HU189244B - Process and equipment for disclosing gibbsit bauxites in reversing recuperating currents - Google Patents

Abstract

Energetic efficiency in a system of gibbsitic bauxite digestion termed counter-current regenerative digestion can be improved significantly by the new method provided by the present invention characterized in submitting the flow of recycled spent liquor, preferably immediately prior to mixing, to at least one evaporation along its preheating path of flow while proceeding to mixing with the flow of ground bauxite. For performing the new improved method the invention provides for an apparatus wherein an evaporator unit is inserted into the preheating path of flow of the recycled spent liquor. The evaporator unit is arranged in said path of flow prior to but in the nearest possible proximity of the mixing point of the preheated liquor with the preferably also preheated flow of ground bauxite. The evaporator unit is heated by high-pressure hot steam introduced to the system from outside source for heating purposes.

Description

BACKGROUND OF THE INVENTION The present invention relates to a process for the digestion of gibbsite bauxites in a countercurrent recuperation system, in particular at a digestion temperature of 140 to 145 ° C, whereby the optionally preheated thick slurry of bauxite additionally milled alkali is preferably is introduced into the autoclave (s), and the digested slurry is generally expanded in multi-stages, and countercurrent pre-heating of the digestate slurry is effected at least in part by steam generated during the digestion slurry expansion and the system to adjust digestion temperature and / or fuel is supplied.

The present invention also relates to an apparatus for carrying out the process, preferably comprising a plurality of exploratory autoclaves and a series of slurry expansion tanks connected thereto, and heat exchangers incorporated in the preheating flow section of the digestion liquor.

For the digestion of gibbsite bauxites at 140-145 ° C, a dual-current solution is generally used. This is characterized by dividing the amount of alkali required for the digestion into two parts; for alkali streams containing 80-85% and 15 -20 °. It is grinded with a 15-20% alkaline stream. slurry the bauxite and then combine this dense slurry without preheating with a preheated stream of 80-85 "in a multi-stage tubular heat exchanger, which is then fed to exploratory autoclaves. Here, if necessary, additional heating is carried out by direct injection of heating steam to adjust the digestion temperature.

A countercurrent recuperation system was set up to carry out the exploration operation. (KW PERRY: Modification to Bauxite Digestion. Proceedings of the Second International Symposium of ICSOBA. Vol. 1, pp. 43-57 Budapest, 1970 and 1. M. SIBLY, LN BUCKETT: Western Australian Alumina Refining Operations 110 the Meeting Annual Meeting in Chicago, Feb. 3, 22-26, 1981, Light Metals 1981, pp. 129-137.) The characteristic of this is that the multi-step expansion of the digested slurry flowing in the series-tube heat exchangers, the expelled slurry (expansion steps = 3-6). ), it is heated in counter-current by vapors evolving in the expansion vessels. Subsequently, the condensation water of the heating steam is used to further heat the alkali, and then the heating steam of the digestion continues to be heated in tube heat exchangers as well.

The excavated slurry from the last expansion tank was ca. It proceeds to the next step of dilution at 107 ° C.

After grinding the first heat exchangers at lower temperatures, the caustic soda used for grinding should be approx. 90 is conducted at a temperature of 95 ° C and the resulting slurry of 70-75 ° C is combined with an alkaline stream preheated to 150-160 ° C.

The heat content of the alkaline condensation water of expansion vapors is generally utilized by evaporation to the following expansion stages.

The pure condensate water from the heating steam is returned to the power plant after being used at the condensation temperature or for further heating of the lye heated with expansion vapors. However, optionally, the heat content of the pure condensate water is further utilized within the digestion system by expanding it in several steps and combining the evolving vapors with the slurry expansion vapors.

A feature of the well-known dual-current digester system described above is that the heat transfer is effected at a high temperature difference during the heating stage of digestion heating steam. This also applies to the indirect preheating of the alkali, where there is a temperature difference of 25-40 ° C between the expansion steam preheated and the digestion heat preheated digestion temperature (ca. 120-125 ° C to ca. 150-160 ° C). ). But even more so when heating the slurry, where approx. Heating the slurry at 75-80 ° C to about 160-170 ° C, approx. 8 bar pressure.

With respect to the preheating of the slurry, this large temperature difference in heat transfer is significantly reduced if the expansion vapors of the digested slurry are also used to preheat the slurry, preferably in multi-stage direct or indirect preheaters. However, the temperature difference does not reach a favorable value for economical heat utilization. Similarly, in these digestion systems, the unfavorably large temperature difference during the preheating of the alkali remains.

The main object of the present invention is to preheat the digestive stream with the heating steam of the digestion, especially in the two-stream digestion system of gibbsite bitoxites! During the flow phase, the temperature differences in heat transfer with respect to the slurry, and in particular the alkali, are significantly reduced, thereby improving the energy efficiency of the entire system. It is a further object of the invention to provide temperature differentials favorable for heat transfer in the pre-heating flow stage heated by the expansion vapor in the digestion system. Finally, it is our goal that other low-temperature operating heat requirements in low-temperature operating areas of the plant can be met by low-pressure vapors.

SUMMARY OF THE INVENTION This object is achieved by providing an object process in which the digestion stream is evaporated at least once during the preheating flow stage leading to the blending with the frozen bauxite, preferably immediately prior to the digestion, and preferably from the lye stream. used for preheating the slurry / or alkali stream and / or for further steam generation from the lye stream and / or for other operational heating, if appropriate. Preferably, the digestion stream is concentrated by heating steam fed into the system. It has been found advantageous to perform several evaporations in succession at different temperature ranges of the preheat flow stream of the digestion stream and to cause the evaporation (s) of alkali in the lower temperature ranges of the preheat stream flow stream to be at least

189,244 are performed in part at higher temperature range (s) using steam (s) generated during evaporation (s) of heating steam fed into the system. The vapor (s) produced by evaporation of the alkali stream can advantageously be utilized in several steps. In a preferred embodiment of the process according to the invention, the vapor (s) generated from the lye stream during vaporization (s) are utilized together with vapors from the digested slurry expansion to preheat the slurry and / or lye stream and / or evaporate from the lye stream at lower temperature ranges. development. Finally, it has proved expedient to expand the alkaline stream branched out from the preheat flow stage for milling in one or more stages before use, and preferably to use the developing steam (s) to directly heat the ground slurry.

A key feature of the apparatus of the present invention for carrying out the process is that a heater is fed directly to the system, at a point in the preheating flow of the digestion stream, directly at the junction of the preheated digestate and the preheated slurry stream. In preferred embodiments of the apparatus, a plurality of vapor units is provided in the evaporator unit (s), which are inserted at least partially in the evaporator unit (s) in the different alkaline temperature ranges of the exploratory lye stream. It is preferred that the apparatus has at least partially an expulsion system, preferably a circulating pump forced circulation evaporator comprising an expansion vessel and heat exchanger (s). Preferably, the apparatus also has an expansion vessel (s) inserted into the lye stream branched out of the digestion stream into its relatively high lye temperature range, the head (s) being provided with at least one stream of condensate mixed with bauxite connected.

The design of the method and apparatus of the present invention is based on the recognition that due to the heat-consuming (endothermic) reaction during digestion and the boiling point of the digestion liquor, heat recovery necessarily becomes unfavorable, and thus the digestion and reducing this leads to significant economic results. The dissolution of gibbsite (A1 (OH) ₃ ) results in a heat consumption of 586 kJ / kg which results in a reduction of the slurry temperature of about 7 ° C during digestion. The increase in the boiling point of the digesting liquor is approx. 4 'C.

In addition to the heat consumption of gibbsite dissolution and the rise of the digestion liquor boiling point, a further unfavorably large temperature difference in heat transfer occurs in two current digesters commonly used in current industrial practice, where the slurry combines with the separately heated lye stream without preheating. We recognize that this is. high temperature differences can be reduced under economically advantageous conditions according to the invention.

Setting a favorable temperature difference for heat transfer in slurry expansion slurry heated with slurry expansion vapors has also proved to be advantageous in preheating systems, mainly multistage slurry, as it does not occur spontaneously. Likewise, the process according to the invention can advantageously be made more energy efficient for various other purposes at different temperatures in other areas of the plant.

In order to carry out the process according to the invention, various, simpler and more complex apparatuses or a switching arrangement may be established, either of which may prove to be more favorable depending on the economic circumstances.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail below with reference to the accompanying drawings, in which exemplary embodiments of various degrees of complexity are illustrated. In the drawing it is

Figure 1 is a schematic diagram of a first exemplary exploration device, a

Figure 2 is a schematic diagram of a second exemplary exploration device, a

Figure 3 is a schematic diagram of a third exemplary exploration device, a

Figure 4 is a schematic diagram of another fourth exemplary exploration device,

Figure 5 is a schematic diagram of an exemplary indirect slurry heater apparatus.

The l. Fig. 4A illustrates an exemplary apparatus of the present invention, the main feature of which is to produce steam from the alkaline stream of the binary digestion by evaporation with digestion steam, which is used to preheat the bauxite cold slurry in a mixing condenser.

In the digester of Fig. 1, the digestion stream 1 is fed to the first unit of a digester heat exchanger line containing heat exchangers 13. The alkaline heater heat exchanger line is heated by expanded vapors 12 in slurry expansion vessels 10. From the digestion stream 1, a portion 2 of the digestion stream 2 is led to the bauxite 3 after the first digester heat exchanger 13. The remainder of the alkaline stream 1 passes through the heat exchange line to the heat exchangers 15 of an evaporator unit, which is heated by the heating steam 8 of the digestion. The lye stream 20 from the heat exchangers 15 is combined with the hot slurry stream 6 from the cold slurry stream 4 in a mixing condenser 5 and, as set slurry stream 7, flows into the first member of the digester autoclave 9, where appropriate. From the exploratory autoclaves 9, the hot digested slurry 14 flows to the first member of the slurry expansion tank row 10, while the cooled digested slurry 11 leaves the last member of the tank row.

The alkali stream 19 leaving the heat exchangers 15 of the evaporator unit expands in an alkaline expansion vessel 17, the evolving steam 18 heats the cold slurry stream 4 into the mixing condenser 5 and the steam 22 is discharged for other operational heating purposes. The cooled lye stream 26 leaving the lye expansion vessel 17 is fed back to the digestion stream 1 by a circulating pump 16. Optionally, the alkaline stream 23 is the explorer

189,244 of the lye stream is drained after the last 13 heat exchangers of the lye heater to dilute the ground slurry to prevent the mixing condenser from settling. Pure condensate water 24 can be recycled directly to the power plant. The alkaline condensation waters 25 are normally discharged after expansion for operational use.

The exemplary apparatus of Fig. 2 differs mainly from that of Fig. 1 in that the steam generated from the digestate stream by the digestive heating steam is used in addition to the direct preheating of the bauxite slurry.

Accordingly, the apparatus of Fig. 2 includes an alkali heater heat exchanger 28 in which the digesting stream 1 is further heated by the vapor 27 evaporated from the lye. In the 29 preheaters, the condensation water of the digestion steam heats the digester 1 further. The vapors generated in the expansion tanks of the pure condensate water are partially combined with the alkaline vapors and utilized. After the last 30 expansion tanks, the clean 24 condensate water can be traced back to the power plant.

The main feature of the exemplary apparatus of Fig. 3 is the multi-stage mix condenser slurry preheating, where the steam generated in the lower stages of the alkaline stream is utilized in the first stages of preheating. Accordingly, the exemplary apparatus of Fig. 3 includes an additional alkaline heater heat exchanger 31 having an increased surface area, the heating of which is carried out by evaporation of steam generated from the digestate stream 1 and generated during the slurry expansion. The heat exchanger 28 may also have an increased heating surface, since its function is not only to preheat, but also to produce evaporative steam.

The main feature of the apparatus of Fig. 4 is that the multi-stage preheating of the slurry is accomplished in part by the vapors developing from the expansion of the lye which has led to grinding and is more heated than usual. An additional alkaline stream 32 serves here to control the optionally lower temperature alkaline stream to the lye expansion vessels 17. In this connection, only a portion of the alkali stream flows through an evaporator assembly consisting of heat exchangers 15, an alkali expansion tank 17 and a circulating pump 16.

Finally, the main feature of the exemplary apparatus of Fig. 5 is that the vapors generated from the digestate stream by evaporation are used to indirectly preheat the digestate and slurry stream.

in slurry preheaters, the slurry is heated by expansion vapors. This continues in a heat exchanger 34, which is heated by the steam 8 from the digestion stream 1 generated by the digestion stream 1. Indirect heating of the slurry is completed in a heat exchanger 35, which is heated directly by the heating steam 8.

The heated slurry stream 37, in combination with the exiting alkaline stream 20, flows as a set slurry stream 7 into the first reservoir of the exploratory autoclave 9. In this connection, two conventional evaporators 38 are used instead of the evaporator unit, which is usually included in the previous figures, consisting of a heat exchanger and an alkali expansion vessel. A line 39 serves to recirculate the alkali.

The expediency of using the process of the invention is particularly pronounced when it is considered that excess water is evaporated in the digestion system compared to the conventional two-stream digestion process without increasing the heat consumption of the plant. This is partly due to the fact that the direct introduction of the digestion steam is significantly reduced or completely eliminated, since steam generated from the alkali stream by evaporation is used for this purpose. In addition, the evaporation of excess water means that the water evaporated from the alkaline stream is used as an indirect heating vapor under reduced pressure in the digestion line. for preheating slurry.

The digestion current or In some cases, with a water evaporator in several stages of the grinding liquor stream, the excess heat without evaporator will be even more efficient, even if the evaporated steam is used outside the digester system.

Other advantages of the process compared to the conventional solution are that heating of the vapor barrier system can be achieved with lower pressure steam, which results in a better utilization of the energy content of the steam in the turbine.

The process of the present invention allows for a better choice of snow transfer conditions by reducing the temperature differences in heating. Although this requires the addition of additional surface area to the exploration system, this surface increase is only approximately equivalent to the standard surface area values incorporated in the alumina evaporator. In other words, the additional surface built into the digestion system can substantially reduce the surface area of the separate evaporation system otherwise required to adjust the alkali concentration. It is also possible that such a separate evaporator is not required, since evaporation in the digester system covers all water evaporation needs.

But not only the additional cost of new seats installed in the exploration system, but also the cost of using additional electricity on the exploration line. The surplus electricity consumption on the exploration line is reduced to a degree corresponding to that of the evaporator.

Another advantage of the process according to the invention is that it can be realized not only when new exploration units are created, but also by supplementing existing ones.

The following example illustrates the more efficient energy management achievable by the process of the present invention based on the exemplary apparatus of Figure 2.

A plant with a production capacity of 600 U / year of alumina,

In the circuit of the circuit shown in Fig. 2, a liquor stream of 1271.4 m ³ / h is fed to the first member of the lye heater heat exchange line. This _{digestion liquor has a} Na ₂ O _hexagonal concentration of 115.2 g / l and a temperature of 82 ° C. (The Na ₂ O _{ki) II / 5} concentration is the NaOH content of NaOH bound to the aluminum content of the _{digestion liquor} and expressed in Na ₂ O.) After the first heat exchanger, which is heated in the order of the last sludge expansion vessel, C, and this liquor lúgáramot 130.8 m ³ / h to 180 t / h gibbsites

189 244 leads to the bauxite stream. (Bauxite A1 ₂ O ₃ content = 47%, SiO ₂ content 3%; Feed moisture = 10%). Temperature of the resulting slurry stream

72.8 C and 202.4 m ³ / h.

1140.6 m ³ / h of the digestate flux is further flowing through the digester heat exchangers and its temperature after the steam-heated heat exchanger of the first slurry expansion vessel is 122 'C. After the next heat exchanger, which is heated by the steam generated at 150 ° C by the expansion of steam at 145 ° C, the alkaline stream temperature is 133 ° C. This alkali is further heated to 135.4 ° C by steam generated from the condensation water of 8 bar exploration heating gas in the next heat exchanger. The alkaline stream is then heated to 150 ° C in the digestion heat exchanger with heated steam, along with the cooled and recirculated lye in the lye expansion tank.

The slurry stream of 202.4 m ³ / h is fed to a mixing capacitor where it is heated to a temperature of 142 ° C by the steam formed at the expansion of the 150 ° C alkaline stream at 145 ° C.

. The amount of digestion heating steam used to heat the alkaline stream and evaporate the water is 89.5 t / h. The amount of steam generated from the alkaline stream is 51.3 t / h; of which 25.5 t / h direct heating of sludge,

At 25.8 t / h, it was indirectly heating the alkali.

After the condensation of steam ³ / h m 227.9 increased zagyáram and after combining ³ / h 1089.3 m lugar decreased after the evaporation of alkali and bauxite ratio takes a positive value, and this set of temperature zagyáramnak 148.8 ° C.

As the heat consumption of gibbsite dissolution follows, the slurry temperature would fall below the desired 143 ° C, therefore 3.5 t / h of 8 bar steam would be blown directly into the first digestion autoclave. Thus, at the end of the exploration line, the slurry temperature is 143 ° C.

The amount of heating steam used for the digestion is 93 t / h with direct injection.

After digestion, the slurry fed to the slurry expansion tanks cools down in four steps, and its temperature after the last slurry expansion vessel is 107.2 'C.

The slurry expands to a total of 88 t / h of water as steam to heat the lye boiler heat exchangers, thus leaving the last expansion tank

1232.7 m ³ / h digested slurry flow is removed from the dilution operation; section.

The so-called indirect steam generated by the exploration system at 89.5 t / h. cooled to 120 ° C with an expansion of pure condensate water

Decreases to 85.7 t / h. Is this condensation water flowing back to the? power plant.

The so-called 113.8 t / h evaporated from the alkaline stream and slurry stream. Alkaline condensate escapes from the digestion system at a temperature of 100 ° C, increased by 3.8 t / h evaporated from pure condensate (total 117.6 t / h).

The plant has an hourly alumina output of 68.5 t.

By way of comparison, the performance characteristics of a conventional two-current exploration system which produces the 68.5 t / h alumina production, which is identical in its technological parameters to the conventional, and the two-current exploration system according to Figure 2 are summarized below.

Process according to the invention Traditional process Digestion temperature, 'C 143 143 Heating steam pressure, bar 8 8 Amount of heater: t / h 93.0 88.8 Of which: indirect heating of the alkali and evaporation of water, 89.5 60.7 for direct heating of slurry 3.5 28.1 Volume of water evaporated from alkaline stream: t / h 51.3 Of which: leading to direct heating of slurry. 25.5 leading to indirect heating of the alkali 25.8 - The amount of water leaving the slurry expansion, t / h 88.0 88.0 Amount of steam fed to steam slurry: t / h 25.5 + 3.5 = 29.0 28.1 Amount of alkali and water leaving the slurry stream: t / h 88.0 + 51.3 = 139.3 88.0 Decrease in water content of alkali and slurry stream, t / h 139.3 - 29.0 = 110.3 88.0 - 28.1 = 59.9 Reduction of excess water content in the new process: t / h 110.3 - 59.9 = 50.4

Evaporative steam demand for excess water reduction: t / h 50.4 x 0.3 = 15.1 (Evaporative steam requirement for 1 t water is 0.3 t)

189,244

Conventional according to the invention procedure procedure Increase in Exploration Steam Use for the new Process: t / h 93.0 - 88.8 = 4.2 Difference between evaporation steam demand and heating steam consumption: t / h 15.1 - 4.2 = 10.9

Based on the foregoing, the exploration method and apparatus of the present invention results in an additional 4.2 t of steam utilization, however, due to the evaporation (evaporation) of the excess water ¹⁰ , a 15.1 t reduction in steam utilization compared to a conventional evaporator . Overall, the process of the present invention reduces steam consumption by 15.1 to 1.2 = 10.9 t, which is 12.3% of the conventional digestion process consuming 88.8 t of heating steam. This

8 means that a = 1.296 t / t. alumina, grade 68.5 by converting a conventional exploration steam consumption system of 2o g to the system of the invention, 1.296 x 0.123 = 0.16 t / t. alumina, a decrease in specific steam consumption occurs.

It is noted that the situation is in fact even more favorable, since the amount of pure condensate water flowing back to the power plant is higher by 85.7 - 60.7 = 25 t / h than the process according to the invention. Similarly, a higher amount of alkaline condensate drainage from the system with a value of 117.6 - 88 = 29.6 t / h. Their heat content results in a further reduction in steam consumption at the plant level.

If the slurry is preheated in several stages, or steam can be produced for other purposes in several temperature stages, the steam consumption can be further reduced.

Claims (11)

Claims 1. A procedure for the exploration of gibbsite bauxites in a ^40- mu recuperative system, in particular 140 ... Digestion temperature of 145 ° C, formed with lúghozzáadással ground bauxite in which optionally preheated thick slurry is preferably ⁴⁵ were combined and fed into a digestion followed by autoclave (s) in question, then the digested slurry is typically preheated exploratory lúgárammal többlépcső- a preheat flow stage expanded többlépcsőben and exploratory lugar counterflow preheating of at least part of the digested slurry is carried out by developing steam during the ⁵⁰ dispersion of expansionist addition to the system of digestion temperature and / or the feltárólúg directly desired prior to bonding temperature setting fütőgőzt fed, characterized in that the exploratory the alkali stream is evaporated at least once during the preheating flow stage leading to unification with the frozen bauxite ⁵⁵ , preferably immediately prior to unification, and Preferably, the expanded steam (s) generated by the process are utilized for dewatering the slurry and / or alkali stream, and / or for steam generation by further evaporation from the lye stream and optionally for other operational heating. 2. A process according to claim 1, wherein the digestion stream is evaporated with heating steam fed into the system. The process according to claim 1 or 2, characterized in that several evaporations are carried out successively over different temperature ranges of the preheating stream of the digestion stream. The process according to claim 3, wherein the evaporation (s) of liquids at lower temperature ranges of the preheat flow stream of the alkali stream is performed at least in part by the vapor (s) generated during evaporation of the heating vapor (s) fed into the system. . Process according to claim 5 or 3, characterized in that the steam (cke) generated by evaporating the alkaline stream is utilized in several steps. 6. A process according to any one of claims 1 to 3, wherein the steam (s) generated from the lye stream during evaporation (s) are utilized together with the vapors from expanding the digested slurry to preheat the slurry and / or lye stream and / or evaporate from the lye stream at lower temperature ranges. steam development. 7. A process according to any one of claims 1 to 6, wherein the alkaline stream branched out from the preheating flow stage for milling is expanded in one or more stages before being used and the steam (s) generated are preferably used to directly heat the ground slurry. 8. Apparatus according to claims 1-7. A method according to any one of claims 1 to 3, which preferably comprises a plurality of exploratory auloclaves and a series of slurry expansion tanks connected thereto, and heat exchangers inserted in the preheating flow section of the digestion stream, characterized in that ) and an evaporator unit with a heated space (8) fed directly into the system as a heat transfer medium directly in front of the junction of the preheated slurry stream (6,37). Apparatus according to claim 8, characterized in that the vapor (27) is introduced into the evaporator unit (s) in the evaporator unit (s) inserted into different alkaline temperature ranges of the preheating flow section at least partially in the evaporator unit (s). ) has a tcbb evaporator unit with a heated heating space. Apparatus according to claim 8 or 9, characterized in that it comprises at least partially an expansion system, optionally an integral expansion vessel (17) and a heat exchanger (s) (15, 28, 31). 189,244 preferably have circulating pump (16) forced circulation evaporator unit (s), 11, pp. 8-10. Apparatus according to any one of claims 1 to 3, further comprising an expansion vessel (17), which is inserted into the alkaline trough (19), branched out from the digestion stream (1) in its relatively high temperature range, having a humidifier (s) ) with at least one preheater connected to the cold slurry stream (94) of the frozen bauxite (3), preferably connected to a mixing capacitor (5).