FR2487325A1 - PROCESS FOR FLOCCULATION OF RED SLUDGE IN THE PREPARATION OF ALUMINA - Google Patents

Abstract

THE OBJECT OF THE INVENTION IS A PROCESS ALLOWING TO EFFICIENTLY ELIMINATE AQUEOUS SUSPENSIONS OF RED SLUDGE FROM THE BAYER PROCESS FOR THE PREPARATION OF ALUMINA CONSISTING OF ADDING IN AT LEAST THE FIRST STAGE OF THE RECOVERY CIRCUIT A SELECTED FLOCCULANT, AMONG HOMOPOLYMERS OF ACRYLIC ACID OR ACRYLATES, COPOLYMERS OF ACRYLIC ACID OR ACRYLATES CONTAINING AT LEAST 80 MOLES OF ACRYLIC ACID OR OF MONOMERIC ACRYLATES AND THEIR MIXTURES, AND THEN ADDED IN LATER STAGES, MORE, RECOVERY CIRCUIT, A COPOLYMER CONTAINING ABOUT 35 TO 75 MOLES OF ACRYLIC ACID OR ACRYLATE AND ABOUT 65 TO 25 MOLES OF ACRYLAMIDE.

Description

The present invention relates to a process for the preparation of alumina making

  call to the Bayer process. The Baver process is the practically universally used process

  for the preparation of alumina. In a very general way

  line, this process is carried out almost exclusively

  in aqueous solution, and is carried out by reaction of bau-

  xite and a strong base like caustic soda or lime in autoclaves heated with steam, which has the effect of transforming the alumina into a soluble aluminate. A considerable quantity of insoluble impurities results from this operation or is released by bauxite, these impurities having to be separated from the alumina constituent sought. These residues, commonly called red mud, include iron oxides, sodium aluminosilicate, titanium oxide and other substances. Generally, this sludge is in the form of

  very fine particles that are difficult to separate. However

  less, the red mud, which usually constitutes about 10 to 50% of the weight of the ore, must quickly

  and clearly be separated from the solu- alumina liquor

  mobilized so that this particular operation is economical

  mically effective. If the separation speed is too high

  the flow decreases significantly and the efficiency

  The whole process is affected. Likewise, if the separation is not clear, the resulting alumina, in the form of aluminate, is somewhat crude and undesirable.

  for a number of end uses. The impure-

  insoluble tees, present in alumina as a residue

  of the preparation process, tend to add materials

  foreign non-active materials in special environments, such as water, treated with aluminate for various purposes. By

  example, low quality sodium aluminate containing

  relatively large proportions of impurities

  muddy, when used to treat water, pro-

  there is an increased tendency to form muddy masses

  its resulting directly from the impurities present, which

  the masses tend to clog the alarm systems. In addition, if the crude sodium aluminate contains significant amounts of impurities in the form of a mixture, the problems of solution are very difficult to solve if the aluminate is introduced in the form of a solid product.

  A method of overcoming pre-

  cited and effectively accelerate the separation of red mud from alumina, as well as achieve a more complete separation of the constituents is described in the patent

  13 of the U.S.A. n 3.390.959 in which it is said that the utility

  addition of acrylic acid hormo- or copolymers and

  acrylates containing not more than 20% of other monomers

  ethylenically unsaturated polymerizable polar res as flocculants of red mud improves efficiency

overview of the Bayer process.

  The aforementioned patent describes the use of copolymers

  acrylic acid or acrylates with unsaturated monomers

  ethylenic turation as flocculants of red mud; however, it also indicates that when the proportion of ethylenically unsaturated monomers is greater than moles% the separation rate drops dramatically and that at a molar proportion greater than 20% no significant separation is obtained. Although these

  indications are correct regarding the initial stage

  tial or "head" of the recovery circuit, we discovered

  surprisingly green that they do not apply to results obtained in later, more diluted stages of

recovery circuit.

  The present invention relates to a new process for recovering alumina by means of the Bayer process according to which the red mud is separated, by flocculation of the recovery circuit by introducing at least

  first stage of the recovery circuit an effective quantity

  of a flocculant chosen from starch, homopolymers

  res of acrylic acid or acrylates, the copolymers of a-

  acrylic acid or acrylates containing at least 80 mole%

  of acrylic acid or of acrylates monomers and their mixtures

  ges, then adding, at one or more subsequent stages, an effective amount of a copolymer comprising approximately

  75 mole% acrylic acid or acrylate and approx.

  ron 65 to 25 mole% acrylamide. The use of a second, separate polymer in the later stages of the recovery circuit in which the system is more diluted and the conditions are less severe has surprisingly been found to achieve efficient flocculation of the red mud. The present invention relates to a new process

  flocculation of red mud produced as a sub-

  product of the Bayer process for recovering alumina from bauxite, this process consisting in using a conventional flocculant of red mud in at least the

  first stage or "head" stage of the recovery circuit

  tion and, at a later stage, to use a copolymer of approximately 35 to 75 mol% of acrylic acid or of acrylate and of approximately 65 to 25 mol of acrylamide as flocculant

red mud.

  The classic flocculant to use at least first

  stage includes, for example: starch, homopoly-

  mothers of acrylic acid or acrylate; copolymers of acrylic acid or of acrylate in which the copolymer

  contains at least 80 mole% acrylic acid or acrylic

  latesmonomers; hydrolyzed acrylamide monomers or polymers, and mixtures thereof. By "acrylate" is intended to denote the salt of an acrylic acid, this salt being a salt

  of alkali metal or ammonium. When using co-

  polymers of acrylic acid or acrylate, one can use

  a wide variety of comonomers in proportions suitable for

  before reaching 20 mole%. As representative examples of comonomers, mention will be made of: acrylamide; methacrylamide; acrylonitrile; lower alkyl esters of acrylic and methacrylic acids; vinyl methyl ether; acidmethacryl] salts maleic anhydride and its salts; isopropenyl acetate; itaconic acid; acetate

  vinyl; alpha-methyl styrene; styrene; acid fuma-

  risk; aconitic acid; citraconic acid; friends-

  any of the above acids; derivatives of alkali metals (e.g. sodium, potassium and

  lithium) as well as ammonium salts of any

  which of the above or other monomeric acids; alcoyl

  ester amides and partial salts of various polycarbon acids

  xylics; vinyl toluene; chlorostyrene; chloride

  vinyl, vinyl formate; ethylene; propylè-

  born; isobutylene, etc. Among the comonomers which have just been mentioned, those containing a hydrophilic group in a side chain of the hydrocarbon group are very preferable.

  ethylenically unsaturated. The monomers which do not contain

  do not have such a hydrophilic solubilizing group should

  be used in smaller quantities, representing for example-

  ple of about 1 to 5% of the total weight of monomers present.

  As other monomeric substances usable in association with the constituent acrylic acid or salt

  acrylic acid, substances such as acryla-

  sulfoethyl te, carboxyethyl acrylate, diethyl vinyl phosphonate, crotonic acid or its salts, vinyl sulfonate or its salts, vinyl alcohol as well

  vinyl aryl hydrocarbons containing groups

  pes solubilizers such as sulfonates, etc.

  To be particularly usable, homopoly-

  mothers or copolymers of the type described above should preferably have a molecular mass greater than 50,000 and better still greater than 100,000. Excellent polymers

  usable as additives have molecular weights for

to reach 10 million.

  The starches which can be used according to the invention are, for example: potato starch, corn starch, tapioca, amylose, sorghum starch and other starches

easily accessible.

  The effective quantity of the conventional flocculant used at least at the first stage of the recovery circuit varies according to the particular composition based on bauxite which

  is treated, the conditions present at the recovery stage

  ration, i.e. pH, temperature, concentration

  tion in solids, etc. and depending on the nature of the red mud flocculant (s) used. However, generally speaking, when using starch or combinations containing starch, the effective amount

  varies between 0.05 and 2.0% of the weight of the dry sludge residue

  they. When using synthetic polymers or copolymers, the effective amount generally represents from 0.0045 to 0.908 kg of chemical per tonne of sludge

residual dry.

  The copolymer added during some of the later stages

  tières of the recovery circuit is specifically a copolymer comprising from about 35 to 75 mole% of acrylic acid or acrylate ("acrylate" having the meaning

  defined above) and about 65 to 25 mole% of acryl-

  amide. Although other unsaturated monomers are believed to be

  ethylenic ration that acrylamide can be effective

  cement used in this copolymer, for

  In terms of economy, accessibility and performance, it is acrylamide which is the preferable comonomer to use in the present invention. These subsequently added copolymers must have a molecular mass such as the Brookfield viscosity of a 0.15% solution of the polymer in 1M NaCl at pH 8, UL adapter at 60 rpm, that is to say at least 2 centipoise and, preferably at least

3.0 centipoise.

  The effective amount of these copolymers added subsequently

  laughter also varies according to the particular nature

  of the bauxite composition treated and the conditions

  in which recovery is performed. Any-

  times, generally speaking, the effective amount of copoly-

  mother is 0.0045 to 0.454 kg per tonne of dry red mud (solids) and preferably about

  0.0226 to 0.226 kg per tonne of dry red mud (material

solid).

  The particular stage, following the initial or "top" stage of the recovery circuit, o the copolymer comprising about 35 - 75 moles% of acrylic acid or acrylate and about 65 to 25 moles % acrylamide will be effective depends on a number of variables in the

  recovery system. It is believed that the operating conditions

  rakes, including temperature, pH, dilution of

  liquor, and concentration of red mud in ma-

  solid bodies play an important role. The type of mine-

  bauxite spoke itself can also contribute to the position of the effective point. As the exact mechanism of i0 the interaction between the flocculant and the solids

  red mud has not yet been elucidated- the predictability

  the stage or the operative variables controlling the effect

  effectiveness of the copolymer is beyond the scope of a simple categorization. In general, it has been observed that the copolymer is very effective at the fourth stage or at the later stages or the number of grams of muddy residue

  per liter of NaOH plus Na2CO3 is less than about 100.

  As this value is extremely approximate, and can be based more on coincidence than on the operating variable

  decisive, it is recommended to perform the following test

  to determine which stage in a recovery circuit

  given peration, is the effective point of introduction of the copolymer. This test consists in operating as follows: General test protocol The stage of the recovery circuit to be tested, which

  stage is not the "head" stage, is called "nth stage"

  of "in the description of this test. The underflow

  of the stage washer (n-l) is diluted to 1/4 with the aid of

  full of the stage (n) washer to produce a simulated load for the stage (n) washer. This high level of dilution is

  necessary to obtain a level of free sedimentation re-

  can be produced in a test tube, preferably a test

graduated vette from 500 to 1000 ml.

  At this simulated load for the stage washer (n) we

  add the flocculant to be tested, in 0.05% by weight solution.

  It can be an aqueous solution or diluted spent liquor (NaOH). The dose of tested flQculant is introduced using a syringe and mixed with the simulated load

  introduced into the washer using five strokes of a spray

  your perforated plunger. The descent of the liquid / solid interface is measured in meters / hour, so as to determine the usefulness of the flocculant vis-à-vis the operating stage tested. The following nonlimiting examples are given by way of illustration of the present invention. Unless otherwise indicated, the parts and percentages are all expressed

in weight.

EXAMPLE 1

  By operating as described in the general test protocol above, a red mud flocculation copolymer, containing 60% by weight of sodium acrylate and 40% by weight

  acrylamide and having a molecular weight of 5 to 10 mil-

  lions, is added to the simulated load for washer of a cir-

  cooked for Australian red mud in which the

  at the initial stage was carried out using a copoly-

  mother containing 95% by weight of sodium acrylate and 5% by weight of acrylamide. The doses used and the sedimentation rates obtained at each stage are indicated in

Table I below.

COMPARATIVE EXAMPLE A

  We operate as described in Example 1, but using

  as a flocculant of red mud, at the test stage, a co-

  polymer containing 95% by weight of sodium acrylate and 5%

  by weight of acrylamide. The results obtained are reported

ties in Table I below.

COMPARATIVE EXAMPLE B

  The procedure is as described in Example 1, but using the flocculant from Comparative Example A + the flocculant from Example 1 as the red mud flocculant. The results

  obtained are reported in Table I below.

COMPARATIVE EXAMPLE C

  The operation is carried out as described in Example 1, but using a sodium acrylate emulsion as red flocculant

  having a molecular weight of 5 to 10 million. The results

  Test states are reported in Table I below.

  Simulated washing load: Red mud stage Solids content

TABLE I

  Flocculant Dose (Example) (g / ton) Sedimentation rate (m / hour) g / liter g / liter g / liter g / liter Comp.A Comp.A Comp.B Comp.A Comp. C Comp. A Comp. C Comp.A Comp. C 18 of 1) 18 of A) No effect 11.13 24.87 4., 97, 27 1.07 0.40 8.29 0.85 0.49 4.54 1.83 0.67 2 3rd 3rd 4th Co 1- (A. nfi

EXAMPLE 2

  We take a sample from a processing circuit

  of Jamaican red mud from the underflow of the sta-

  of washing and diluted as described in the general test protocol. A flocculant is added to it in a 0.05% by weight solution of NaOH. The flocculant tested is a copolymer of sodium acrylate and acrylamide in the proportions indicated in Table II, having a molecular mass of 5 to 10 million. By operating exactly as described in the general test protocol, we obtain the

  results reported in Table II below.

  It follows from the above that the use of copolymers of sodium acrylate and acrylamide dramatically increases flocculation at the final stages of the recovery circuit. Although a copolymer containing 40% by weight of sodium acrylate and 60% of acrylamide had no appreciable effect at the stages of the start of the test, the same copolymer produces excellent overwhelming results.

  at a later stage, thus highlighting the importance of

  tance of suitability assessment of separate stages

vis-à-vis the copolymer used.

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Claims (8)

  1. Flocculation process of red mud from an alumina recovery circuit by the process   Bayer, characterized in that we introduce into at least the first   1st stage of the recovery circuit an effective quantity   of a flocculant chosen from starch, homopoly-   mothers of acrylic acid or acrylates, the copolymers of acrylic acid or acrylates containing at least 80 mole% of acrylic acid or acrylate monomer and their mixtures, then to be introduced in one or more stage (s)   subsequent (s) of the recovery circuit an effective quantity   of a copolymer containing from about 35 to 75 mol% of acrylic acid or of acrylate monomer and from about 65   25 mole% of acrylamide monomer.   2. Method according to claim 1, characterized in that the flocculant introduced into at least the first stage of the recovery circuit is a copolymer of acrylic acid or acrylate containing at least 90 mole% of acrylic acid or acrylate monomer and no more than 10 mole% of acrylamide monomer.   3. Method according to claim 1, characterized in that the flocculant introduced in one or more subsequent stage (s) of the recovery circuit is a copolymer   containing 50 to 70 mole% acrylic acid or acrylic   late monomer and from 50 to 30 mol% of acrylamide monomer 4. Method according to claim 1, characterized in that the flocculant introduced into at least the first stage of the recovery circuit is a copolymer of acrylic acid or acrylate containing at least 90 mole% of acrylic acid or acrylate monomers and not more than mole% of acrylamide monomer, and the flocculant introduced into one or more subsequent stage (s) of the recovery circuit is a copolymer containing from 50 to 70 mole% of acrylic acid or acrylate monomers and from 50 to 30 moles% of acrylamide monomer.   5. Method according to claim 1, characterized in that the effective amount of the first flocculant added   is 4.5 to 908 g loc; iant per tonne of sludge resi-   dry dual and the effective amount of flocculant added later is 4.5 to 454 g of flocculant per tonne dry residual sludge.   6. Method according to claim 1, characterized in that the flocculant added subsequently is added in one or more stages o the weight of residual sludge   per liter of NaOH plus Na2CO3 is less than 100 g.   7. Method according to claim 1, characterized in that the flocculant added subsequently is added in   the fourth stage or in later stages of the cir- cooked recovery.   8. Method according to claim 1, characterized in   what the flocculant added later is a copolymer-   re containing 70 mole% sodium acrylate and 30 mole%   of acrylamide having a molecular weight of 5 to 10 million.