Removable of non-water soluble substances from solutions of aqueous metal extracts

Abstract

The invention relates to methods and a device for the removal of non-water soluble substances from solutions of aqueous metal extracts, by addition of at least one water-soluble cationic polyelectrolyte, containing a diallylaminoalkyl(meth)acrylamide, neutralised or quaternised with mineral acid and subsequently polymerised, preferably dimethylaminopropylacrylamide.

1 . A process for removing water-insoluble substances from solutions of aqueous metal extracts by addition of at least one water-soluble cationic polyelectrolyte, characterized in that the polyelectrolyte comprises at least 50 mol % of dialkylaminoalkyl(meth)acrylamide, preferably dimethylaminopropylacrylamide, which has been neutralized or quaternized by at least one mineral acid and subsequently polymerized, based on the total amount of the polyelectrolyte.  
     
     
         2 . The process as claimed in  claim 1 , characterized in that the cationic polyelectrolyte comprises at least 70 mol %, preferably at least 90 mol %, of dialkylaminoalkyl(meth)acrylamide, preferably dimethylaminopropylacrylamide, which has been neutralized or quaternized by means of mineral acid and subsequently polymerized, based on the total amount of the polyelectrolyte.  
     
     
         3 . The process as claimed in  claim 1  or  2 , characterized in that the cationic polyelectrolyte is at least one homopolymer of dialkylaminoalkyl(meth)acrylamide, preferably dimethylaminopropylacrylamide, which has been neutralized or quaternized by at least one mineral acid and subsequently polymerized.  
     
     
         4 . The process as claimed in  claim 1  or  2 , characterized in that the polyelectrolyte is at least one copolymer of acrylamide and dialkylaminoalkyl(meth)acrylamide, preferably dimethylaminopropylacrylamide, which has been neutralized or quaternized by at least one mineral acid and subsequently polymerized.  
     
     
         5 . The process as claimed in  claim 4  in which the quotient of limiting viscosity of the copolymer and the molar ratio of acrylamide to dimethylaminopropylacrylamide is at least 200 ml/g.  
     
     
         6 . The process as claimed in any of  claims 1  to  5 , characterized in that the polyelectrolyte has a degree of polymerization of at least 90%, preferably at least 95% and particularly preferably at least 98%.  
     
     
         7 . The process as claimed in any of  claims 1  to  6 , characterized in that the polyelectrolyte has, at a proportion of 40% by weight of polyelectrolyte and 60% by weight of water, based on the total mass, a viscosity of from 1000 to 14 000 mPa*s, preferably a viscosity of from 4000 to 8000 mPa*s, determined by the Brookfield method at 20° C. using a Spindle IV.  
     
     
         8 . The process as claimed in any of  claims 1  to  7 , characterized in that the aqueous metal extract is an extract for the preparation of titanium dioxide, preferably by the “sulfate” process.  
     
     
         9 . The process as claimed in any of  claims 1  to  7 , characterized in that the aqueous metal extract is an extract for the preparation of aluminum oxide, preferably by the Bayer process.  
     
     
         10 . The process as claimed in any of  claims 1  to  9 , characterized in that the polyelectrolyte is used as a liquid or as a water-in-water dispersion or as a granulated material.  
     
     
         11 . The process as claimed in any of  claims 1  to  10 , characterized in that the final concentration of the polyelectrolyte is from 1 to 500 ppm, preferably from 2 to 250 ppm and particularly preferably from 5 to 50 ppm, based on the digestion solution.  
     
     
         12 . The process as claimed in any of  claims 1  to  11 , characterized in that the digestion solution is at an elevated temperature, preferably from 80 to 110° C.  
     
     
         13 . The process as claimed in any of  claims 9  to  12 , characterized in that the Bayer process comprises at least the following steps: 
 a) a digestion of milled bauxite with hot aqueous sodium hydroxide  
 b) the coarse separation of solids by filtration, preferably by means of a sand filter,  
 c) the fine separation of solids,  
 d) the precipitation of hydroxyaluminate from the digestion solution by means of cooling,  
 and in that the addition of at least one cationic polyelectrolyte is carried out prior to and/or after the coarse separation of step b).  
 
     
     
         14 . The process as claimed in  claim 13 , characterized in that the clear mother liquor from the separation of step d) is at least partly worked up by purification and/or addition of sodium carbonate and/or concentration by evaporation and/or heating and is at least partly reused for the digestion of step a).  
     
     
         15 . The process as claimed in  claim 13  or  14 , characterized in that at least one anionic water-soluble polyelectrolyte is added at a time after the addition of the cationic polyelectrolyte in the same process step or in a subsequent process step, but before the fine separation of step c).  
     
     
         16 . The process as claimed in  claim 15 , characterized in that the anionic polyelectrolyte comprises 
 a) from 90 to 30% by weight of acrylic acid and/or methacrylic acid,    b) from 10 to 60% by weight of a compound of the formula    CH 2 ═CR 1 CONH—X—NR 2 R 3      where R 1  is hydrogen or a methyl radical, R 2  and R 3  may be identical or different and are each a methyl or ethyl radical and X is a branched or unbranched alkylene radical having from 1 to 5 carbon atoms,    c) if necessary, from 0 to 50% by weight of acrylamidomethylpropanesulfonic acid,    d) from 0 to 10% by weight of another ethylenically unsaturated compound in copolymerized form, and whose molecular weight measured at a pH of 8.0 is less than 100,000.    
     
     
         17 . The process as claimed in  claim 15  or  16 , characterized in that the anionic polyelectrolyte has a degree of polymerization of at least 90%, preferably at least 95% and particularly preferably at least 98%.  
     
     
         18 . The process as claimed in any of  claims 9  to  17 , characterized in that the polyelectrolytes, in particular the cationic polyelectrolyte, are introduced according to the proportion of humic acid in the digestion solution.  
     
     
         19 . The process as claimed in any of  claims 1  to  14 , characterized in that the cationic polyelectrolyte and/or the anionic polyelectrolyte are/is added a plurality of times in various steps of the Bayer process.  
     
     
         20 . An apparatus for carrying out the Bayer process for the preparation of aluminum oxide from bauxite, which comprises at least one digestion vessel, a solids separation and a precipitation vessel or a filtration apparatus and has an addition point for a cationic polyelectrolyte comprising at least a proportion of 50 mol % of dialkylaminoalkyl(meth)acrylamide, preferably dimethylaminopropylacrylamide, which has been neutralized or quaternized by means of mineral acid and subsequently polymerized, based on the total amount of the polyelectrolyte.