Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Group iiia metal or beryllium
Utility Patent
1999-01-13
2001-01-02
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Group iiia metal or beryllium
C423S121000, C423S127000
Utility Patent
active
06168767
ABSTRACT:
This invention relates to the recovery of alumina from pregnant Bayer process liquors by crystallisation.
In the Bayer process, bauxite is digested with aqueous caustic to form a caustic liquor containing sodium aluminate and red mud, the red mud is separated by settling and filtration to produce the pregnant liquor, alumina trihydrate is crystallised from the pregnant liquor, and the supernatant liquor is recycled.
It is known that the yield and properties of the crystals can be significantly affected by deliberate process conditions such as the temperature profile in the process and the amount and nature of crystal seed that is added to the process, and by impurities, especially organic salts such as sodium oxalate and sodium humate.
Although it is naturally desirable to obtain a high yield of crystals, it is undesirable if the yield is provided mainly by very fine (below 45 &mgr;m) crystals. However the presence of some fine crystals can be desirable for seeding purposes.
Optimum performance requires optimisation of the yield and the average particle size, and the latter may be affected also by the strength of the crystals, i.e., the resistance to attrition. Merely increasing the yield while obtaining reduced crystal size is unsatisfactory, as is obtaining increased crystal size but reduced yield.
It is well known to add a crystallisation modifier to the pregnant liquor in order to impose a deliberate modification on the crystallisation characteristics. The modification that is imposed usually involves a reduction in the proportion of fines, i.e., an increase in the average particle size.
JP-A-54/158398 describes classification of aluminium hydroxide particle suspensions with anionic surfactants such as sodium alkyl benzene sulphonate, sodium alkyl naphthalene sulphonate and sodium dinaphthylmethane sulphonate. These surfactants are generally not good for controlling foam and are limited by their molecular weight and consequent adsorption capability.
Other systems have also been described which incorporate mineral or silicone oils. For instance, in Chemical Abstracts 108:135508 and Hungarian Patent No. 203,852B, a complex mixture of surfactants, silicone oil and hydrocarbon oil is described for crystallisation of alumina trihydrate from pregnant Bayer process liquor. The exemplified mixture in the patent (but not the Chemical Abstracts reference) includes a nonyl phenyl polyglycol ether ester surfactant, the hydrocarbon decahydronaphthalene and the silicone oil dimethyl siloxane. The exemplified mixture contains approximately 39 wt % mineral oil and silicone oil and the minimum total amount of these materials indicated in the patent is 25 wt %. U.S. Pat. No. 4,737,352 also describes a mixture of oil and anionic surfactant for the same purpose. The anionic surfactant is a tall oil fatty acid and the oil can be paraffinic oil, naphthenic oil, mineral seal oil, fuel oil or residue from a C
10
alcohol distillation. EP-A-631,985 also describes use of a blend of oils as a crystallisation modifier during the separation of alumina trihydrate from pregnant Bayer process liquor. In this publication, the crystallisation modifier is a combination of a silicone oil and a mineral oil.
Crystallisation modifier systems which contain oils of these types (silicone oils and mineral oils) have the disadvantage that if the oils contain any impurities they tend to cause discoloration of the trihydrate crystals. Avoiding this necessitates expense in using only very pure oils. Even pure oils increase the total organics load in the Bayer process circuit, which is preferred to be avoided. Those working in the Bayer process would prefer to see lower organics load in order to reduce the possibility of impurities directly added or impurities generated in the highly caustic environment which could have an adverse effect on process kinetics/cycles etc as a result of organics building up and acting as poisons.
It would be desirable to be able to provide a crystallisation modifier system which does not cause discoloration of the crystals but which in use gives good crystal size and yield.
According to a first aspect of the invention we provide a process of recovering alumina from pregnant Bayer process liquor by crystallisation after adding to the liquor a crystallisation modifier, characterised in that the crystallisation modifier comprises:
(a) a polyalkoxylated non-ionic surfactant,
(b) a surfactant, or a precursor thereof, which is not a polyalkoxylated non-ionic surfactant, and
(c) water.
This combination of materials has various advantages. In particular, it gives excellent improvement in crystal size without decreased yield results in use. Furthermore, it has been found that the crystallisation modifier can be formulated with substantially no mineral oil or silicone oil and is formulated as an aqueous composition. Thus the disadvantages discussed above of including these can be avoided.
Additionally, the composition contains water which allows the production of a stable, homogeneous composition containing the two surfactants (a) and (b). Inclusion of water further dilutes the active surfactant materials and provides easier dosing.
One essential component of the crystallisation modifier is a polyalkoxylated non-ionic surfactant. Generally it is formed from units comprising ethylene oxide (EO) units. Preferably it contains ethylene oxide and propylene oxide (PO) units, and is for instance an ethylene oxide-propylene oxide block copolymer. Suitable non-ionic surfactants of this type are available under the trade names Pluronic, Synperonic PE, Dowfax and Monolan. In preferred ethylene oxide-propylene oxide block copolymers the content of ethylene oxide units generally ranges from 10 to 60%, preferably 10 to 40%, (by weight) and the content of propylene oxide units generally ranges from 40 to 90%, preferably 60 to 90% (by weight).
Other preferred polyalkoxylated non-ionic surfactants are formed from units selected from ethylene oxide, propylene oxide, butylene oxide (BO) and functional moieties such as alcohols and amines. Preferred surfactants of this type contain blocks of ethylene oxide units and blocks of propylene oxide units.
Suitable functional alcohols include mono-, di-, tri- and tetrols, and phenols and sorbitols. Suitable examples of surfactants which include a functional alcohol unit are the Plurafac LF series of surfactants which are formed from synthetic fatty alcohol (mono-ol) plus EO/PO chain. Suitable diol functional groups are ethylene and propylene glycol. Suitable triols are based on glycerol and trimethylol propane. Some of the Ukanil and Dowfax range of surfactants are based on triols of these types. Suitable tetra-ols are based on pentaerythritol.
Other functional moieties include amines, such as ethylene diamine. Suitable examples of surfactants which include a functional amine unit are the Tetronics, based on ethylene diamine.
In non-ionic surfactants which include ethylene oxide, propylene oxide and butylene oxide monomer units, the amount of butylene oxide units is often up to 40% (by weight), for instance at least 1% (by weight).
Preferred polyalkoxylated non-ionic surfactants have molecular weight of the EO/PO (and optionally BO) chain of 600 Daltons or greater, preferably 2,000 to 5,000 Daltons.
It is preferred that the polyalkoxylated non-ionic surfactant (a) has a cloud point tested in 1% solution in water of from 10 to 100° C., preferably 20 to 60° C. If the polyalkoxylated surfactant is a mixture, the cloud point is that measured for the mixture.
The polyalkoxylated non-ionic surfactant (a) is preferably water-soluble. That is, it has a solubility at 25° C. of at least 10 g/100 ml deionised water, preferably at least 20 g/100 ml deionised water.
The crystallisation modifier also comprises (b) a surfactant of a type different from type (a) or a precursor thereof. By a precursor of such a surfactant we mean a material which forms such a surfactant material on addition to the caustic pregnant liquor.
The surfactant (b) is chosen such that it is capable of dispersing the p
McColl Philip
Welton Roger
Ciba Specialty Chemicals Water Treatments Limited
Crichton David R.
Griffin Steven P.
Ildebrando Christina
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