HU182807B - Process for the continuous digestion and further treatment of the aluminium content of raw materials containing aluminium according to the bayer technology - Google Patents

Abstract

The invention relates to a process for the continuous dissolution and further processing of the aluminium content of aluminium-containing raw materials by the Bayer process by mixing the material streams of the raw materials and the material stream or the material streams of the sodium aluminate liquor and/or sodium hydroxide and/or solution(s) containing additive(s), and optionally water, by heating material streams at least partially during flow, optionally by subsequently keeping said streams hot between or after the heating processes and by further processing the heat-treated mixture, in which material streams of the solution(s) and/or slurry or slurries and optionally of inert gases and/or air are repeatedly mixed, at least in a static mixing element, and optionally allowed to stand in a delay vessel and, if necessary after mixing and/or allowing to stand in the delay vessel, are at least partly recycled, at least an individual or a mixed material stream being heated in one or more stages during dissolution or digestion. The invention also relates to an arrangement for carrying out the process. <IMAGE>

Description

BACKGROUND OF THE INVENTION The present invention relates to a process for the continuous exploration and further processing of aluminum-containing raw materials according to Bay er technology.

In Bayer's alumina production technology, aluminum-containing ores and raw materials (the aluminum content of which is predominantly in the form of alumina-aluminum oxide hydrate) are mixed with a solution of aluminum alumina and sodium hydroxide in is dissolved in the form of sodium aluminate. The alumina content in solution during digestion is precipitated in the form of alumina hydrate during mixing. The heating can take place in preheaters and eg. in high-pressure and parallel-coupled tubular, mechanical mixing autoclaves, in which the slurry has a longer residence time and consequently significant crust formation, particularly on heating surfaces.

The exploration of bauxite is influenced by the combined effect of many factors. These include - the mineral composition of the oxidic clay minerals, i.e. aluminum hydrates, their crystalline structure and structure - the fineness and uniformity of bauxite grinding - the chemical composition (A1 ₂ O ₃ , SiO ₂ , Fe ₂ O ₃ , TiO ₂ as the main constituents) and V ₂ O _S , CaO, MgO, Ga, C, CO ₂ , SO ₃ , etc. as other co-agents) - kinetic and equilibrium relationships of the heterogeneous reaction of sodium aluminate (NaA10 ₂ ) formation - with A1 ₂ O ₃ and ratio of Na ₂ 0 concentration, hydrolysis decomposition of alumina, etc.

In the course of the process, Bayer is determined by all the factors that determine the discoverability of A1 ₂ O ₃ . In a conventional, batch-fired, autoclave tube-heated autoclave, high quality bauxite is reacted optimally for 2-3 hours at 170-180 ° C for 2-3 hours. With the deterioration of the quality of bauxite, even at about 200 ° C, the digestion process is difficult and the operating conditions for post-digestion sedimentation screening deteriorate. The deterioration in the former economic efficiency of production brought to the fore the idea of intensifying exploration by moving from batch to continuous operation. In known pipe exploration processes, a continuous flow of material streams is excavated.

- DT 579.114; according to German patent specification

At temperatures above 190 ° C and at pressures of 10 to 15 bar, the bauxite can be digested with an alkali density of 1.17 to 1.20 kg / m ³ , which allows the transition from autoclaves to a 'tube of heating coil'. The bauxite and the aluminate liquor are moved forward by a high-pressure pump and are forced out of the reaction area by a slurry valve or back-pressure pump.

The process did not go through the practice because the deposition crusting impaired the heat transfer conditions of the pipes so that digestion had to be stopped within a short period of time, and sedimentation and filtration of the effluent slurry was also difficult. it has become very slow.

- DT 665,495. According to German patent specification Bauxite was discovered by a "tower procedure". The piece bauxite was treated continuously with counter-current and steam-heated alkali. Because the extraction of lumpy bauxite is a much slower process than the leaching of fine bauxite flour, very large volumes were obtained on the solvent drums, which made the process uneconomical.

- DT 830.945; According to German Patent No. 4,198, they seek to bypass the direct heat transfer and thus the deterioration of the heat transfer conditions caused by crusting. According to the method, the majority of the preheated lye is forced through a centrifugal pump heat exchanger into a series of exploratory autoclaves. A smaller portion of the preheated lye is mixed with the bauxite powder, the suspension being fed to at least two pre-autoclaves heated with fresh steam, from where they are fed in batches to the digestion autoclaves connected in series. However, with this process, even with more complex and costly technical solutions, only high-quality bauxites can be explored continuously. However, this procedure is inappropriate for detecting inferior bauxites.

Further, a pipe exploration process is described in U.S. Patent No. 149,514. Hungarian Patent Specification, or German Patent Specification DT 1.920.222. However, even in these processes, the heating of the slurry results in a hard, mechanically removable deposit on the slurry side of the hot tubs, which is constantly thicker, reducing the heat transfer surface and reducing the heat transfer coefficient. Occasionally, deposits must be removed from both the pre-heater and autoclave digger line and the pipe digger. This is done by bypassing and purifying a preheater, autoclave, hot tub, or an exploration line (mechanical cleaning or acidification). All of this means loss of production, significant material costs and wages. Due to the shrinking heat transfer surface and the heat transfer coefficient, and the fact that units are being permanently dismantled, more equipment than necessary is needed, resulting in additional investment costs. Finally, it is disadvantageous that previous processes do not allow for the complete digestion of all aluminum-containing ore components (eg bohemian), which reduces yield.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of reducing deposition on heat transfer surfaces and significantly improving heat transfer and increasing the efficiency of digestion.

The invention thus provides a method for continuously digesting aluminum content of aluminum-containing raw materials according to Bayer technology, by one or more liquid streams of one or more liquid material streams of the aluminum and optionally additive raw materials in solid state and / or sodium hydroxide and optionally containing solutions. and / or by heating at least part of the separate and / or combined material streams in at least one heat step during the flow, optionally maintaining the heat treated mixture between or after the heatings and processing the heat-treated mixture in a known manner.

SUMMARY OF THE INVENTION It is an object of the invention to subject one or more separate and / or combined streams of material to one or more stages of heat treatment in one or more stages during mixing, staging or further processing through one or more streams of static mixing elements. below.

182,807

Surprisingly, it has been found that the intense mixing of the material stream (s) through static mixing elements results in the surprising effect that, by heating, the excellent materials do not settle on the heat transfer surface but continue to move through the material flows. The collision of the slurry particles, as well as the intense mixing and collision, as well as the absence of deposition, is increased and the heat transfer through the wall remains constant over time and digestion improves. Unexpectedly, in tubes or 10 autoclaves, etc. the concentration of solids to be digested by advanced slurry can be significantly increased, nearly doubled. Local solid enrichment by static agitators promotes collision of the particles and, as a result of the mechanical energy of the collisions, active sites are formed on the particles 15 which increase the desired chemical reaction. Surprisingly, silicates, etc. can be controlled via static mixing elements. location and amount of precipitation.

The process of the present invention accelerates the process inhibited by surface diffusion of particles by means of static mixing elements arranged in the material streams. Partly due to their collision energy with the tube wall and with one another, the particles increase the degree of dispersion of the system and the interface between the phases, which increases the rate of digestion. The faster than normal warming and higher end temperatures, and the fact that during solidification the solid particle concentration is not precipitated on the heat transfer wall, but forced to remain in the slurry, allows other, if not very low, aluminum to be exposed. - for example, a significant dissolution of 80-90% of the aluminum content of barrier-free aluminum.

In the process of the present invention, the dissolution and re-release of the silicate mineral constituents can be accomplished in a direction desired by the technology.

The process according to the invention has the advantage that mechanical stirrers are omitted, that by means of recirculation the stirring is better, that the heat transfer in the autoclave and thus the separation on the walls can be omitted. Finally, there is an advantage in increasing the productivity of the plant with the same set of server equipment, which in practice results in a 2-4% higher yield.

In the process of the invention, the energy requirement for moving the material streams is increased by the use of static mixers. However, this excess energy is well covered by the known production methods and reduced investment costs for known pipe exploration processes, as well as the inevitable shutdown losses for cleaning that are unavoidable in the conventional process.

Detailed Description of the Invention The present invention will be described in detail by way of exemplary embodiments.

Example 1

The bauxite meal and the so-called bauxite meal. The streams of recycled alumina liquor are recycled once or more through an expansion steam heated pipe system with static stirrers. The slurry during the passage eg. It heats from 81 ° C to 85 ° C to 195 ° C to 200 ° C. Due to the measured heat transfer coefficient of 3000 - 3500 kcal / m ² h ° C, the heating is very intensive and the heating area required is economically small. Due to the vigorous internal mixing of the slurry, no continuous crusting occurs on the inner pipe wall or static mixing elements during long-term operation (5-6 weeks). In this latter tube, the initial heat transfer coefficient is only 700-800 kcal / m ² h ° C and due to the continuous crust deposition it has decreased to 500 kcal / m ² h ° C in 20 days. This low heat transfer coefficient no longer provided the 195 to 200 ° C temperature required for the process heat flow, so the process had to be stopped and the equipment had to be dismantled to clean up deposits. Acidification for cleaning with 2-3 days of downtime and the consequent loss of production makes operating uneconomical.

Example 2

After the recuperative heat exchangers of Example 1, the bauxite is digested in a sharp steam-heated tube reactor with static stirrers at a final temperature of 240-250 ° C over a period of one minute. Due to the intense internal turbulence caused by static mixers and the friction and collision of crystalline and amorphous particles in the slurry, heating to the digestion temperature is characterized by a high heat transfer coefficient of 3300-3500 kcal / m ² h ° C. sediment or the non-alkaline soluble components of bauxite remain suspended. Surprisingly, the reaction results in a 2-4% increase in the yield. The latter phenomenon can be explained by the kinetic and equilibrium conditions of sodium aluminate formation as follows:

- Due to the intensive heat transfer, the digging temperature is slightly increased. This, on the one hand, increases the rate of digestion and, on the other hand, since the higher temperature the solution is leaner from the equilibrium, more A1 ₂ O ₃ can be dissolved. In addition, the higher temperatures favor not only the digestion of hydrargillite, but also the digestion of significantly less soluble bohemite and diaphoresis.

- The difference in dissolution for each mineral is related to the difference in energy of the crystal lattice. Thus, the kind of mineral that requires more energy to break the grid is more difficult to dissolve. In the exploration phase, the mixed collision of the material particles also mechanically facilitates the dissolution of the less soluble bohemian.

- The collision of particles also causes dispersion. A system with a higher degree of dispersion has a larger dissolution surface, which also increases the rate of digestion.

- Other ingredients - at lower temperatures, approx. Silicic acid soluble at 120 ° C, certain organic components - can form a coating on the aluminum hydroxide particles, which physically inhibits the passage of caustic soda to the surface of the dissolved Al ₂ O ₃ . The collision of the particles causes the shells to burst, thus improving the digestion.

Finally, due to flow resistances, it is advantageous to reduce the hydraulic pressure by 2.3-2.5 bar along the length of the spout because it reduces the precipitation of sodium oxide formed from the SiO ₂ content of the bauxite and thus the loss of A1 ₂ O ₃ bound to the sodium oxide. It is known that the volume of the sodium molecule is smaller than the sum of the components, ie the increase in pressure, which is an autoclave of the Bayer process.

182,807 is characterized by an increase in the excretion of soda ash, while in this example, a decrease in the length of the nyc source along the tube decreases the segregation of soda as well as the loss of A1 ₂ O ₃ taken with soda.

Example 3

It is known that the exploration of bauxite is a time-consuming process and the equilibrium is slowly established. During the digestion, the precipitation of sodium aluminum hydrosilicate is also slowly reaching the dynamic equilibrium characteristic of the process. In the Bayer process, e.g. it is only after the second hour of digestion that the rate of precipitation of the silicic acid compound increases, leading to a sharp decrease in the SiO ₂ content of the alkali. It is therefore advisable to dispose of the sludge from the digestion tube reactor into larger vessels. This will bring the digging closer to equilibrium on the one hand and adjust the 1.7 to 1.9 molar ratio of alumina, which is favorable for post settling and filtration, on the other. It is advantageous for the slurry to be transported through static mixing elements integrated in the remaining vessels to keep the sediment in the slurry floating and hydraulically transported. Another function of the residence vessel is to cool the slurry. Due to post-digestion operations, the slurry with a final temperature of 240-250 ° C is approx. Cool to 100 ° C. Due to the static mixing elements, the cooling can also be carried out very intensively, with a heat transfer coefficient of 1200-1500 kcal / m ² h ° C. The heat energy released during cooling, like the Bayer process, can be utilized in recuperative heat exchangers. Deposits should be removed from the wall of the vessel (s) after 1-2 years of operation.

Example 4

In a state-of-the-art 200,000 tonne plant per annum, the material remains in the reactor for 40-60 hours, while 40-60% of the total alumina content of the alumina is precipitated but below 50% on average, with an average air demand of 200-300 Nm ³ . h. To produce 1 Nm ^{3 of} 4-5 bar air, 0.09 kWh of electricity is needed, ie 18-27 kWh in total. This energy requirement is approx. can be halved if the air is not typical of the Bayer process mixers, approx. It is dispersed through the 0.007 m boreholes into the riser pipe, but is injected into the air through an agitator tube with static mixing elements. The good dispersion of the air and the efficiency of the mixing is ensured by the good contact caused by the static mixing elements, while the rate of crystal precipitation and crystal growth is increased, since the injector-like introduction of air causes the entire mixing fluid volume to circulate vigorously. The flow characteristics outlined increase the efficiency of the mixer, reaching an average of 50-52%.

Claims (1)

A patent claim A process for the continuous digestion of aluminum content of aluminum-containing raw materials according to Bayer technology, comprising the step of single or multiple liquid streams of the aluminum and optionally added raw materials in the form of liquid sodium, sodium or sodium hydroxide and / or additive solutions, 30 heating at least partially one or more separate and / or combined streams of material at least in one stream during the flow and optionally maintaining the heat treated mixture between or after the heatings and further heating in a known manner, 35 characterized in that during digestion, residence, or further processing, at least one or more streams of one or more separate and / or combined materials are subjected to one or more stages of heat treatment through static mixers. (Without drawing)