GB2121394A

GB2121394A - Method of recovering zinc using a water-in-oil emulsion

Info

Publication number: GB2121394A
Application number: GB08313629A
Authority: GB
Inventors: Rolf Marr; Manfred Protsch
Original assignee: Chemiefaser Lenzing AG
Current assignee: Lenzing AG
Priority date: 1982-06-04
Filing date: 1983-05-17
Publication date: 1983-12-21
Also published as: GB2121394B; ATA217082A; DE3318109A1; DE3318109C2; AT373859B; GB8313629D0

Abstract

A method of removing zinc ions from waste water comprises contacting the water in counterflow with a water-in-oil emulsion, said oil acting as a membrane and containing an organic phase-transfer aid. Dezinced water is removed from the system, the emulsion is broken in an electrostatic field to render a zinc- rich aqueous phase and an oily phase which is recycled to form a fresh emulsion.

Description

SPECIFICATION Improvements in or relating to a method of recovering zinc The invention relates to a method of recovering zinc from industrial waste waters.

The waste waters from viscose fibre factories constitute an important portion of the industrial waste waters and are particularly characterised by a strong inorganic (metal salts) and organic (modifiers) load. In the aqueous spin baths, in particular zinc salts are contained.

To purify waste waters and to recover valuable elements, such as Zn, from waste waters, two courses have basically been followed so far, i.e.- precipitation of Zn, for instance with H2S, and subsequently either storage of the precipitated product on a waste dump or processing of the product;-extraction, for instance according to the known Valberg method.

At any rate, the precipitation is very expensive.

The direct extraction with waste waters of the type mentioned fails because of the many organic and inorganic contaminations contained therein.

It is known that ions can be exchanged between two immiscible liquids and that dissolved metal salts from an aqueous phase can be converted, by means of liquid substancespecific carriers under appropriate conditions, into an organic medium and again stripped off the same with appropriate solutions.

With the socalled liquid membrane permeation, which was first applied on a technical scale by N.N. Li (U.S. patent No. 3,779,907), the conditions are chosen such that through the "exterior" water-oil interface of a water in oil emulsion a species present in an aqueous solution is converted into the organic membrane phase (oily phase) and is simultaneously stripped at the "interior" oil-water interface.

It has now appeared that by means of liquid membrane permeation, by using appropriate substance-specific carriers, a plurality, or selectively one of the metal ions present in the waste waters can be separated in the membrane phase so that, on the one hand, the waste waters are freed from these metal ions and, on the other hand, it is possible to recover the corresponding metals or their salts in an economical manner in sufficiently pure forms.

Thus, it was proposed in German Offenlegungsschrift No. 28 29 163, by way of the example for recovering copper from an aqueous solution, to adjust the droplet size of the aqueous interior phase of the water in oil emulsion to predetermined values and to subject the emulsion separated from the exterior aqueous solution to an electrostatic field for the purpose of a coalescence of the droplets. The aqueous exterior solution to be extracted, e.g. a copper-ore leaching liquor, in this case is brought into contact with the emulsion in a mixer-settler.

Due to the difficulties pointed out in connection with the recovery of zinc from very heterogenously composed waste waters, there has been the endeavour for quite a long time to develop a continuously operatable, low-cost method with which, on the one hand, zinc can be selectively removed from the waste water and, on the other hand, the zinc can be recovered in a sufficiently pure form-primarily with a view to the freedom of calcium and of organic components. It is desirable to obtain zinc salts that are directly soluble in water.

This object is achieved according to the invention with a method of the initially defined kind in that: waste water is brought into contact with an emulsion comprised of a dispersed aqueous interior phase in an organic membrane phase immiscible with the waste water and the interior phase and containing a carrier based on liquid ion exchangers, wherein a volume ratio of the waste water to the interior aqueous phase of between 20:1 and 500:1 as well as a volume ratio of the organic membrane phase to the interior aqueous phase of between 1:1 and 4::1 are adjusted, the waste water and the emulsion are guided in counterflow in a stirred column having at least 80% free cross section between the column installations and the column inner wall, the emulsion is separated from the dezinced waste water and is broken by applying an electric field between a grid electrode and the already coalesced earthed portion of the aqueous interior phase, the voltage gradient amounting to at least 0.3 kV/cm, whereupon the coalesced aqueous interior phase rich in zinc is drawn off and the coalesced membrane phase is recycled for the preparation of the emulsion.

In columns of the described design the occurrence of osmotic processes is largely prevented.

Since waste waters occur in large amounts, the realisation of the method according to the invention in a stirred counterflow column is of a particular advantage, since in this manner a continuous process operation is ensured, with an intensive contact between the waste water and the emulsion prevailing at the same time. The dimensions of the column and the number of stirring means may be chosen according to the special conditions present in each case with a view to the desired flow rate, contact time as well as to the desired initial and end concentrations of Zn in the waste water and in the dispersed aqueous interior phase.

The coalesced aqueous interior phase enriched with zinc either can be further used directly, e.g., added to a spinbath, or the zinc can be recovered from this aqueous solution in a known way, for instance by electrolysis.

Preferably, IN to 6N mineral acid, in particular an approximately 5N sulphuric acid, is used as the aqueous interior phase. However, other components may also be contained in the aqueous interior phase. These compounds must be chosen such that they make possible a transition of the Zn ions against the concentrations gradient of the Zn ions.

If the zinc is removed from the acidic aqueous phase, also the dezinced aqueous interior phase can be recycled with the organic membrane phase for the preparation of the emulsion.

As a carrier, advantageously an organic phosphor compound, a hydroxy oxime or a longchain carboxylic acid, i.e., particularly suitably bis (2-ethylhexyl)phosphoric acid or its alkali metal or ammonium salt, is added to the membrane phase.

As the diluent or solvent, which constitutes the main portion of the membrane phase, i.e. mostly about 80%, a paraffin-kerosene mixture is preferably used. However, halogenated hydrocarbons, for instance, may also be used as diluents.

According to an embodiment of the method according to the invention, an interface-active substance is additionally present in the membrane phase in a dissolved state.

The liquid organic ion exchangers or substancespecific carrier means and the membrane composition are always chosen such that the zinc can be selectively separated and recovered under the waste water conditions present in each case.

Suitably, the mean droplet diameter of the emulsion dispersed in the waste water is adjusted to a value of above 0.8 mm. The mean droplet diameter in this case is determined according to J. Sauter, Forsch. Geb. Ing. Wesens 279 (1926).

The emulsion most suitably is pre-dispersed already when entering the column; by stirring means, suitable shearing conditions are maintained. In this manner, breaking up of the emulsion is largely prevented.

It has furthermore proved favourable if both the organic membrane phase and the emulsion have slighter densities than the waste water, so that the emulsion can be supplied to the bottom part of the column and drawn off the top of the column.

The waste water usable with the method according to the invention may contain a portion of up to 1.5 g of settable solids/l or even more and may be organically loaded. Organically loaded i.a.

means a content of-polar organic solvent, such as dimethylformamide (DMF).

By the method described it is thus possible to dezinc the frequently heavily contaminated waste waters (e.g. from the viscose fibre industry) in an economical way.

The method sequence will be illustrated in the following by way of an operational chart schematically illustrated in Fig. 1.

The zinc-containing waste water to be treated is supplied via a conduit 1 to an intermediate container 2, where a possible adjustment of the pH is carried out via conduit 3, and is then conveyed through conduit 4 closely below the column top into the permeation column 5. From the bottom of this column the waste water, which has been dezinced to a major extent, is drawn off via conduit 6. Closely above the widening column section that follows upon the bottom, the socalled "settler", the emulsion is supplied to the column through conduit 7 and is pre-dispersed by means of a distribution organ 5c.

Stirring organs fastened to a shaft Sb are denoted by 5a. The emulsion droplets ascending in counterflow to the waste water selectively take up the zinc present in the waste water, with a simultaneous enrichment or upgrading in the interior aqueous phase, and the emulsion droplets unite in the widening top of the column. The emulsion is supplied through the conduits 8 and 10 of an emulsion breaking plant 1 A partial stream may be guided in circuit through conduit 9 illustrated in broken lines and through conduit 7.

The emulsion breaking plant 11 includes a grid electrode 12 fed with a high voltage; the earthed, already coalesced portion of the dispersed interior aqueous phase of the emulsion constitutes the second pole 13. The coalesced interior phase upgraded with Zn is supplied for reuse via conduit 14.

The organic membrane phase is taken to an intermediate container 16 through conduit 15, where incurred losses are complemented via conduit 17. The membrane phase, through conduit 18, finally reaches an emulgator 19, where the desired water in oil emulsion is prepared with the interior aqueous phase from conduit 20. This aqueous interior phase is prepared in the container 21 of water or of a possibly present aqueous solution having a low zinc content from conduit 22 and complementary acid from conduit 23.

The invention will be explained in more detail by the following examples.

Example 1 Waste water containing 180 p.p.m. of zinc, 11 5 p.p.m. of calcium and having a pH of 4 as well as a settable solid portion of 1 g/l was brought into contact with an emulsion at a flow rate of 30 I/h in a stirred 50 mm counterflow column having a free cross section of more than 95% and a height of 1.4 m, the emulsion being comprised of an aqueous 30% by mass sulphuric acid as the interior phase in a solution of 2.5% by mass of Span 80 (sorbitane monooleate) and 5% by mass of bis-(2-ethylhexyl)phosphoric acid (D2 EHPA) in Shellsol T (high-boiling hydrocarbons with a high portion of isoaliphates) as the membrane phase. The flow rate of the emulsion was 1.25 or 2.5 I/h, respectively, the volume ratio of the membrane phase to the interior phase was 4 each. The emulsion, through a nozzle system, which caused an additional unification of the emulsion on account of the prevailing laminar flow, was introduced into the column and predispersed. The stirring organs available in the column make uniform the dispersion of emulsion in waste water thus formed and bring about both an increase of the dispersed phase portion and also an intimate thorough mixing of the waste water, on the other hand.

The waste water drawn off the bottom of the column then had a Zn-content of only 45 or 32 p.p.m., respectively, the calcium content remained unchanged. The pH of the largely dezinced waste water was 2.6. A repeated use of this pre-purified waste water in the permeation plant yielded a residual zinc content of less than 3 p.p.m. in the waste water treated.

The emulsion collecting in the top of the column was supplied to a continuously operating emulsion breaking plant. With a voltage gradient of 0.5 kV/cm and an electrode surface of 30 cm2, the emulsion was broken at a mean dwell time of 4 or 2 min, respectively. It broke into the membrane phase recycled to the new preparation of the emulsion and into the interior aqueous phase, whose zinc concentration amounted to 15 or 8.3 g/l, respectively. The coalesced, zinc-loaded interior phase was directly added to a viscose spin bath.

In Fig. 2 the concentration values of Zn and Ca in the waste water in dependence on the column height from the point of entry of the waste water into the counterflow column is graphically illustrated. At the point of entry, the concentration of the waste water of Zn is 180 mg/l and of Ca is 115 mg/l.

Curve a) represents the concentration course for a flow rate of emulsion of 1.25 I/h, curve b) results from an amount of 2.5 1 of emulsion/h.

By c) the concentration course for Ca is denoted. As already mentioned, the calcium content remained unchanged during the extraction.

Example 2 It was operated analogously to Example 1 , the waste water used, however, had a zinc concentration of 47 p.p.m., a calcium concentration of 35 p.p.m., a pH of 2.25 as well as a content of settable solids of 0.1 g/l.

The flow rate of the waste water was adjusted to 25 I/h.

Composition of the organic membrane phase: 7.3% by mass Span 80 7.3% by mass D2 EHPA 17.0% by mass paraffin thick dissolved in Shellsol T Flow rate: 1 I/h Composition of the interior phase: 30% by mass H2SO4 30 g of zinc/l as zinc sulphate Flow rate: 0.2 I/h The dezinced waste water has a residual Zn-content of 8 p.p.m. and a pH of 2.15. After emulsion breaking, the interior phase contained 34 g of zinc/l.

Example 3 500 ml of waste water containing 2,800 p.p.m. of zinc (II) ions as zinc sulphate were brought into contact in a stirrer vessel, with 75 ml of an emulsion comprised of an organic membrane phase (50 ml solution of 4% by mass of Span 80 and 4% by mass of D2 EHPA in paraffin thin) and an aqueous interior phase (25 ml of 5N H2SO4) in a manner that the dispersion produced by stirring had a Sauter diameter of 1 mm. The waste water was buffered and had an approximately constant pH of about 4 during the transfer of mass.

After 15 min stirring was stopped, whereupon the emulsion coalesced due to its density difference relative to the waste water. The refined product contained 3 p.p.m. of zinc.

The coalesced emulsion was transferred into the emulsion breaking arrangement, which comprised a grid electrode fed with a high voltage and was earthed where the aqueous interior phase united. At a voltage gradient of 300 Volt/cm the emulsion broke. By circulation of the emulsion layer, a more rapid sinking of the larger aqueous droplets on account of the formation of a slower flow in the arrangement was reached.

The zinc concentration of the coalesced interior phase was 50 g/l.

In Fig. 3 the increase of the Zn-concentration in the interior aqueous phase (curve a) and the decrease of the Zn-concentration in the waste water (curve b) in dependence on the contact time t of the dispersed emulsion in the waste water is graphically illustrated.

Claims

1. A method of recovering zinc from industrial waste waters, characterised by the combination of the following measures: waste water is brought into contact with an emulsion comprised of a dispersed aqueous interior phase in an organic membrane phase immiscible with the waste water and the interior phase and containing a carrier based on liquid ion exchangers, wherein a volume ratio of the waste water to the interior aqueous phase of between 20:1 and 500:1 as well as a volume ratio of the organic membrane phase to the interior aqueous phase of between 1:1 and 4: 1 are adjusted, the waste water and the emulsion are guided in counterflow in a stirred column having at least 80% free cross section between the column installations and the column inner wall, the emulsion is separated from the dezinced waste water and is broken by applying an electric field between a grid electrode and the already coalesced earthed portion of the aqueous interior phase, the voltage gradient amounting to at least 0.3 kV/cm, whereupon the coalesced aqueous interior phase rich in zinc is drawn off and the coalesced membrane phase is recycled for the preparation of the emulsion.

2. A method according to claim 1, characterised in that a 1 N to 6N mineral acid is used as the aqueous interior phase.

3. A method according to claims 1 and 2, characterised in that an organic phosphor compound, a hydroxy oxime or a long-chain carboxylic acid is added as a carrier to the membrane phase.

4. A method according to claim 3, characterised in that bis-(2-ethylhexyl)phosphoric acid or its alkali metal or ammonium salt is added.

5. A method according to claims 1 to 4, characterised in that a paraffin-kerosene mixture is used as the diluent for the organic membrane phase.

6. A method according to claims 1 to 5, characterised in that in the membrane phase an interface-active substance is additionally contained in a dissolved state.

7. A method according to claims 1 to 6, characterised in that the mean droplet diameter of the emulsion dispersed in the waste water is adjusted to a value of above 0.8 mm.

8. A method substantially as hereinbefore described with reference to the accompanying examples.

9. A method substantially as hereinbefore described with reference to the accompanying drawings.