WO1997029047A1

WO1997029047A1 - Production of alumina

Info

Publication number: WO1997029047A1
Application number: PCT/GB1997/000280
Authority: WO
Inventors: Stephen John Adkins
Original assignee: Allied Colloids Ltd
Current assignee: Ciba Specialty Chemicals Water Treatments Ltd
Priority date: 1996-02-06
Filing date: 1997-01-31
Publication date: 1997-08-14
Anticipated expiration: 1998-08-06
Also published as: AU1608197A; GB9602343D0

Abstract

In a Bayer red mud process for producing alumina, contamination of the sodium aluminate liquor with organic contamination is reduced by contacting the digestion product or the red mud with bone char, macroporous carbon or macroporous fly ash or other adsorbent and separating the resultant loaded adsorbent from the digestion product or wash liquor.

Description

PRODUCTION OF ALUMINA This invention relates to the reduction of hu ate and other organic impurities in Bayer process liquor.

The Bayer process for producing alumina comprises digesting bauxite in aqueous caustic digestion liquor in a digestion stage to form a digestion product, separating the digestion product in a primary separating stage into a sodium aluminate liquor and red mud, recovering alumina from the separated sodium aluminate liquor in an aluminium recovery circuit and cycling caustic from the circuit to the digestion stage, subjecting the separated red mud to counter-current washing with wash liquor in a first and subsequent washing stages and thereby producing a used wash liquor and a washed red mud, cycling the used wash liquor into the primary separating stage or in the digestion stage, and depositing the washed mud, for instance by dumping in a red mud lake or in a stack.

The bauxite tends to be contaminated with humic matter and some degradation of this to materials such as oxalic acid occurs primarily in the digestion stage. The presence of oxalic acid, humic matter and other organic residues can interfere with the recovery of alumina, in particular it can interfere with the crystallisation of aluminium hydrate from the caustic liquor. It is therefore well known to include steps in the overall process that are designed to remove humic matter and organic residues.

Various methods have been proposed for reducing the humate and other organic content of the caustic liquors. Many of the methods involve some combination of precipitation, coagulation and flocculation, often conducted on the sodium aluminate liquor after the primary settler and before crystallisation.

Some methods include adsorption. See, for instance, US4275043. Another process in which the separated sodium aluminate liquor is treated is described in US4578255 in which organic impurities are coated onto dispersed solids in the separated sodium aluminate liquor and the coated solids are removed. In U.S. 4,046,855, magnesium and aluminium hydroxides are precipitated in the aluminate liquor and then removed from the liquor with organic impurities.

In Australian application no. 66094/94 we describe a process in which the bauxite is washed with aqueous alkaline wash liquor before it enters the digestion stage so as to wash humate and other organic components out of the bauxite, and the organic components are separated from the bauxite wash liquor prior to recycling that liquor into the digestion stage or the alumina recovery circuit or the separating stage. The separation of the organic components from the bauxite wash liquor can be by adsorption onto an adsorbent such as bone char or fly ash and then filtration, flocculation or other removal of the loaded adsorbent (carrying the organics) from the wash liquor, followed by the recycling of the wash liquor from which the adsorbent and organics had been separated. Although each of these processes can have a beneficial effect, there still remains a need to provide a simple and cost effective way of reducing humate and other organic contamination in the Bayer process liquors.

The Bayer process is conducted for months or years on end without interruption with continuous charging into the process of fresh bauxite ore and with an accompanying minimum loss of liquor from the process. Accordingly, irrespective of the effectiveness of any particular humate removal stage, there remains a tendency for humate and other organics to accumulate in the process. This has an undesirable effect on the process, and in particular the separation by crystallisation of alumina from the separated sodium aluminate liquor, and it would be desirable to provide an alternative or additional cost effective way of reducing humate and other organic contamination in the

Bayer process liquors. In the invention organic contamination in the separated sodium aluminate liquor is reduced by contacting the digestion product or the red mud with adsorbent material during the counter-current washing and thereby adsorbing organic contamination by the adsorbent material, and separating the loaded adsorbent material from the digestion product or wash liquor. Generally the separation of the loaded adsorbent is effected by depositing the loaded adsorbent with the washed red mud. Thus, in the invention the adsorbent becomes loaded with organic contaminants (by adsorption) during the contact with the digestion liquor or the red mud wash liquor, and the loaded adsorbent is removed from the system. Preferably the adsorbent material is particulate. Preferably it is present during one or more of the counter¬ current washing stages in admixture with the red mud and is dumped or otherwise deposited with the washed red mud. Thus the loaded adsorbent is deposited with the washed mud and thus takes the organic contamination (which is adsorbed onto it) out of the Bayer process as a mixture of the red mud and the adsorbent carrying the organic contamination.

When the adsorbent is to be contacted with the digestion product, this can be effected in the digestion stage, for instance by including adsorbent with the bauxite which is fed to the digester and/or by adding particulate adsorbent to the materials in the digester and/or by immersing a support carrying adsorbent into the digester. However the conditions in the digester may tend to degrade the adsorbent and it is generally more efficient to contact the digestion product with adsorbent by contacting the adsorbent with the digestion product in the primary separation stage. Thus the preferred way of making the contact with the digestion product is by adding particulate adsorbent to the primary settler or to other processing equipment in the primary separation stage. Instead of or in addition to contacting the digestion product with the adsorbent, it is useful to contact the red mud with the adsorbent after it has been separated at the primary separation stage and before the end of the washing stages. The contact is usually in the first washing stage or sometimes the second stage but it can be made at any stage down to the last washing stage since even the reduction in organics that is achieved at this stage can have a long term beneficial effect on the process. The washing stages can be sedimentation or filtration stages. Preferably the adsorbent is a particulate material which is added into the primary settler or into the first or second washers.

The adsorbent material must have porosity such that it is effective in the invention. Preferably it has a significant porosity due to the presence of macropores, having a pore size (diameter) above 50nm and often above lOOnm or even above 500nm. It may alternatively, or preferably additionally, have a significant porosity due to the presence of mesopores (e.g., 2 to 50nm) and/or micropores (e.g., below 2nm) .

Since adsorption of the contaminants in the inention appears to rely in part on migration of the contaminants into the internal porous structure of the adsorbent, it is probably more accurate to refer the material as also being absorbent, rather than as being solely adsorbent.

The preferred material is bone char. This has a wide range of pore sizes (diameters) typically ranging from around lnm or less up to several thousand nm, for example 6500nm. Activated carbons can also be used in the invention. Macroporous carbons having a pore size of greater than 50nm are more effective than mesoporous carbon having a pore size of 2 to 50nm or microporous carbons having a pore size of less than 2nm, by virtue of pore diameter, but a mixed activated carbon of varying pore size is preferred. Fly ash can be used, provided it has appropriate pore sizes, typically macroporous preferably with some mesoporosity and/or microporosity.

In preferred processes of the invention, the adsorbent material is bone char (bone charcoal) . So far as we are aware all normal grades of bone char are suitable irrespective of the pore size distribution and so precise measurement of the pore size is not essential.

The particle size of the particulate adsorbent can be selected from very fine (for instance 90% below lOOμm but generally above 30μm) up to a size of 90% 0.1 to lmm. Particles having a size above lmm can be used.

The amount of adsorbent which is used to contact the liquors can vary widely. When adsorbent is added continuously throughout prolonged operation of the process, adequate adsorption can be achieved by the addition of, typically, at least 0.2 and generally at least 0.5 grams bone char or other adsorbent per litre of suspension being treated. Typically the dose is 0.5 to 5g/l. However when the bone char or other adsorbent is added intermittently the rate at which it is added can be higher, for instance 5 to 50g/l.

It is particularly desirable to contact the adsorbent with the digestion product or the red mud which is being washed while the product or mud has an elevated temperature, for instance above 50°C and preferably at least 90°C, and/or a relatively high content of sodium hydroxide, for instance at least lOOg/1 and preferably at least 200g/l. Rapid adsorption of the organics occurs onto the adsorbent under such conditions and yet the organic impurities do not significantly de-adsorb from the particles at lower temperatures and caustic levels, ie as the particles move down the series of washers to the final washing stage.

The overall Bayer process may be conducted in conventional manner except for the addition of the adsorbent described above.

The following are examples. Example 1

A synthetic slurry was prepared to simulate a primary thickener slurry and was made up as follows:

35g/l Red Mud solids

200g/l NaOH

50g/l Na₂C0₃

125ppm sodium humate

500cm samples were taken in 500cm cylinders and placed in a water bath and allowed to reach a temperature of 90°C.

One sample was left untreated and to the others, various dose levels of bone char were added and mixed by plunging for the length of time indicated. lmg/1 of sodium polyacrylate flocculant was added to flocculate all the solids which were allowed to settle. A sample of supernatent was taken for sodium humate determination.

The humate level was determined by measuring colour absorbance at 420nm and comparing with a set of standards.

Sodium Humate content (rag/1) at time (mins)

Bone Char 1 5 15 dose (g/1)

0 127.9 127.9 127.9

10 53.1 29.3 24.9

50 25.5 23.3 23.0

100 20.3 16.4 14.7

Example 2

The synthetic slurry was prepared as in Example 1. 100cm samples were taken in 150cm beakers and stirred mechanically. Various dose levels of bone char were added and stirring continued for the time indicated. After this elapsed time, a sample was removed, filtered to remove the solids and the filtrate was analysed for humate as in Example 1. Sodium Humate content (mg/1) at time (mins)

Bone Char 15 60 120 180 240 dose (g/1)

0 91.0 91.0 91.0 91.0 91.0 0.1 86.8 89.5 86.2 84.6 80.9

1 66.5 57.2 * 51.8 48.7 10 11.7 12.6 13.9 12.7 11.5

* anomalous result

Example 3

A synthetic slurry was prepared to simulate a final washer slurry as follows:

50g/l Red Mud solids

9g/l NaOH

6g/l Na₂C0₃

250ppm sodium humate

The test method used was identical to that in Example 2 except that only the 240 minute conditioning time was used.

Bone Charcoal Residual Sodium dose (g/1) humate (mg/1)

0 268

0.51 258

1.13 241

1.49 227

2.00 213

2.53 204

3.05 194

5.02 156 Example 4

A sample of first washer feed was taken from plant and used as test substrate. Tests were conducted by adding the appropriate weight of bone charcoal to each 250cm plastic bottle. 200cm of first washer slurry was added and the contents shaken by hand. The bottle was placed in a rotating water bath set at a temperature of 95°C. At the appropriate time, 25cm of sample was extracted, filtered to remove the solids and the absorbance measured at 690nm. The lower the absorbance the lower is the colour (humate) intensity.

Claims

1. A process for producing alumina by digesting bauxite in aqueous caustic digestion liquor in a digestion stage to form a digestion product, separating the digestion product in a primary separating stage into a sodium aluminate liquor and a red mud, recovering alumina from the separated sodium aluminate liquor in an aluminium recovery circuit and cycling caustic liquid from the circuit to the digestion stage, subjecting the separated red mud to counter-current washing with wash liquor in a first and subsequent washing stages and thereby producing a used wash liquor and washed red mud, and cycling the used wash liquor into the primary separation stage or into the digestion stage and depositing the washed mud, characterised in that organic contamination in the separated sodium aluminate liquor is reduced by contacting the digestion product or the red mud with an adsorbent material during the counter-current washing and thereby adsorbing organic contamination by the adsorbent material and providing loaded adsorbent material, and depositing the loaded adsorbent with the washed red mud or separating the loaded adsorbent material from the digestion product or wash liquor.

2. A process according to claim 1 in which the adsorbent is selected from macroporous activated carbon, macroporous fly ash, and bone char.

3. A process according to claim 1 in which the adsorbent is bone char.

4. A process according to claim 1 in which the adsorbent material is macroporous.

5. A process according to claim 4 in which the adsorbent is also mesoporous and/or microporous.

6. A process according to any preceding claim in which the adsorbent material is particulate and is present during one or more of the counter-current washing stages in admixture with red mud and is deposited with the washed red mud.

7. A process according to claim 6 in which the adsorbent material is mixed into the digestion product in the primary settler or is mixed into the red mud in one or more of the washing stages.

8. A process according to claim 6 in which the adsorbent material is mixed into the primary settler or the first or second washing stages.