US3738411A

US3738411A - Treatment of aluminate digester liquor

Info

Publication number: US3738411A
Application number: US00059954A
Authority: US
Inventors: F Lazar
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-08-05
Filing date: 1970-07-31
Publication date: 1973-06-12
Anticipated expiration: 1990-06-12
Also published as: AU1827770A; AT291197B; FR2056990B1; SE360638B; AU458474B2; FR2056990A1; DE2038706A1; DE2038706C3; DE2038706B2; HU163357B; RO55560A

Abstract

Alumina-containing minerals, such as bauxite, are digested in sodium aluminate liquor, wherein the digestion liquor leaves the digester at a temperature above 150*C and is conducted, without flashing, in a heat exchanger in a countercurrent flow with respect to the feed liquor for the digester so that the temperature difference between the two liquors is less than 10*C.

Description

United States Patent 1 Lazar TREATMENT OF ALUMINATE DIGESTER LIQUOR Inventor:

Ferenc Lazar, Tatabanya, Hungary Assignee: Tatabanyai Szenbanyak, Tatabanya,

Hungary Filed: July 31, 1970 Appl. No.: 59,954

[30] Foreign Application Priority Data Aug. 5, 1969 Austria A 7521/69 US. Cl. 159/47 R, 159/2 MS, 159/46,

Int. Cl 801d 7/00, BOld 9/00, F26b 3/12,

'B0ld 3/02, C01f7/34, BOld 11/04 Field of Search 159/2, 2 MS, 4 MS,

159/46, 17 VS, 45, 47, DIG. 8, DIG. 25; 202/173; 23/143, 297-299, 305-306 56] References Cited UNITED STATES PATENTS 2,806,766 9/1957 Anderson 23/143 BAUXITE- CAUSTIC SLURRY I HOLDING TANK r-usu vzsssrs 11] 3,738,411 June 12, 1973 2,852,343 9/1958 Scandrett et al 23/143 3,497,317 2/1970 Tusche 23/143 2,459,302 1/1949 Arouson 159/47 WL X 2,824,058 2/1958 Zimmerman 159/47 WL X 3,332,470 6/1967 Chirico et a1. 159/47 FOREIGN PATENTS OR APPLICATIONS 780,291 3/1968 Canada 23/143 851,900 10/1952 Germany 159/47 WL Primary Examiner-Norman Yudkoff Assistant Examiner-J. Sofer Attorney-Gabriel P. Katona [57] ABSTRACT Aluminum-containing minerals, such as bauxite, are digested in sodium aluminate liquor, wherein the digestion liquor leaves the digester at a temperature above 150C and is conducted, without flashing, in a heat exchanger in a countercurrent flow with respect to the feed liquor for the digester so that the temperature difference between the two liquors is less than 10C.

4 Claims, 1 Drawing Figure PRESSURE R EGULATOR l DIGESTER LIQUOR 4 SLURRY V II 1 DILUTION TANK 2Q TREATMENT OF ALUMINATE DIGESTER LIQUOR The digestion of alumina containing minerals, e.g., of bauxite, is carried out, as known, in a continuously operating digester line with sodium aluminate liquor at temperatures above 150C and under pressure according to the Bayer process. An increase of the concentration of caustic Na O (e.g., above 220 g/l caustic Na O) to be designated hereinafter in the specification and the claims as kxNa O in the digesting liquor is advantageous for the digestion, since higher caustic concentrations allow the application of lower temperatures and reduced pressure, further specific pressure may also be lowered compared to the process in which the digesting liquor has a lower concentration of kx Na O.

Nevertheless the recent trend is to perform bauxite digestion with dilute liquors (below 220 g/l kXNa O), since thereby the amount of dilute liquor to be evaporated will be diminished resulting in a saving in steam and elimination of the maintenance costs which compensate for the drawbacks of the digestion process.

If, however, digestion is carried out with a dilute liquor a smaller amount of wash water will be sufficient to eluate the caustic which has been introduced into the cycle and this fact causes caustic losses. Compared to the saving in steam this loss causes, however, only a relatively insignificant rise in costs, provided multistep settlers are used and at the end of the line the filtration of the red mud is carried out e.g., by means of filter presses. Increasing the concentration of the digesting liquor is achieved by means of evaporating the dilute liquor which has been obtained in the course of precipitation.

In alumina production water has to be evaporated not only when concentrating the dilute liquor obtained after precipitation of aluminum hydroxide, but evaporation results also due to the expansion of the digester liquor leaving the digester, which is at a temperature above 150C. Since the evaporation of the digester liquor takes place only after digestion, its occurrence does not result in any particular advantage in the bauxite decomposition process. This is because during the flashing the temperature of the resulting concentrated slurry becomes so low that it has no significance in the digestion. Digestion with dilute liquor involves as a rule the evaporation ofless water-from the spent liquor than the quantity of water which evaporates when the di gester liquor is subjected to expansion. This fact is illustrated on an example in the following table.

liquor liquor liquor Digesting after after after liquor mixing expansion of precipitation prior to the digester of the alumidigestion liquor nate liquor kxNa,O 180 g/l 172 236 145 m of water per metric ton of alumina 12 12.57 9.17 14.90

In case of a digester liquor with 180 g/l of kxNa O concentration the total quantity of water leaving the cycle will therefore be 6.30 m per metric ton of alumina of which 2.90 m of water per metric ton of alumina can be removed by evaporation in an evaporator and 3.40 m of water per metric ton of alumina by means of the expansion of the spent liquor.

Evaporation of the dilute liquor is absolutely necessary, for a digesting liquor with a concentration lower than 170 g/l kxNa O is uneconomical due to a deterioration in digestion efficiency. Another method of eliminating the evaporation of the liquor may be carried out by diluting the digester liquor only to a degree which will ensure that after decomposition and precipitation of the aluminum hydroxyde, a concentration of 180 g/l of kxNa O will obtain. This solution is however uneconomical partly because of the less satisfactory settling of the red mud and partly because of a lowering of the decomposition efficiency.

By applying the method and apparatus in accordance with the present invention evaporation of the dilute liquor can be eliminated without causing a reduction in the efficiency of the process.

The method for the processing of digester liquor slurry obtained in the continuous digestion of alumina containing minerals, especially of bauxite, with sodium aluminate liquor according to the present invention is characterized by leading the liquor leaving the digester at a temperature above 150C without expansion in indirect countercurrent heat exchange with respect to the dilute liquor used for digestion through heat exchangers or through series connected heat exchangers preferably so that the temperature difference between the dilute liquor exiting from the heat exchanger and of the feed digester liquor slurry entering the heat exchanger should be less than 10C and evaporating the heated dilute liquor in one or several steps by expansion (flashing).

The sodium aluminate liquor used for the digestion of bauxite is a liquor which is obtained by the dilution of the digester liquor and separation of the red mud by decomposition.

The digester liquor is diluted to a value suitably of about to g/l of kxNa O.

The dilute liquor to be evaporated and used for digestion is preferably led at pressure values of 5 to 60 atm. through the heat exchanger, thereafter the flashing of the heated dilute liquor leaving the heat exchangers is effected in several steps.

The super-heated steam formed by the flashing of the dilute liquor is used for pre-heating the starting mixture.

The apparatus for the implementation of the method according to the present invention is characterized by the used of at least one heat exchanger between the digester and the flashing (expansion) equipment said heat exchanger being fitted on one end with a conveyor pump to feed the dilute liquor to be evaporated and at the opposite end of the heat exchanger with a pipe for feeding the digester liquor, and with automatic temperature and pressure control devices.

As heat exchanger preferably a shell tube heat exchanger is used which withstands operation pressures up to 100 atm., or heat exchangers with laminar plates or of helical construction withstanding operation pressures pressures between 15 and 50 atm.

For most effective heat exchange it is advantageous to have on the one hand a desilicated digester liquor and on the other hand solids of small particle sizes in the digester liquor. The diameter of the particles is between of about 1 to about 45 microns.

According to one embodiment of the present invention in cases in which part of the liquor recirculated into the digestion process for the removal of the inherent salts must be adjusted to a higher Na O concentration than the concentration of the digesting liquor, it is possible to proceed in such a way, that a part of the flashed liquor is fed back into the container fitted before the heat exchanger Where said liquor is reheated, i.e., evaporated, and part of the dilute liquor is led not through the heat exchanger, but mixed directly with the concentrated liquor obtained after desalting and used as digesting liquor.

The dilute liquor to be evaporated is transported by means of piston or centrifugal pumps capable of producing over-pressures of 5 to 60 atm. into the heat exchanger, while at the opposite side of the heat exchanger the pressure of the digester liquor may be used for transporting it.

An embodiment of the equipment for the application of the method in accordance with the present invention is shown on the attached FIGURE.

The digester liquor slurry leaving the digesters 17 and 18 at a temperature above 150C and under pressure is led through one of the chambers la, 2a and 3a of the series connected

heat exchangers

1, 2 and 3, while in the other chambers 11), 2b and 3b of the series connected heat exchangers dilute liquor flows in countercurrent. The dilute liquor originates from tank 6 which is designed to store the decomposed dilute liquor and is transported by means of pump 5 into the heat exchanger series. In the series connected

heat exchangers

1, 2 and 3 part of the heat content of the digester liquor slurry is transmitted to the dilute liquor preferably in such a way that the temperature difference between the dilute liquor leaving from the series connected heat exchangers and the introduced digester liquor slurry should be less than 10C. In the heat transfer process the desired temperature difference can be adjusted by means of a temperature regulator 7, and a pneumatic valve 8. Automatically controlled pressure regulators 19 and 20 serve to ensure the heat transfer. The dilute liquor which leaves the heat exchangers at a temperature higher than its boiling point and under pressure is led through

flash vessels

9, 10 and 11 in which it is stepwise expanded and finally through pipe 12 into a holding tank, for the concentrated liquor which is kept under atmospheric pressure for use in the treatment of bauxite. The caustic is mixed with the bauxite and the resulting slurry 13 is introduced through preheaters 14-16 into the digesters.

The digester liquor slurrywhich leaves the part 3a of the heat exchanger 3 and has been cooled to below its boiling point is led through pipe 4 into a dilution tank (not shown) which is also under atmospheric pressure. The red mud is separated from the slurry, the aluminate is precipitated and separated by filtration and the canstic liquid is introduced into the tank 6. The overheated vapors which leave the flash vessels 9, l0 and 11 are led as heating steam into

preheaters

16, 15 and 14, respectively, of the initial liquor. The starting bauxite slurry arriving through pipe 13 is heated in these preheaters.

The following examples illustrate the process of the present invention. Example 2 is intended as a comparison with the known process.

Example 1 3.1 metric ton of moist bauxite per metric ton of alumina is mixed with 12 or digesting liquor per metric ton of alumina, said digesting liquor having a concentration of 180 g/l of kxNa O. in this way the quantity of the liquor increases after mixing to 12.57 m per metric ton of alumina, while its concentration drops to 172 g/l of kxNa O. This starting mixture is treated in a digester at a temperature of 240C and under pressure of 40 to 60 atm. According to the invention the digester liquor obtained in this way is led into a surface heat exchanger in which the liquor is cooled without flashing in. such a way that in the heat exchanger the dilute liquor obtained after decomposition is led in countercurrent to the digester liquor. From the surface heat exchanger the cooled digester liquor is led into the dilution tank, while the dilute liquor flowing in countercurrent is led in 10 steps through a flash equipment in which it expands to atmospheric pressure and the flashed steam of the dilute liquor is used for heating the starting mixture. Flashing of the dilute liquor is performed in such a way that 3.4 m of water per metric ton of alumina is removed.

In the dilution tank the digester liquor is diluted to g/l of kxNa O concentration by means of the red mud liquor. The quantity of wash water used for dilution is 3.4 m per metric ton of alumina. The smaller quantity of wash water which can be used for the washing of the red mud may raise in this case the loss of caustic which can be washed out from the red mud, therefore at the end of the settling-washing line the mud is filtered to compensate for the caustic losses. The concentration of the spent liquor obtained after decomposition increases as a result of the removal of the hydrate crystal water to g/l of kxNa O, whereby in the dilute liquor flash equipment of the digester plant by means of flashing of 3.4 m of dilute liquor per metric ton of alumina a digester liquor with a kxNa O concentration of 180 g/] is obtained.

Hence the method according to the present invention eliminates the application of a separate liquor evaporator unit.

Example 2 3.1 metric ton of bauxite per metric ton of alumina is digested with 12 m digesting liquor of a kxNa O con centration of 180 g/l. After mixing of the bauxite with the digesting liquor the quantity of the liquor is increased by the moisture content of the bauxite to 12.57 m per metric ton of alumina, while at the same time the concentration of the liquor drops to 172 g/l of kxNa O. The starting mixture is heated in the digester to 240C under a pressure of 40 to 60 atm. Under these conditions the alumina content of the bauxite dissolves in the liquor. The digester liquor under pressure is led through a flash line having 10 stages where it is ex panded to atmospheric pressure and the flashed steam is used for heating the liquor prepared for digestion. The digester liquor which has been cooled in this way to 107C is led then into the dilution tank. The digester liquor is concentrated in this way to 236 g/l of kxNa O. The degree of water removal amounts to 3.4 m per metric ton of alumina, the volume is reduced to 9.17 m per metric ton of alumina. 1n the course of dilution the digester liquor is diluted with red mud water to 140 g/l of kxNa O, but is concentrated during decomposition to g/l of kxNa O. The water requirement of dilution amounts to 6.3 m per ton of alumina. From the dilute liquor obtained after decomposition a quantity of water of 2.90 m per metric ton of alumina is evaporated in the liquor evaporator whereby a digestion liquor of a concentration of 180 g/l of kJcNa O is obtained. The water removal of 6.3 2.9 3.4 in per metric ton of alumina required for the water balance of the cycle is ensured by the flashing of the digester liquor.

What we claim is:

1. In a process for the treatment of aluminacontaining minerals, wherein the aluminum content of the mineral is extracted in a reaction zone with a caustic solvent under elevated temperature to form a hot liquid slurry, the improvement which comprises transferring heat without expansion in an indirect heat exchanger from the hot liquid slurry after it leaves the reaction zone in countercurrent to a dilute caustic liquor, and concentrating the thus heated dilute caustic liquor by flashing, wherein said dilute liquor is obtained by separating the solids content and precipitating at least a major portion of the dissolved aluminum content of said liquid slurry after it leaves the heat exchanger.

2. The improvement of the process of claim 1, wherein the temperature difference between the hot liquid slurry entering the heat exchanger and the dilute caustic liquor leaving the heat exchanger, is not greater than about 10C.

3. The improvement of the process of claim 1, wherein the dilute caustic liquor which enters the heat exchanger contains from about to about g/l kx Na O and the pressure of the dilute caustic liquor in the heat exchanger is from about 5 atm. to about 60 atm.

4. The improvement of the process of claim 1 wherein the dilute caustic liquor which was concentrated by flashing is slurried with an alumina-containing mineral and the slurry is preheated prior to the digestion-extraction thereof, the preheating being carried out by utilizing the steam liberated during flashing.

Claims

2. The improvement of the process of claim 1, wherein the temperature difference between the hot liquid slurry entering the heat exchanger and the dilute caustic liquor leaving the heat exchanger, is not greater than about 10*C.
3. The improvement of the process of claim 1, wherein the dilute caustic liquor which enters the heat exchanger contains from about 120 to about 150 g/l kx Na2O and the pressure of the dilute caustic liquor in the heat exchanger is from about 5 atm. to about 60 atm.
4. The improvement of the process of claim 1, wherein the dilute caustic liquor which was concentrated by flashing is slurried with an alumina-containing mineral and the slurry is preheated prior to the digestion-extraction thereof, the preheating being carried out by utilizing the steam liberated during flashing.