WO1993016003A1

WO1993016003A1 - Process for the treatment of red mud

Info

Publication number: WO1993016003A1
Application number: PCT/AU1993/000048
Authority: WO
Inventors: Clay Mitchell Cardile
Original assignee: Alcoa Of Australia Limited
Priority date: 1992-02-06
Filing date: 1993-02-05
Publication date: 1993-08-19
Also published as: IE930076A1; GR930100044A

Abstract

A process for the treatment of red mud, the red mud having an amount of soda entrained therein, the process comprising passing carbon dioxide through a stream of red mud to lower the pH of the red mud and to retrieve a portion of the soda. The carbon dioxide may be provided in the form of waste flue-gas from the Bayer process.

Description

"PROCESS FOR THE TREATMENT OF RED MUD"

THIS INVENTION relates to a process for the treatment of red mud, and in particular relates to a process capable of both ameliorating the pH of red mud and allowing soda recovery from red mud.

Red mud is produced as a by-product in the Bayer process for production of alumina, being an iron-rich residue from the digestion of bauxite with caustic soda. The red mud provides two main issues for the alumina refining industry: firstly, the disposal of the red mud represents a major loss of valuable soda; and secondly, the red mud has a high equilibrium pH value as a result of the entrained soda.

Traditionally, reactive silica, in the form of kaolin, is added in conjunction with bauxite to the Bayer process in order to effectively desilicate the Bayer liquor and to recover soda from the red mud. The reactive ^' silica addition has the benefit that although it consumes large quantities of soda, the resulting desilication product (DSP) formation locks up impurities, such as sulphate anions, into its structure thus immobilizing them and removing them from the liquor.

The main process used for soda retrieval has historically been to perform a lime sinter process on the DSP and then leach out the liberated soda. However in such a leaching step, the sulphate and other anionic impurities trapped in the DSP are also retrieved.

An aim of the present invention is to provide a process for the treatment of red mud that overcomes, .or at least partly alleviates, the above mentioned problems.

The present invention provides a process for the treatment of red mud, the red mud having an amount of soda entrained therein, the process comprising passing carbon dioxide through a stream of red mud to lower the pH of the red mud and to retrieve a portion of the soda. The carbon dioxide may be in the form of waste flue-gas from the Bayer process or may be any other C0₂ containing gas. In this respect, the CO„ containing gas may be further processed to increase or decrease its CO^ content as necessary.

The carbon dioxide may be passed through the stream of red mud by bubbling the gas through the mud, or by reaction in a sealed vessel to increase the pressure of the system to a desired level. In this respect, the extent of pH reduction and the amount of DSP dissolved is related to the rate of supply of carbon dioxide, as a function of time, in the reacting system.

Another variable controlling the reaction between the red mud and the carbon dioxide is the extent to which the mud has been washed. In this respect, the final pH able to be achieved is also directly proportional to the total alkali (TA) concentration of the red mud. Thus, lower TA muds will achieve a final lower pH value after treatment. Therefore, in the preferred form of the invention the red mud may be water washed prior to treatment.

In the process according to the invention, the structure of the DSP is not degraded enough to enable the release of the anionic impurities from the DSP structure. Thus, the bulk of the soda is retrieved into solution but without the bulk of the impurity phases.

Furthermore, because the DSP structure is partially destroyed, the mobile caustic fraction is also released, It is the mobile caustic fraction which results in the red mud having the high pH equilibrium value of about 12.5 in its untreated state. Thus, release of this caustic into the treatment solution not only retrieves the soda but effectively ameliorates the mud. This results in lower equilibrium pH values, for the treated mud, of the order of 6.5-8.0 depending upon which variant of the process is used.

The invention will now be more specifically described by way of four examples. However, it is to be understood that the following description is not to limit the generality of the invention as described above.

The examples presented demonstrate that for unwashed mud there is often little DSP dissolution, although carbonation of the liquor does occur resulting in a lower pH value of about 8.4. While such a pH is often acceptable for amelioration of mud, in some instances it may not be acceptable for retrieving the soda value.

Further, removal of mud liquor appeared to significantly improve DSP dissolution from the mud, and a lower pH (6.4) was achieved. Therefore, it is apparent that a low TA value is preferred to achieve a significant level of DSP dissolution.

However, water washing the mud first, to zero TA, resulted in recovery of about 87% of the DSP soda while retrieving only about 20% of the DSP impurities (e.g. chloride and sulphate). Thus, a mud washing regime may be instigated which will produce a mud with a final near-neutral pH, with or without soda recovery, depending upon the extent to which water washing is acceptable.

The soda recovered into solution is presented in a carbonated form amenable to lime causticization allowing retrieval of the original soda value consumed during the precipitation of the DSP. The following examples illustrate some embodiments of the invention.

EXAMPLE 1

Approximately 600 g of last stage washer underflow mud from a Bayer refinery, which contained 11.3% DSP, was placed in a sealed autoclave reactor vessel. A 600 psi overpressure of CO- was applied to the reactor vessel and the mud was constantly stirred at 35 rpm with a double-bladed impeller. The reaction was continued for 20 hours. Analysis of the liquor indicated that only 0.59% of the available DSP had dissolved but that the pH was 8.4. This pH was stable and as such represents a suitably ameliorated mud able to be disposed of as land-fill.

EXAMPLE 2

Last stage washer underflow mud was filtered using a water-jet suction device and a Buchner funnel and flask, to give a moist filter-cake with no free liquor. Approximately 100 g of this mud was placed in the stirred reactor, as above, along with about 600 mL of water. Treatment for 23 hrs with C0₂ at 600 psi resulted in 13.39% of the DSP dissolving, and a final stable pH of about 6.4 was attained. Thus while little soda was retrieved, a stable, neutral pH mud was obtained.

EXAMPLE 3

Bayer process last stage washer underflow mud was water washed to zero TA, and about 600 L of water was placed in the stirred reactor described above. Treatment for 6.5 hrs with an overpressure of about 600 psi of CO„ resulted in about 87% of the soda available in the DSP being dissolved into solution. The resulting pH of the liquor was 6.5. Only 16.6 % of the available sulphate was also dissolved out of DSP into solution. Moreover only 19.41% of the DSP was dissolved. This also represents a small gain of alumina back into the leach solution.

EXAMPLE 4

Following reaction of the Bayer process mud with CO_, a solid/liquor separation was carried out. The separated liquor was then heated to 80°C before passing C0„ through the solution. This resulted in dawsonite being precipitated. After the dawsonite had been filtered off, it provided an impurity-free form of sodium carbonate amenable to soda recovery processes such as lime sintering or lime causticization. The precipitation of dawsonite also provided some extent of alumina recovery.

It will be appreciated that various modifications and alterations may be made to the process described above without departing from the scope of the present invention.

Claims

THE CLAIMS defining the invention are as follows:-

1. A process for the treatment of red mud, the red mud having an amount of soda entrained therein, the process comprising passing carbon dioxide through a stream of red mud to lower the pH of the red mud and to retrieve a portion of the soda.

2. A process according to claim 1 wherein the carbon dioxide is provided in the form of waste flue-gas from the Bayer process.

3. A process according to claim 2 wherein the waste flue- gas is further processed to increase or decrease its carbon dioxide content.

4. A process according to any one of claims 1 to 3 wherein the total alkali concentration of the red mud is lowered before passing the carbon dioxide therethrough by washing the red mud.

5. A process according to claim 4 wherein the washing of the red mud is conducted as a water-wash.

6. A process according to any one of claims 1 to 5 wherein the soda retrieved is presented in a carbonated form amenable to lime causticization allowing subsequent retrieval of the original soda value consumed.

7. A process according to claim 1 substantially as herein described in relation to any one of the Examples.