IE41496B1 - Process for reducing the titanium and aluminium content of aqueous wate liquid - Google Patents

Description

This invention relates to a process for reducing the titanium and aluminium content of aqueous waste liquid, especially of waste liquid obtained from the polymerisation of olefins by, for example, a Zeigler process.

The process proposed by Ziegler allows for polymerising ethylene and a 1-olefin in an inert solvent or dispersant, for example, a hydrocarbon, in the presence of a titanium halide and of an aluminium alkyl compound as catalyst under a low pressure and at a relatively low temperature to yield polyolefins. This process results in polymers in finely-dispersed form which have to be submitted to further treatment for the elimination of dispersants and catalysts. Such further treatment consists of the elimination of the catalyst residue attached to, or incorporated in, the polymer preferably firstly by mixing the polymer suspension with an alcohol, the catalyst components being 41486 converted into water-soluble titanium and aluminium compounds, and subsequently carrying out a thorough extraction with water, diluted acids or bases.

The resulting dispersant-free extraction water, hereinafter referred to as aqueous waste liquid, which occurs upon further treatment of polyolefins, is thus polluted by alcohols, titanium and aluminium compounds and inorganic acids or bases. The alcohol may be separated from these contaminants by, for example, steam jet stripping and may either be recovered or eliminated by biological degradation. The other contaminating components remain in the waste water; the result is a high degree of pollution of water courses by a solution of catalyst metal salts.

The waste liquids of a polyolefin production plant are generally first conveyed through a piping system into a waste liquid collecting tank, from where they are transferred to further waste liquid treatment installations, for example, to a distillation plant for recovering alcohol. A serious disadvantage is the fact that in biological sewage purification plants and/or other types of treatment plants a pH of the waste liquid of less than 10 causes titanium and aluminium alike, which are dissolved in the waste liquid, gradually to build up linings inside the pipe walls. For this reason the flow of waste liquid decreases with time. Finally, the whole piping system (including pumps, regulating devices, waste liquid collecting tanks of the adjacent purification steps, for example, a distillation column for recovering alcohol) has to be cleared from these deposits by means of costly and complicated operations, - 3 namely either by mechanical means or by boiling out with alkaline lyes. Since no waste liquid must be produced during the cleaning operation, the polymerisation process has to be reduced or shut down altogether, depending on the capacity of the biological purification plant, unless a supplementary purification plant is available. These operational steps mean a considerable decrease in production.

It is known that aluminium and titanium salts may be precipitated as hydroxides in an aqueous medium by addition of alkaline substances, the degree of completeness of the precipitation depending on the pH. Aluminium hydroxide, for example, precipitates at a pH varying from 5 to 9,- a pH higher than 9 brings about the dissolution of the hydroxide in the presence of alkaline substances due to the formation of aluminates. Titanium compounds may precipitate as hydroxides at a pH varying from 4.5 to 7.5.

In contrast to aluminium hydroxide, titanium hydroxides do not dissplve in alkali and so do not form a titanate. ι However, titanium hydroxide ocpurs most frequently as a colloid which settles only hesitantly.

An object of the present invention is to provide a process for further treatment of the waste liquid resulting from the Ziegler polymerisation in such a way that, for protection of the environment, the contamination of water courses with titanium and aluminium salts is kept as low as possible and that possibly an undesirably clogging of the pipes through which the waste liquid flows is avoided. Another object is to provide a process which can be operated continuously.

The invention provides a process for reducing the titanium and aluminium content of aqueous waste liquid from tho polymerisation of olefins, which comprises adjusling 41486 the pH of the waste liquid, after having separated the polymer suspension, to a value in the range of from 4.5 to 9.5, preferably 5 to 9, and removing the resulting precipitate which comprises a titanium and aluminium hydroxides.

It will be appreciated that the process of the invention may be applied to waste liquid obtained from processes other than the polymerisation of olefins, and that the pH values are as measured at the operating temperature.

The precipitation tendency and the settling speed of the hydroxides may be improved by addition of a suitable flocculating- agent.

The process of the invention may be carried out as follows: The waste liquid obtained after extraction of the polymer suspension may contain from 0.5 to 3 weight % of alcohols in addition to the titanium and aluminium compounds present and possibly hydrogen halide acid, (for example, hydrochloric acid) or inorganic bases, (for example, sodium hydroxide solution), and alkali halide (for example, NaCl). In a vessel equipped with an agitator this waste liquid is adjusted to a pH of from 4.5 to 9.5 by adding acid or base either prior to or after separation of the alcohols, at a temperature of from 40 to 80°C, preferably from 50 to 70°C. Suitable bases are, for example, dilute aqueous solutions of alkali, or alkaline earth, metal hydroxides, ammonia or salts which produce an alkaline reaction upon hydrolysis, for example, sodium carbonate. Suitable acids are, for example, hydrochloric acid or sulphuric acid or salts which produce an acid upon hydrolysis. On so adjusting the pH, the dissolved titanium and aluminium compounds may precipitate as hydroxides in a floc5 <1496 culation sediment which depends on the precipitation conditions, such as temperature and precipitation speed. However, it is possible that partially colloidal solutions may form. In order to avoid the formation of colloidal solutions and also to improve the settling speed or rather the sediment-forming capacity of the hydroxides, a flocculating agent is preferably added to the waste water. Especially suitable flocculating agents are those based on polyacrylamide, such as non-ionic or partially saponified polyacrylamides which are commercially available under various names. The flocculating agents may be used as aqueous solutions in concentrations varying from 0.05 to 1%, preferably from 0.08 to 0.5%, calculated on the waste liquid.

For economic reasons it is very advantageous that the hydroxides should settle at the highest possible speed, especially if the waste water purification is to be carried out by a continuously operated process, since the purifying capacity of a waste liquid treatment plant is essentially determined by the residence time of the waste liquid in a thickening device. The waste liquid may then be supplied, for example, by means of a pump or by free flow, into a thickening device in which the hydroxides settle out and from where they are withdrawn from the lower part either discontinuously or continuously. The tank of the thickening device is not heated. Several forms of thickening device may be employed. A preferred form is as illustrated in the accompanying drawing and comprises a vessel equipped with a feeder shaft and two weirs. Having adjusted the pH of the waste liquid to a value in the range of from 4.5 to 9.5 and optionally having added flocculation agents, the liquid is fed in al the inlet. (1) through the feeder shaft (2) into the treatment vessel. In order to ensure that the hydroxides settle completely, the treatment 41498 zone itself should be as free as possible from currents and turbulences. This requirement can be satisfied to a great extent by directing the waste liquid first under a weir (3) and causing it to flow into the treatment zone between the two weirs (3) and (4). In the lastmentioned zone, the hydroxides settle on the bottom of the tank and the treated supernatant liquid flows out over the weir (4) at the outlet (5). The deposited hydroxides are withdrawn from time to time at the outlet (6).

Another possible suitable form of thickening device is a cylindrical tank with a conical lower part equipped 1 with a rabbling device. Waste liquid adjusted to a pH of from 4.5 to 9.5 and an aqueous solution of flocculating agents are metered into the upper part of the thickening device simultaneously and continuously. The hydroxides which settle are collected in the conical part of the thickening device by means of the rabbling installation.

The thickened mud is fed into a decantation apparatus by a pump, and further quantities of flocculating agent are added for improving the filtration properties. The filtration water is combined with the treated waste liquid, the mud is rejected and possibly used as starting material for recovering titanium and aluminium.

A special advantage of the process according to the invention consists thereof in precipitating the titanium and aluminium salts as sparingly soluble hydroxides at a pH of from 4.5 to 9.5, optionally while adding flocculation agents, so that the precipitated hydroxides may be separated from the waste liquid by sedimentation. In such a way a waste liquid is obtained which has a much lower metal salt content, a fact which is especially significant for environmental protection. The precipitation may be carried out, in principle, prior to or after special waste liquid purifying plants, for example, a waste liquid distillation for separating alcohol and/or a biological purification plant.

A further advantage of the process according to the invention is that, due to the elimination of titanium and aluminium salts from the waste liquid, the formation of linings in piping, the waste liquid tank and the distillation column for recovering alcohol, which normally occurs at a low pH in a polyolefin producing plant, is prevented or suppressed to a large extent. Consequently, a cleaning operation of the waste liquid treatment plant, which would otherwise be necessary at regular intervals, can be dispensed with. This is especially advantageous when the polyolefin production process and the waste liquid treatment operation are coupled, because the hitherto required cleaning operations meant a reduction in, or interrup tion of, the polymerisation process during the cleaning process.

The following examples, which refer to the accompanying drawing, illustrate the invention.

EXAMPLES The purification tests are carried out with waste liquid provided by a continuously operated polypropylene production plant and having the following composition: Solid matter contents about 1 g/Iitre or about (catalyst residues) 3¾ by volume pH from 3-4 or from 11-12 density of the solid matter about 3 g/cm - 8 41496 alcohol content from 1-2 weight % titanium content from 200-300 ppm. aluminium content from 300-400 ppm.

EXAMPLE 1 Waste liquid was stored in a 250 litre vessel equipped with agitator and steam coil and, by means of steam heating, was maintained at a temperature of from 60 to 65°C corresponding to the operating conditions.

By the addition of diluted sodium hydroxide solution or diluted hydrochloric acid a pH of from 4.5 to 9.5 was established, while stirring continuously so as to neutralise concentration fluctuations and so present a premature precipitation of the hydroxides. A vessel as shown in the accompanying drawing was used as a treatment tank. The tank was made of transparent plastics material, had a volume of 74 litres and measured 59 X 50 X 25 cms. The purifying zone itself between the two weirs had a volume of 54 litres. The vessel was not heated.

The warm waste water was then pumped through (1) into the feeder shaft (2) by means of an adjustable rotating piston pump and filled the entire treatment vessel up to the overflow at the weir (4). The average residence time of the waste liquid in the treatment vessel was adjustable and depended on the output of the rotating piston pump.

A throughput of 54 litres per hour, equivalent to one hour of residence time, proved to be satisfactory. At the end of a test lasting for about half an hour to one hour the titanium and aluminium hydroxides settled as mud at the bottom of the treatment vessel, and clear supernatant water flowed over the weir (4). The temperature of the waste water decreased to about 50°C during the test.

The purifying effect Was measured by a test specimen selected prior to and behind the purifying basin and was subjected to analysis in respect of titanium and aluminium Selected results are specified in Table 1.

Under the specified conditions the titanium contents may decrease from about 225 ppm to about 20 ppm, corresponding to an elimination of the titanium at the rate of about 91%. The elimination rate of aluminium from the waste water depends upon the pH. At a pH of 11.5 practically the entire aluminium remains as an aluminate solution, whilst at pH 6 and 9 the aluminium compounds precipitate, too, so that the aluminium contents at pH 6 decreases accordingly from 360 ppm to 14 ppm corresponding to an elimination of aluminium at a rate of 96%.

The mud which settled at the bottom of the treatment vessel was withdrawn at the outlet (6) either discontinuously or continuously.

EXAMPLE 2 Waste liquid which was adjusted to a pH of 9 by means of diluted sodium hydroxide solution or hydrochloric acid was fed through a pump into an unheated thickening device equipped with a rabbling installation and having a 50 litre volume. So as to speed up the settling of the titanium and aluminium hydroxides a flocculation agent based on polyacrylamide was added at a concentration rate of 0.1 weight %. This addition of a flocculation agent may enhance the settling speed to tenfold the initial value - such as Table 2 shows.

Table 3 specifies that waste water containing titanium at the rate of from 200-300 ppm and aluminium from 41436 300 to 400 ppm prior to purification had contents of merely 2 ppm of titanium and 20 ppm of aluminium after purification by means of a flocculation agent.

The mud which had been separated in the thickening device was then fed into a decantation device for concentration, whereby a further quantity of flocculation agent was added (about 15 g/m of waste liquid) for improving the filtration properties. After centrifugation the matter contained from 10 to 13% of drier; the centrifugation discharge contained about 25 ppm of solid matter. 11496 TABLE t—I P G> TJ •P GJ (0 •p s ip •P G) P -P P cn Ch nJ s T5 GJ •P 4-i P •H GJ P -P P nj CL s Ό GJ β •P nJ ω nJ >d S£ GJ •P GJ Ή •P •H tp P •P P P ft 0 J cu β 1 o β c 0 β m 0 (0 jj c GJ -P Ci υ g •P ci •P g P i-I nJ τί β nJ g •P •P •P in CM o co o *4* ΓCM o o CM o o CM ω ρ ffi+J ¢) ft(O-P re nJ £ £ GJ W G) Ο P tn fi x: nJ G) Ρ Ό GJ v4 GJ > W g < OJ -P Ρ -P m •P «η •P •P s β p 0 P U) Λ •H -P \ P GJ H nJ i—1 *·—' Ui β B H ϋ o of flocculation agents on

Claims (23)

1. CLAIMS:1. A process for reducing the titanium and aluminium content of aqueous waste liquid, which comprises establish ing a pH in the range of from 4.5 to 9.5 in the waste > liquid and separating the resulting titanium-aluminiumcontaining deposit.

2. A process as claimed in claim 1, in which the said pH range is from 5 to 9.

3. A process as claimed in claim 1 or claim 2, in which a flocculating agent is incorporated with the waste liquid to assist formation of the said deposit.

4. A process as claimed in claim 3, in which the flocculating agent is one based on polyacrylamide.

5. A process as claimed in claim 4, in which the flocculating agent comprises.a non-ionic, or partiallysaponified, polyacrylamide.

6. A process as claimed in any one of claims 3 to 4, in which there is incorporated with the waste liquid from 0.05 to 1% of flocculating agent calculated on the quantity of waste liquid.

7. A process as claimed in claim 5, in which the quantity of flocculating agent is in the range of from 0.08 to 0.5% calculated on the quantity of waste liquid.

8. A process as claimed in any one of claims 1 to 7, in which a pH in the said range is established by incorpor14 ating a base with the said waste liquid.

9. A process as claimed in claim 8, in which the said base comprises one or more of the following: an alkali metal hydroxide, an alkaline earth metal hydroxide, 5 ammonia or a salt which provides an alkali upon hydrolysis.

10. A process as claimed in claim 9, in which the said salt comprises sodium carbonate. r

11. A process as claimed in any one of claims 1 to 7, in which a pH in the said range is established by incorpor10 ating an acid with the said waste liquid.

12. A process as claimed in claim 11, in which the said acid comprises one or more of the following: hydrochloric acid, sulphuric acid or a salt which produces an acid upon hydrolysis. 15

13. A process as claimed in any one of claims 1 to 12, in which the step of establishing a pH in the said range is effected at a temperature in the range of from 40 to 80°C.

14. A process as claimed in claim 13, in which the 20 said temperature range is from 50 to 70°C.

15. A process as claimed in. any one of claims 1 to 14, in which the step of establishing the said pH is effected continuously in a continuous flow of waste liquid.

16. Λ process as claimed in claim 15, in which the said resulting hydroxides are separated continuously from the treated waste liquid.

17. A process as claimed in claim 15, in which the '> said resulting hydroxides are separated discontinuously from the treated waste liquid.

18. A process as claimed in any one of claims 1 to 17, in which waste liquid which has been established at a pH in the range of from 4.5 to 9.5 and optionally incorporated 10 with a flocculating agent, is supplied to a first zone of a vessel and flows under a barrier into a second zone of the vessel, the titanium and aluminium deposit settling out in the second zone, the supernatant liquid flowing from the second zone over a weir, and the said deposit being removed 15 through an outlet in the second zone.

19. A process as claimed in any one of claims 1 to 18, in which alcohols present in the waste liquid are removed before or after establishing the said pH.

20. A process as claimed in any one of claims 1 to 19, 20 in which the said waste liquid is that obtained from polymerisation of olefins.

21. A process as claimed in claim 20, in which the said polymerisation is a Ziegler polymerisation.

22. A process for reducing I Ik- titanium and aLuminiuiii 25 content of aqueous waste liquid, carried out substantially 41486 as hereinbefore described with reference to the accompanying drawing.

23. A process as claimed in claim 1, carried out substantially as described in Example 1, or Example 2, 5 herein.