water. with 57. The method according to claim 56, where the stage of washing with deionized water is carried out using hot deionized water. eat 58. The method according to claim 52, which also includes the step of using at least a portion of the removed hydrocalumite 7 50 for causticizing the Bayer solution. syu" F) name) 60 b5