DE941610C - Extraction of the clay content of minerals - Google Patents

Description

The present invention relates to improvements the treatment of alumina-containing minerals for the purpose of obtaining their alumina content, in particular according to the Bayer process and similar processes in which the extraction of the alumina from Bauxite and similar alumina-containing ones. Minerals by digesting the minerals using caustic solutions he follows; the greater part of the alumina content is dissolved by the caustic solution and the greater part

ίο the unwanted mineral components remain undissolved, so that it can be separated from the liquor. More specifically, the invention relates to those processes in which the nature and nature of the minerals treated require the separation after the Digestion made more difficult, d. H. on the filtering of the alumina-containing solution from the insoluble.

In procedures based on the Bayer process for the treatment of alumina minerals, the Mineral, e.g. B. bauxite, usually digested with a caustic solution that is used in the technical implementation z. B. usually one recycled from previous digestion and alumina precipitation stages, is depleted aluminate liquor; by digesting the alumina content of the mineral is extracted and as a result, a relatively high concentration of the liquor in dissolved alumina is achieved again. One Effective operation required digestion at temperatures well above normal boiling point of the solution (i.e. the boiling point at normal atmospheric pressure), i.e. at pressures well above normal atmospheric pressure. In the next, the digestion of the mineral with caustic solution

The step was then to cool the sludge obtained by relaxation, d. H. by rapid lowering the digestion pressure in one or more stages, which is accompanied by rapid evaporation of water of a drop in temperature. Is z. B. by one or more sudden sudden changes When the digestion vessel is opened, the pressure in it is suddenly reduced, so that it cools with the usual surge of the Sludge promptly down to the normal boiling point or ίο near it. After relaxing, the alumina-containing lye from the undesired and undissolved constituents of the suspended in it Minerals, the so-called red mud, separated. If other methods, e.g. B. Sediment with subsequent decanting or other combinations of measures, sometimes suggested it has generally been found that for the technical and economic implementation, the Usually in large quantities, filtering is the most economical and effective means of doing this Separation of the alumina-containing liquor, d. H. the aluminate solution, from the undesirable one suspended in it and undissolved red mud is. In practice, this is how the sludge is removed from the digestion vessel (after its relaxation) to filter presses or the like, from which an alumina-rich liquor for the subsequent precipitation of alumina drains off, while the red mud is used as a usual filter cake on the Filter cloth remains.

When trying to separate the digestion liquor from the red mud, however, in the case of certain mineral types, especially the "Jamaica" e-bauxite (in Jamaica mined bauxite) or minerals of a similar type, present special difficulties, as detailed below will be executed. The main difficulty is achieving an effective separation speed, or in other words, obtaining (by filtering or otherwise) one Satisfactory yield of clear and pure aluminate solution in an economically cheaper, short Time. It was found that the rate of filtration is that of minerals of this type after digestion and the sludge obtained from the normal pretreatment described above is very slow in comparison for filtering the corresponding sludge of others, e.g. B. that in industry as "Demerara" - and "Arkansastf-Bauxite known, d. H. the one in British Guiana and certain -in the United States, especially Arkansas, occurring bauxite. To remedy this extraordinary slowness of the Filtration speed in »Jamaican bauxites, the seriously hinders the normal operation of the manufacturing equipment and in some cases the usability Such minerals in an economic sense strongly impaired, is not yet a means found.

A main object of the present invention is to propose improved digestion and filtering methods for obtaining the alumina content which the difficulties just discussed are at least greatly reduced and in some cases even largely overcome. Another item is the suggestion of measures to improve the filtration speed made for the same purpose certain alumina sludges.

It has been found that the excessively slow rate of filtration in Jamaicacc bauxites at least in the main, the consequence of a rapid clogging of the pores of the filter cloth and / or is the result of a dense, tightly packed cake building up on the filter cloth. These Difficulties, in turn, seem to be the result of the extreme fineness of the solid particles in the normal process · resulting sludge to be contained red mud. Furthermore was however, found that these difficulties are unexpectedly overcome when the mud Is transferred from the digestion vessel to the filter device while it is kept relatively at rest will, d. H. especially if the sludge from the digestion vessel is passed directly into the filter without Relaxation and preferably at a high temperature or at best after gradual cooling. Comprehensive Investigations showed that the separation rate of the desired alumina-containing Solution increased considerably and the difficulties mentioned are largely overcome. One an important procedural measure is avoidance of relaxation (or, in other words, avoidance any sudden change in pressure or temperature), e.g. B. by filtering at digester pressure and -temperature or-by slowly lowering the sludge temperature while the sludge is being prepared sudden decrease in digestion pressure is prevented from filtering.

It was found that - in some respects not yet fully understandable - by relaxing the digested sludge of " Jamaicacc bauxite" a suspended red mud is formed compared to that obtained by relaxing the digested sludge of " Arkansas" or "Demerara" bauxites Particles of extremely fine particles and that the rate of filtration is increased substantially by avoiding relaxation of the digested sludge. After microscopic examination, however, it can now be assumed that Jamaica bauxite has a residual skeletal structure, so to speak, which is almost completely destroyed by the relaxation, presumably by the relatively strong movement or other disturbances as a result of the surge caused by the sudden drop in digestion pressure caused. In contrast, the residual skeletal structure of the bauxites commonly used up to now, e.g. B. the Demerara and Arkansas bauxites not to show this sensitivity. Obviously, this distinctive difference in the nature of the red mud particles of these two mineral types has not yet been observed, nor has its significance and scope been recognized.

With the expression residual skeleton structure is the structure after the removal of the alumina content remaining undissolved by digestion with caustic solutions (usually consisting of particles of relatively considerable size) components or parts meant. The starting mineral is usually (at least when entering the digestion vessel) of moderate, but not excessive

moderate particle fineness, d. H. the bauxite is made with a view to possible savings in grinding costs and easy handling usually only ground to a fineness that is almost extracted all clay permitted. For the sake of simplicity, the following minerals to which the invention is particularly applicable, as minerals with a residual skeletal structure, which is meant by relaxation ( relaxation after digestion) is almost completely destroyed, denotes with which Jamaica bauxite and similar, the same, just discussed, to the peculiar, peculiar structure of their insoluble in caustic solution Partly attributable behavior like how this showing bauxite should be meant. Further, it has been found that elevated temperatures when filtering sludges of the above described Type lead to an unusually sharp increase in the filter speed. The effect of this measure, namely filtering the sludge at well above the normal boiling point of the Temperatures lying in the sludge, and easiest at the digestion temperature, is particularly pronounced in the case of bauxite of the Jamaica type, especially in contrast to other types of mineral such as demerara bauxite, whose red mud particles do not have the easily destructible structure.

The advantages of the invention can be applied in economic and technical terms through direct filtration of the digested sludge at digestion pressure and temperature and subsequent cooling of the obtained Find red mud filter cake and the alumina filtrate. In this way of working z. B. the still under the digester pressure sludge without relaxation or blowing from the Digestion vessel fed directly into the filter. Then the mud is at a suitable, comfortable pressure filtered, while at the same time the temperature drops considerably in the course of the filtration is kept above normal boiling point. If this is the way to work, then Jamaica and similar bauxites, the red sludge of which has a residual skeletal structure of the easily destructible ones discussed above Kind have achieved a significantly improved filter speed.

The improved process can be used with advantage on Jamaica and similar bauxites of the particular discussed Type also with a slow decrease in the temperature of the sludge in or after the digestion vessel be used if the sludge is kept from sudden pressure drop during this time until the temperature has dropped to or near normal boiling point (at atmospheric pressure). Conveniently, the digestion vessel is passed through with internal or other cooling coils or a jacket the cooling water or other coolant is sent, so that the temperature of the sludge can be slowly lowered to the desired point while the digestion vessel is closed remains and stronger disturbances of the sludge are avoided. The one so lowered in temperature Sludge is then sent to the filters and treated there in the usual manner, i.e. i.e., as in the Bayer process usual, filtered under pressure. It should be noted that it is immaterial whether during the slow Cooling of the sludge and before it reaches the filter cloth under pressure, the digestion vessel is closed remains or is open to the atmosphere, provided, of course, that it has already been cooled to a temperature is, in which by lowering the pressure above or in the solution stronger disturbances of the liquid do not occur can.

It can be seen that a notable advantage of the invention is that it applies to the Bayer process, such as it is currently being carried out technically and practically without any major changes to the systems can be.

The following table I shows the values obtained in small tests. In each of these experiments, a sludge was worked up by digesting a batch of a certain bauxite measured in such a way that using a solution of 100 g of caustic soda per liter resulted in a lye with 45 g of alumina per liter. The sludge was filtered through a vertical filter of 6.5 cm ² filter area under a pressure of 3.5 at. Digestion was carried out in the usual way, ie for 10 minutes at 150 ^° . In the experiment, in which relaxation was avoided before the filtration temperature had fallen below 100 °, the sludge was slowly cooled in the digestion vessel that remained closed. In each experiment the volume of the filtrate was measured at 5 minute intervals for one hour, but the total volume reached after 1 hour is sufficiently indicative.

50 number of
try Table I. Filtration temperature
in ⁰ C Volume of the after 1 hour
accumulated filtrate, in cm ³ ,
average Bauxite type 2
I.
8th Digerierschlam m
relaxed 85
85
85 6l, 6
204.5 55
Jamaica
Jamaica
Demerara Yes
no
Yes

As can be seen, the processing of Jamaica bauxite increased the yield obtained in the given time of aluminate solution quite considerably if, according to the present process, in which the relaxation avoided and the digestion batch is slowly cooled down.

As a further and more detailed example of the new process, Table II below gives the values obtained in a test plant. In these experiments, samples of the selected bauxites were digested with waste liquor that had accumulated in the process, depleted of alumina or exhausted. The initial amount of bauxite was measured in such a way that the alumina content of the 50 l of waste liquor used in each experiment rose by 45 g / l. The waste liquor was the caustic liquor commonly used in the Bayer process (solution of sodium hydroxide, sodium carbonate and sodium aluminate), taken directly from the circuit of the technical system in which the tests were carried out. In each case the digestion was carried out in a steam-jacketed autoclave for 10 minutes at about 143 °. A filter cell was connected directly to the bottom of the autoclave and contained a vertical round filter with a filter area of about 240 cm ² . The volume of the filtrate was measured at regular, appropriate intervals for 1 hour. If necessary, the filtrate was cooled by passing it through a condenser. The autoclave was set up in such a way that the charge could be cooled down for the experiments with relaxation by decompression (blowing out) into a container, from which it was then passed back into the autoclave. In the lower temperature filtrations without flash, the batch was cooled by passing cold water through the jacket of the autoclave. When filtering at high temperatures, the filter was designed to receive the sludge from the digester (ie, from the autoclave) without a sudden drop in pressure. Only in the experiments with a high filtration temperature after expansion was the digestion vessel depressurized (in order to achieve a sudden, substantial reduction in pressure), closed again and then reheated to the specified temperature before filtration.

Table II

Bauxite type Yes 100 Jamaica 108 no 130 109
120 0 0 0 0 Demerara Yes 147 112 in 1 no 1 ^ 3 relaxed 117 HQ -101 2.3 2.6 3.3 4.1 121 O 14 ¹ y Filtration temperature ^{in 0 C} -90. -102 -II3 3.6 4.3 5.5 7.0 -106 3.4 -124 average 126 6.1 5.7 7.5 9.2 12.0 5.9 6.0 Temperature range 6 ι 147 6.2 6.1 6.7 7.7 10.3 12.8 16.3 6.3 5.8 10.8 5.8 6.4 -112 6.8 7.0 -5.6 9.4 12.8 15.5 20.3 6.1 15.2 6.5 Filter pressure in at -5.6 -5.0 -5.6 5 10.8 15.1 18.4 23.9 4th -5.6 19.2 -5, o average 2 5.7 2 2 Average total 12.1 17.2 21.1 27.4 of the filtrate 2 22.7 4th Pressure area .; 6, i O 26.2 0 -3.5 4.6 0 Number of attempts i, 7 2 8.1 4.5 Filtration time in minutes 2.7 13.8 8.0 O 4.1 18.4 13.6 5 5.6 22.3 18.2 IO 6.8 25.7 22.4 20th 7.9 28.9 26.0 3 ° 8.8 29.7 40 5 ° 6o

The values in Table II indicate that the process improved according to the invention d. H. for the treatment of Jamaica bauxite without relaxing the digestion batch, clearly that recognize the high degree to which filtering speeds up at both low and high temperatures will. The unusual advantage of this measure is particularly noticeable when making a comparison with the relatively small effect of the same measure in the treatment of demerara, Bauxite obtained sludges. After these last attempts, the substitute for relaxation shines through slow cooling can even lead to a slight slowing down of the filtration. The results The above and further experiments showed that this was the case

lying process provides a unique and extraordinary improvement in the liquid separation efficiency of obtained from Jamaica-type bauxite Brings sludge.

Claims (4)

PATENT CLAIMS: i. Process for extracting the alumina content of alumina minerals which contain the Have characteristics of the Jamaica bauxite and have a residual skeletal structure that is caused by relaxation is essentially destroyed, the mineral being added by heating with a caustic solution elevated temperature, preferably significantly above the normal boiling point of the caustic solution and increased, preferably substantially above Atmospheric pressure is digested to form a dissolved alumina and in this sludge containing suspended undissolved particles, characterized in that a sudden relaxation between digestion and filtering off the sludge avoided will. 2. The method according to claim i, characterized in that that is filtered at substantially the digestion temperature. 3. The method according to claim 2, characterized in that filtered without a rapid drop in pressure will. 4. The method according to claim 1, characterized in that that in the processing of minerals, which can be almost completely destroyed by relaxing Have residual skeleton structure, the digestion pressure through slow filtration Lowering the temperature is lowered. Referred publications:
Swiss techn. Magazine No. 34, 1950, P. 540 ff. »Encyclopedia of techn. Chemistry «by Ulimann, 1928, Vol. I, p. 304; "
German patent specifications No. 556 925, 665 495. © 509 688 4.56