HU186395B - Process for flocculating red mud - Google Patents

Abstract

Aqueous suspensions of red mud are effectively removed from the Bayer process for making alumina by the addition to at least the first stage of the recovery circuit of a flocculant selected from starch, homopolymers of acrylic acid or acrylates, copolymers of acrylic acid or acrylates containing at least 80 molar percent acrylic acid or acrylate monomers and combinations thereof and subsequent addition to later, more dilute stages in the recovery circuit of a copolymer containing from about 35 to 75 molar percent of acrylic acid or acrylate and from about 65 to 25 molar percent of acrylamide monomers.

Description

The present invention relates to a process for flocculating red mud in the alkaline digestion process of Bayer alumina production.

The Bayer process is used almost everywhere to produce alumina. Generally speaking, this process is carried out almost exclusively in aqueous solutions by reacting bauxite in a steam-heated autoclave with a strong base such as caustic soda or lime to convert the alumina to a soluble aluminate. In this step, significant amounts of insoluble contaminants are formed or released from the bauxite and this slag must be removed from the recoverable alumina component. The residues containing these iron oxides, sodium silicate, titanium oxide and other substances generally consist of very fine particles which are very difficult to separate. However, in order for this separation step to be economical, the red sludge, usually 10 to 50% by weight of the ore, must be rapidly and cleanly separated from the aluminum alkali. If separation is slow, the yield is significantly reduced and the overall efficiency of the process deteriorated. Similarly, if the separation is not pure, alumina obtained in the form of aluminate will be quite crude and will be unusable in many areas. The impurities present in the alumina that remain throughout the manufacturing process can cause difficulties in many cases when treating a medium, such as an aqueous solution, with aluminate, since the impurities are introduced into the media as foreign inactive material. For example, the use of a low purity aluminate containing relatively high amounts of impurities in the treatment of water causes the insoluble impurities present directly to produce a slurry which tends to clog the passages of the equipment. Similarly, if the crude sodium aluminate contains a significant amount of impurities, solubility problems can be quite difficult to overcome when the aluminate is fed as a solid.

To overcome these drawbacks and to significantly accelerate the separation of cane sludge from the bulk and to better separate the components, U.S. Pat. No. 3,390,959. U.S. Pat. Accordingly, homo- and copolymers of acrylic acid and acrylates containing less than 20% of other ethylenically unsaturated, polymerizable polar monomers are used as flocculating agents for red mud. In this way, the overall efficiency of the Bayer process is improved.

The above patent discloses the use of copolymers of acrylic acid or acrylates with ethylenically unsaturated monomers to flocculate red sludge, but it is also mentioned that more than 5 mol% of ethylenically unsaturated monomer already substantially decreases the separation efficiency of this cannot be performed. Although this teaching is true for the early stages of the processing cycle, it has been surprisingly found that this procedure is not applicable at later stages of the recovery process, which use dilute solutions.

The present invention relates to a novel process for flocculating red mud produced as a by-product from the bauxite extraction of bauxite by the Bayer process. According to the invention, a conventional red sludge flocculating agent is used in the first stage of the processing process, and in a later stage, a flocculating agent comprising 35 to 75 mol% acrylic acid or acrylate and 65 to 25 mol% acrylamide is used as the flocculating agent.

The conventional flocculants used in the first stage may be: starch; homopolymers of acrylic acid or acrylate; copolymers of acrylic acid or acrylate, in which the copolymer contains at least 80 mol% of acrylic acid or acrylate monomer; hydrolyzed acrylamide monomers; and combinations thereof. Acrylates refer to salts of acrylic acid, such as alkali metal salts or aminonium salts. When using acrylic acid or acrylate copolymers, many comonomers may be used up to 20 mol%. Among the most commonly used monomers are: lower alkyl esters of acrylamide, methacrylamide, acrylonitrile, acrylic acid and methacrylic acid, vinyl methyl ether, methacrylic acid salts, maleic anhydride and its salts, iso-propenylacetate, styrene, fumaric acid, aconitic acid, citraconic acid, amides, alkali metal salts (such as sodium, potassium and lithium salts) and ammonium salts of the acids listed above, and partial alkyl esters and salts of various polycarboxylic acids, vinyl toluene, chlorostyrene, ethylene, propylene and isobutylene. Of the above comonomers, those which carry a hydrophilic group on the side chain of the ethylenically unsaturated hydrocarbon group are particularly preferred. Monomers which do not contain such a hydrophilic solubility-promoting group are used in smaller amounts, in the range of 1 to 5% by weight, based on the total weight of the monomer present.

Other monomers that may be used in combination with acrylic acid or acrylic acid salts include sulfoethyl acrylate, carboxyethyl acrylate, diethyl vinyl phosphonate, crotonic acid and its salts, vinyl sulfonate and its salts, vinyl alcohol and solubilising aryl groups, -szénhidrogének.

Among the homopolymers or copolymers of the types listed above, those having a molecular weight greater than 50,000, more preferably greater than 100,000, are preferred. The best additive polymers have a molecular weight of 10 million.

Among the starches useful in the process of the present invention are potato starch, corn starch, tapioca starch, amylose, sorghum starch and other readily available starches.

The appropriate amount of conventional flocculant used in the first stage of the alkaline digestion cycle depends on the specific composition of the bauxite being processed, the parameters of the particular processing stage, such as temperature, pH and solids content, and the red sludge flocculating agent (s) used. Generally, when starch or starch-based mixtures are used, the amount of flocculating agent that provides the appropriate effect will be 0.05 to 2.0% by weight of the dry sludge residue. If synthetic polymers or copolymers are used, the amount is generally3

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In the United States, about 0.0045-0.9 kg / tonne dry sludge residue.

The copolymer added in the later stages of the alkaline digestion process is well defined as a copolymer comprising 35 to 75 mol% of acrylic acid or acrylate, the exact meaning of acrylate being given above, and 65 to 25 mol% of acrylamide. Although ethylenically unsaturated monomers are generally effective in addition to acrylamide in this copolymer, acrylamide comonomer is preferably used in the process of the present invention for economic, availability and feasibility reasons. These subsequently added copolymers should have a molecular weight such that the Brookfield viscosity of a 0.15% solution of the polymer in a 1 molar sodium chloride solution and a UL adapter at 60 rpm and a pH of at least 2 0, preferably at least 3.0 centipoise.

The amount of these later copolymers also depends on the specific composition of the bauxite being processed and the parameters of the particular processing step. However, the amount of copolymer providing a good effect is about 0.0045-0.45, preferably 0.0225-0.225 kg per tonne of dry red sludge.

The specific stage after which the addition of a copolymer containing 35 to 75 mol% acrylic acid or acrylate or acrylate and 65 to 25 mol% acrylamide to the system can be carried out after the initial stage of the alkaline digestion process depends on many variables of the processing system. The parameters of the particular processing operation, such as temperature, pH, alkali concentration and red mud content, play an important role in this. Naturally, the type of bauxite ore also influences effective exploration. Until we know the exact mechanism of interaction between red mud and flocculant, it is not possible to say with absolute certainty at what stage of the processing process which variables will determine the copolymer efficiency. In general, it has been found that the addition of the copolymer can be carried out with high efficiency in the fourth or subsequent stages of the processing process when the sludge content of one liter of NaOH + Na ₂ CO ₃ solution is less than 100 g. Because this number is only an approximation and is sometimes based on random coincidence rather than control of variables, it is advisable to perform the test described below to determine at which stage of the processing process the copolymer is added.

In the test, we will call the "nth stage" the different processing stage to be investigated and the initial or "head" stage. The lower flow of the n-stage wash tank is diluted 1: 4 with the upper overflow of the n-stage wash tank to produce a "simulated" feed of the n-stage wash tank. This high level of dilution is necessary to ensure reproducible, free settling in the test cylinder, preferably a 500 to 1000 ml measuring cylinder.

To this, a simulated feed of the washing tank of step n, is added to the flocculant to be tested in 0.05% w / w solution. The solution may be diluted with water or dilute backwash (NaOH). The test dose of flocculant is added via syringe to simulated feeding of the wash tank and mixed with 5 strokes using a perforated plunger. The rate of liquid / solid interface settling (meters / hour) is measured in units to determine the applicability of the flocculant in the test section.

The following is a non-limiting example of the process of the invention and methods for evaluating the process of the invention. Unless otherwise indicated, the percentages are by weight.

Example 1

Following the above assay, a molecular weight copolymer of between 5 and 10 million molecular weight of 60% sodium acrylate and 40% acrylamide was tested. The copolymer was added to the simulated feed of a wash tank of an Australian red sludge processing process. In the initial step of the processing, the red mud was treated with a copolymer containing 95% by weight of sodium acrylate and 5% by weight of acrylamide. The dosages used and the sedimentation rates obtained at each stage are summarized in Table I below.

A) Comparative Example

The procedure of Example 1 was followed except that a copolymer containing 95% by weight of sodium acrylate and 5% by weight of acrylamide was used to flocculate the red sludge during the test phase. The results obtained are summarized in Table I below.

B) Comparative Example

The procedure of Example 1 was followed in all cases, but in addition to the flocculant of Example 1, the flocculant of Comparative Example A was also used. The results obtained are summarized in Table I below.

C) Comparative Example

All of the procedures of Example 1 were followed except that a flocculant of red sludge was used in an emulsion of 5 to 10 million molecular weight sodium acrylates. The results obtained are summarized in Table I below.

Table I

Simulated feed to nth wash tank [n] The dry matter content of red mud is fe / ii flocculant [example] Dose fe / t] sedimentation speed [M / hr] Second 70 I 36 effect- maybe A) total Example A Total A). example 18 36 11.1 24.8

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Simulated feed into nth wash tank [nj Dry matter content of red mud fg / 1] flocculant [examples Dose fe / t] deposition rates speed lm / hr] B) total example 18 4.9 (Example 1) 18 - Total A) as in the example Third 100 A) total example 13 1.0 C) total example 30 0.4 First 13 5.2 4th 100 First 13 8.3 A) total example 13 0.8 C) total example 30 0.5 5th 100 First 13 4.5 A) total example 13 1.8 C) total example 30 0.6

Example 2

Samples from the Jamaican red sludge processing process were taken from the lower wash tank outlet and diluted according to assay 5. The flocculant was added to the sample in a 0.05% w / w solution of sodium hydroxide. Sodium acrylate and acrylamide, having a molecular weight of 5 to 10 million, are used as a flocculant in Table II. The stubble copolymer was used in the proportions given in Table II. In all, the general assay procedure was followed. The results are summarized in Table III.

As is readily apparent from the above data, the use of copolymers containing sodium acrylate and acrylamide greatly increases the rate of flocculation later in the process. Although in the first stages of the study, 40% by weight of sodium II. spreadsheet

Simulated feed to nth wash tank [n] Dry matter content of red sludge fe / 1] Sodium acrylate flocculant [S ° / ol acrylamide [Wt% l Dose DAG / t] sedimentation speed [M / hr] First 38 95 5 0.9 1.9 First 38 70 30 0.9 ineffective First 38 60 40 0.9 ineffective First 38 50 50 0.9 ineffective Third 27 95 5 0.9 2.3 Third 27 70 30 0.9 9.1 Third 27 60 40 0.9 1.6 Third 27 50 50 0.9 0.9 Third 27 40 60 0.9 ineffective 6th 34 95 5 0.9 1.3 6th 34 70 30 0.9 4.3 6th 34 60 40 0.9 4.9 6th 34 50 50 0.9 2.5 6th 34 40 60 0.9 1.1 7th 34 95 5 0.45 3.1 7th 34 60 40 0.45 5.2 7th 34 50 50 0.45 22.8 7th 34 40 60 0.45 22.8

The copolymer containing acrylate and 60% by weight of acrylamide does not show a noticeable effect, and the same copolymer provides surprisingly excellent results, thus emphasizing the importance of evaluating the suitability of the individual sections and the copolymers used.

Claims (3)

1. A process in the alkaline digestion process of Bayer alumina production, in the first stage of which 0.0045 per tonne of dry sludge residue— 0.9 kg flocculant - preferably starch, homopolymers of acrylic acid or acrylates, copolymers of acrylic acid or acrylates containing at least 80 mole% acrylic acid or acrylate monomer, preferably a copolymer containing at least 90 mole% acrylamide mole and 10 or more acrylamate mole - or mixtures thereof, for flocculating red sludge, characterized in that a copolymer is used as a flocculating agent in one or more of the first stages or stages of the digestion process, wherein the sludge content is less than 100 g per 60 liters or one liter of NaOH + Na ₂ CO ₃ solution. Containing 35 to 15 mol% of acrylic acid or acrylate monomer and 65 to 25 mol% of acrylamide monomer and a molecular weight of 5 to 10 million per tonne of dry sludge 0.0045-0.45 kg / -4186395 is used. 2. A process according to claim 1, wherein the flocculant used in the first stage or stages of the digestion process comprises a copolymer comprising 50 to 70 mol% acrylic acid or acrylate monomer and 50 to 30 mol% acrylamide monomer. 3. The process of claim 1, wherein the flocculant added thereafter is added to the system during the fourth or subsequent stages of the digestion process.