GB2150923A

GB2150923A - Dechlorination of zinc starting-materials

Info

Publication number: GB2150923A
Application number: GB8422211A
Authority: GB
Inventors: Sven Santen; Sune Eriksson
Original assignee: SKF Steel Engineering AB
Current assignee: SKF Steel Engineering AB
Priority date: 1983-12-09
Filing date: 1984-09-03
Publication date: 1985-07-10
Also published as: SE8306804L; SE8306804D0; IT1176628B; JPS60125337A; DE3431381A1; ES8506357A1; DK390284A; BE900757A; AU3283084A; GB8422211D0; DK390284D0; FR2556368A1; NO844047L; IT8422409A0; ES535856A0

Abstract

The method for dechlorinating and oxidizing secondary zinc starting-materials comprises charging the chlorine-containing zinc starting-material to a reactor (1) and heating to 750 DEG C by means of a hot gas (4) charged to the reactor, such as to vapourize the zinc chloride, whereafter the gaseous zinc chloride is withdrawn through an outlet (6) located at the top (7) of the reactor, and a dechlorinated zinc and zinc-oxide product is withdrawn from the reactor (1) at a lower level (at 5) thereof. <IMAGE>

Description

SPECIFICATION Dechlorination of zinc starting-materials The present invention relates to a method for dechlorinating and oxidizing secondary zinc startingmaterials in the form of ash, oxides or other secondary products.

It is estimated that there are between 40,000 and 60,000 tons of zinc ash in Europe alone, in addition to all other zinc-containing waste products. Zinc ash is obtained as a waste product in the manufacture of zinc and contains about 60% zinc in metal and oxide form. One serious disadvantage with regard to present day known processes, is that the zinc ash contains large quantities of chlorine in the form of volatile zinc chlorides and other chlorides.

The zinc ash is a highly valuable starting material in the manufacture of zinc, primarily for secondary industries, but only provided that the ash can be completely freed from the chlorine, since chlorine is a poisonous substance. In present day methods, the zinc ash is mainly subjected to thermal treatment in a rotary furnace, to eliminate volatile chlorine compounds. The dechlorinated product is then used to manufacture zinc oxide, for use as a pigment or in the manufacture of fillers.

The aforementioned known process, however, has limitations which affect the possibilities of recovering the valuable constituents of the zinc ash in any great yields. Rotary furnaces have a very low efficiency and are not readily made gas-tight, and are consequently unsatisfactory with respect to environmental care and protection. Processes effected in rotary furnaces also produce large quantities of gas, chiefly because the heat is generated by combustion.

Consequently, the object of the present invention is to provide a process which enables the extensive recovery of valuable constituents of chlorinecontaining starting materials, and with which the disadvantages associated with known processes are eliminated or minimized.

This is achieved by means of the method according to the invention, mainly by feeding the chlorinecontaining zinc starting-material to a reactor vessel and heating said material to about 750 C, the thermal energy required herefor being supplied by an energy-rich gas heated in a plasma generator; and by withdrawing zinc chloride at the top of the reactor and dechlorinated zinc-oxide at a lower level in the reactor. The gas rich in energy is preferably an oxidizing gas, so as to oxidize any zinc metal present to zinc oxides, and therewith prevent cladding.

According to a first embodiment ofthe invention, the chlorine-containing zinc starting-material is charged in powder form, above a gas-permeable grate, beneath which there is supplied a gas which has been heated by means of a plasma generator, the gas being supplied in such quantities and at such rates of flow as to maintain a fluidized bed above said grate, while at the same time controlling the amount of energy charged to the plasma generator in a manner to maintain a temperature of about 750"C in the upper part of the reactor, a dechlorinated product comprising zinc oxide being withdrawn via a spillway and gaseous zinc chloride being withdrawn through a gas outlet in the top of the reactor.

According to another embodiment of the invention, the chlorine-containing starting material is charged in briquette form or in lump form through a gas-tight charging inlet at the top of the reactor vessel, and dechlorinated zinc-oxide product is withdrawn from the reactor at the bottom thereof, through a gas-tight outfeed means, the expulsion of formed gaseous zinc-chloride being effected by supplying thermal energy by means of a gas heated in a plasma generator, the quantity of gas supplied being such as to maintain temperature of about 750"C at the top of the reactor.

Other advantages and characteristics of the invention will be apparent from the following detailed description of two embodiments of the invention made with reference to the accompanying drawings, in which Figure 1 is a schematic flow sheet relating to the dechlorination and oxidation of secondary zinc starting material in powder form, and Figure 2 is a schematic flow sheet relating to the dechlorination and oxidation of secondary zinc starting-materials in briquette form.

As mentioned in the introduction, the zinc startingmaterials may derive from zinc-manufacturing processes, and then comprises zinc ash. Secondary zinc-containing waste, however, is also obtained in the form of chemicals, and in the form of scrap, plastics etc. Depending upon the kind of waste concerned, the material can be charged, as required, in a finely divided state to a reactor which operates in accordance with fluidized bed principles, c.f.

Figure 1, or the material can be briquetted and charged to a reactor vessel operating in accordance with the principles of a shaft furnace, c.f. the process according to Figure 2.

In the process according to Figure 1 there is used a fluidized-bed reactor. The chlorine-containing zinc starting-material is charged in a finely divided form to the reactor through an inlet 2 located above a gas-permeable bottom or grate 3.

The term 'finely-divided' is here used to refer to starting material which has been crushed to a particle size which preferably does not exceed about 5 mm. The particles or grains, however, should be sufficiently large to prevent them from being entrained with the fluidizing gas, and the size of the particles should be such so as to obtain the requisite residence time in the reactor. The particles are heated by means of the heated fluidizing gas, which is given a desired high energy content in a plasma generator 4 and then blown into the reactor 1 at a location beneath the grate, at a pressure above ambient pressure.

A dechlorinated product in the form of zinc oxide is then withdrawn via a spillway 5, while gaseous zinc chloride formed is withdrawn through a gas outlet 6 at the top 7 of the reactor.

The gaseous zinc chloride then passes through a dust separator 8, such as a hopper, whereafter it is brought into contact with water in a venturi-scrubber 9. The pH of the resultant zinc-chloride solution is adjusted in a vessel 10, so as to obtain zinc dihydroxide. The cleansed process gas can thereafter be recycled to the process or released to atmosphere.

The dust 8 separated in the dust hopper 8 comprises mainly zinc oxide, and is recovered together with the dechlorinated zinc-oxide product.

The gas used should preferably be an oxidizing gas, and may be air for example.

The temperature in the upper part 11 of the fluidizing bed is preferably about 750on. The temperature in the venturi-scrubber is approximately room temperature. The dechlorinated zinc-oxide product, which is withdrawn through the spillway 5, is discharged through a gas-tight gate-valve arrangement of a known kind.

In the process illustrated in Figure 2, briquetted or lump-form chlorine-containing zinc starting-material 21 is charged to the top of a reactor vessel 22, through a conventional gas-tight inlet means 23. The reactor operates as a shaft furnace, in which the briquetted starting-material forms a filling 24, which continuously sinks down, through the reactor. As with the process previously described, the requisite energy is supplied by means of gas heated in a plasma generator 25, the supply of energy being regulated in a manner to maintain a temperature of about 750 C at the top 26 of the reactor. Gaseous zinc chloride is withdrawn through a gas outlet 27 located at the top of the reactor, while dechlorinated zinc-oxide is discharged through a gas-tight gatevalue arrangement 29 located at the bottom 28 of said reactor.

In this latter case, in which the starting material is in briquetted form, no dust separator is required, since any minor quantities of dust possibly entrained by the gas can be readily captured in the venturiscrubber 30, in which zinc-chloride contained by the gas is taken up in the form of an aqueous solution, which can be converted to zinc hydroxide, by adjusting the pH of said solution in a vessel 31.

During the aforesaid scrubbing process, the temperature is about 30"C.

The resultant waste gas can either be released to atmosphere or compressed and recycled in the process.

Thus, both of the aforesaid embodiments of the process according to the invention enable the recovery of valuable zinc and zinc-oxides from all conceivable industrial waste products, something which present day techniques are unable to do in an economically viable manner.

Claims

1. A method of dechlorinating and oxidizing secondary zinc starting materials comprising heating chlorine-containing zinc starting material in a reactor vessel to a temperature of about 750"C, passing hot gas to the reactor so as to vaporize the zinc chloride present; removing gaseous zinc chloride from the reactor vessel through an outlet located at the top of said vessel; and withdrawing dechlorinated zinc and zinc oxide products from the reactor vessel through an outlet located beneath the outlet for gaseous zinc chloride.

2. A method according to claim 1, wherein the gas is heated in a plasma generator located externally of the reactor vessel.

3. A method according to claim 1 or claim 2, wherein the reactor vessel is a fluidized bed reactor.

4. A method according to claim 3, comprising charging the chlorine-containing zinc starting material to the reactor vessel in a finely divided form art a location above a gas-permeable grate; charging the hot gas to a location beneath the grate at a rate of flow so controlled as to maintain a fluidized bed above the grate, while controlling the energy content of said gas to maintain a temperature of about 750"C in the upper part of the reactor vessel; with drawing a dechlorinated zinc oxide product via a spillway and withdrawing gaseous zinc chloride through an outlet at the top of the reactor.

5. A method according to claim 3 or claim 4, wherein the zinc starting material has a maximum particle size of about 5mum.

6. A method according to any one of claims 1 to 5, wherein the gaseous zinc chloride is passed through a dust filter.

7. A method according to claim 1 or claim 2, wherein the chlorine-containing zinc starting material is charged to the reactor in lump form and the reactor comprises a shaft.

8. A method according to any one of claims 1 to 7, wherein the gaseous zinc chloride is treated with water in a venturi scrubber at a temperature of about 30"C, to obtain an aqueous zinc chloride solution.

9. A method according to claim 8, wherein the pH of the zinc chloride solution is adjusted so as to convert said solution to zinc dihydroxide.

10. A method according to claim 8 or claim 9, wherein the gas washed in the scrubber is compressed and recycled through the reactor.

11. A method according to any one of claims 1 to 10, wherein the hot gas is oxidizing.

12. A method as claimed in claim 1 andsubstan- tially as herein described with reference to and as illustrated in either of the Figures of the accompanying Drawings.