NO130323B - - Google Patents

Description

Process for purifying sludge obtained by the electrolytic production method of zinc, which contains basic sulphates and hydroxides of iron as well as zinc ferrite to extract a raw material suitable for the production of iron and to recover zinc.

In the case of conventional electrolytic zinc extraction, a zinc ferrite-containing solution residue is obtained, the content of zinc and other valuable metals (Cu, Cs, Ag, Au, Pb, ...) is quite significant.

Both pyro- and hydrometallurgical treatment methods have been developed for the further treatment of this discharge residue and for the recovery of the valuable metals contained therein.

In the case of the hydrometallurgical treatment methods, which are described, among other things, in the American patent no. 1,834,960, the Norwegian patent no. 108,047, the Canadian patent no. 663,664, the Spanish patent no. 304,601 and the Belgian patent no. 724,214, the ferrite-containing solution residue has been dissolved in the return electrolyte from the zinc electrolysis. The solid residue left by this dissolution, which contains the solvate, gold and lead in the original dissolution residue, has usually been separated from the solution and added to processes,

in which the above-mentioned metals have been able to be separated and extracted. The solution, whose iron (III) and sulfuric acid content usually varies in the range 20 -. 35 g/l, for most solutions, has been added to the precipitation step for iron directly or after pre-neutralisation.

The ferritic iron is thereby precipitated as basic sulphate and/or oxide-hydroxide. Rusted zinc has usually been used to neutralize the sulfuric acid released during the precipitation reaction. The ferrite component of the roasting products does not dissolve during the precipitation phase, but remains in the iron sludge. In some methods (Norwegian patent no. 108,047, Canadian patent no. 304,601), an alkali or ammonium compound is also added to the iron's precipitation phase, whereby the ferrite iron is precipitated as alkali or ammonium jarosite (A[Fe^ (SO^ ) ^ (OH) ^] 5

A = Na, K, NH4).

All the methods mentioned give a high yield of zinc, copper and cadmium (solvent, gold and lead). With all these methods, a zinc-ferrite-containing iron sludge remains, whose zinc content amounts to 5 - 7%, and which corresponds to approx. 3 - 4% of the zinc content in all the material used in the process. Although the relatively large quantities of zinc carried away by the iron sludge can be considered a disadvantage of these methods, the large amount of iron sludge obtained in the process (with zinc production of 100,000 t approx. 40,000 - 50,000 t iron precipitation) causes

and which in most cases must be considered a waste, the most serious disadvantage of these methods.

For partial recovery of the zinc that the iron sludge contains, an acid washing of ferrite-containing jarosite sludge has been developed (Norwegian utlegrJ r.gsskrift no. 123.248), and this procedure is described in more detail by G. Steinveit in the article "Die Eisen-fållung als Jarosit und ihre Anwendung in der Nassmetallurgie des Zinks", Erzmetall 23 (1970), pp. 532-539). With acid washing, the zinc content of the precipitate can be reduced to a value of 2 - 3%, but even here the sludge problem remains due to the large amount of iron sludge, and the zinc yield is also unsatisfactory.

The leach residue from the conventional process and the above-mentioned ferrite-containing iron sludges have also been treated by a combination of chlorinating and sulphating roasting (A. Roeder, H. Junghanss, H. Kudelka, Process for Complete Utilization of Zinc Leach Residues, Journal of Metals 21 (1969) pp. 31-37). This treatment yields an iron oxide ore whose non-iron content is so low that it is directly suitable as raw material for iron production. But even with this method, the solution from the leaching of the rusted goods cannot be fed to the electrolytic zinc extraction process due to the high chloride content,

and the application of this method in direct combination with an electrolytic zinc extraction process is not possible.

Direct sulphating roasting of the primary solution residue, which mainly consists of zinc ferrite, has also been used, whereby the sulphation is carried out either with sulfuric acid or with a sulphating gas atmosphere which is formed from sulphide-containing material, e.g. pyrite. A process, where pyrite is used as fuel and for the formation of a sulphating atmosphere, is described by E. Moriyama and Y. Yamamoto in Akita Electrolytic Zinc Plant and Residue Treatment of Mitsubishi Metal Mining Company Ltd., Proceedings of AIME World Symposium on Mining and Metallurgy of Lead and Zinc (1970) Volume 2.

In this method, after the leaching of the sulphated roasting product, an iron oxide sludge has been obtained, the zinc content of which is 2-3%. Such a jeroxide sludge is not directly suitable as a raw material for iron production. Moreover, with this method, the excess sulphate typical for the electrolytic zinc extraction process does not disappear without special precautions being taken, which is the case with the above-described hydrometallurgical treatment method of the primary leaching residue.

The disadvantage of all the methods described above has either been insufficient recovery of zinc and the quantities of iron sludge representing almost a waste, or if these problems could be resolved satisfactorily, one has had the disadvantage that the treatment process of the primary discharge residue or the ferrite-containing iron sludge has not been able to be combined directly with the electrolytic zinc extraction process.

In the U.S. patent no. 1,834,960, a case has been treated where the iron is precipitated as basic sulphate. For the neutralization, a roasted zinc product has been used, whose ferrites,

as described above, has remained in the iron sludge. The iron sludge is then roasted in a Wedge furnace in the temperature range 500 - 6dO°C. During this treatment, the basic sulphates have been decomposed and part of the zinc in the zinc ferrittan has changed into water-soluble sulphate form. After water washing, the zinc content of the roasting products was approx. 5%, of which 50 - 60% has occurred in water-soluble or ferrite form.

In the above-described roasting, as also in the described sulphating roasting of the primary solution residue, one has not succeeded in producing a sufficiently extensive sulphation of the zinc ferrites. After water washing of the rusted goods, the zinc content of the iron oxide sludge, i.e. the zinc occurring in ferrite form, has in all cases been 2 - 3%, so

that the iron oxide sludge cannot be directly used for iron production.

The disadvantages stated above have been avoided with the method according to the present invention, and whose mainly characteristic features appear from patent claim 1.

In this method, it has been assumed that the iron sludge from a hydrometallurgical treatment process of the primary leaching residue has mainly consisted of (a) alkali jarosite (e.g. Na Fe., (SO^) 2 (OH) ^) or zinc ferrite;

(b) basic ferrite sulfates (e.g. hydronium jarosite

H30 <Fe>3(SO4)2(OH)6, glockerite Fe4 S<0>4 (OH) Q), ferric oxy-hydroxide (FeOOH) and zinc ferrite.

In the first case, the sludge was submitted directly and in

the later case together with an alkali compound (e.g. sodium sulfate) a thermal treatment. in the temperature range 200 - 400°C, the jarosites, glockerites and oxyhydroxides emit crystal water. The finely divided ferrites in the iron sludge are now in very intimate contact, e.g.?, in case (a) where the sodium jarosite has given off crystal water, with the remaining compound NaFe (S04) ^ •Fe2°3 * N^r temperature rises, the zinc ferrite ZnO.Fe20^ begins to react with NaFe(S04)2« When the temperature has risen to 600 C, the zinc ferrite's zinc oxide is very completely sulphated after the ferrite has reacted with it in intimate contact thus standing NaFe(S04)2- If the temperature further bkes, ZnS04 begins to decompose slowly, whereby zinc oxide is formed, which further reacts with ~ £e. £>^ during the formation of zinc ferrite. The temperature at which zinc ferrite formation begins depends on SC^-, SO3~ °<? °2~ the content of the gas phase surrounding the grains. Hereby, it is important that the partial pressure and temperature of the oxygen and sulfur dioxide are regulated in such a way that one operates within the system's Zn-Fe-S-0 thermal stability range ZnS04 + Fe ?0-.. In practice, this means that when using low SC^- partial pressure is a suitable temperature approx. 600°C, and with SC^ content of 5 - 10%, a suitable temperature is 670 - 680°C. The above is illustrated by Figures 1, 2 and 3. Figure 1 shows the typical X-ray diffraction diagrams for a sludge of type (a) and for those thereof in air at temperatures of 400, 500, 600 and 700°c over the course of 2 hours. solids obtained by thermal treatment. Figures 2 and 3 show the stability diagrams for the system Zn-Fe-S-0 (R.L. Benner, H. Kenworth<y>, Bureau and Mines, Report of Investigations 6769).

From the diagrams according to figures 1, 2 and 3, it can thus be read that: 1) when Pc„^ 10~<5> atm and P_ = 0.05-0.10 atm, the suitable dry temperature range is approx. 550-600 c (experimentally one can determine that the appropriate temperature range is 580 - 600°C) and 2) when Pcn^ 0.10 atm and P_ = 0.05-0.10 atm it is appropriate-so2 o2

normal temperature range 670-750°C.

If, using thermodynamic data, a corresponding stability diagram was drawn for the sulfur dioxide's partial pressure P = 10 atm, then it should be possible to establish that the advantageous temperature interval was 600-650°C. One can then experimentally, within the framework of the mentioned temperature interval, find the most advantageous temperature range for each P„ - and P -value (cf. the examples).

In order for the zinc content in the iron oxide sludge obtained as the final product after the water washing to be brought to the order of 0.1 - 0.2%, a very extensive sulphation of the zinc ferrite is necessary, and at the same time the reaction conditions must be closely regulated, so that the re-formation of zinc ferrite can be prevented as completely as possible. The achievement of these goals is promoted by: 1. Intimate contact between the zinc ferrite grains and the NaFetSO^^ • Fe203 phase present around them, 2. sufficient SO^ content for sulphation of the zinc oxide in the phase surrounding the zinc ferrite grains, 3. sufficient alkali content in the phase that surrounds the zinc ferrite grains, so that the zinc ferrites react with their surroundings, and double sulfates of sodium and zinc are formed around the ferrite grains, and in one step also a phase containing sodium pyrosulfate, which melts already at a relatively low temperature during rapid and effective sulphation of the ferrite grains, 4. execution of the thermal treatment in a fluidized bed furnace or fluidization reactor, whereby close control of the reaction conditions can be achieved.

In order to maintain a thermal equilibrium, sulphide-containing fuel is preferably used, e.g. pyrite, whereby also gy.ssf asens

S02~ and O2~ contents can vary within fairly wide limits. It is thereby possible to also use non-ferrous metal-containing (mostly Zn- and Cu-containing) pyrites (FeS), FeS ores and -stbv, or

in extreme cases also iron-containing zinc sulphide, whereby the fuel's non-ferrous metal content (mostly Zn and Ci content) can also be taken care of; elemental sulfur can also be used as fuel.

In this context, the necessity of close regulation of the gas phase's S02~ and O2~ content, as well as a temperature that corresponds to this content, should be further emphasized. This close regulation can best be carried out in a fluidization reactor.

Regarding sulphation of the non-ferrous metals in the fuel and the regulation of the conditions, reference is made to the Finnish patent no. 41,874.

The products obtained during the thermal treatment are dissolved

in water, and the solution can be returned after filtration to the solution system for the zinc corrosion products, to which the alkali of the iron sludge also returns. In the process, whereby in connection with the treatment of the primary solution residue the iron is precipitated as alkali jarosite, the normal alkali addition costs will be saved by circulating alkali.

In a similar way, compared to most of the methods mentioned in the introduction, a significant saving of SO^ is achieved. When, in the aforementioned methods, the iron is precipitated as basic sulphate and the sludge is removed from the system without thermal treatment, a significant amount of SO^ is lost together with the sludge, which clearly exceeds the amount that enters the system together with the waste products. The sulphate deficit is usually replaced by H^O^ addition. In the method according to the present invention, a significant part of the SO^ addition required by the system is obtained from the SO^ content in the sludge from the thermal treatment.

EXAMPLE 1

EXAMPLE 1

Analysis of the roasted products from the iron sludge in a laboratory furnace (vi = water-soluble, vol = water-insoluble). EXAMPLE 2

Na2S04~addition 12% of the iron sludge

Analysis of the roasted products from the iron sludge and Na2S04 addition in a laboratory furnace

EXAMPLE 3 Analysis of the pyrite used as fuel

Analysis of a roasted product from iron sludge and pyrite in a ratio of 1:1 in a fluidization reactor Production conditions:

Claims (3)

1. Process for cleaning sludge obtained by electrolytic production of zinc, which contains basic sulphates and hydroxides of iron as well as zinc ferrite, which sludge is suitable as raw material for iron production and for the recovery of zinc, characterized by thermally treating the alkali-containing sludge so that the partial pressure and temperature of the sulfur and oxygen in the system Zn-Fe-S-0 are regulated within the thermal stability range of ZnS04+Fe203, whereby the zinc and certain other non-ferrous metals are converted into water-soluble sulphates, and whereby the iron that is included in the sludge's basic sulphates, such as f .ex. in the alkali jarosite MeFe3(S04)2(OH)^, the hydronium jarosite H3OFe3(S04)2(OH)6 and/or in the glockerite Fe4S04(OH)6 and in the ferric oxyhydroxide FeOOH, is converted into the oxide form, and whereby the iron in the ferrites, which e.g. in the zinc ferrite ZnO. Fe203 or in the cadmium ferrite CdO. Fe2O3 is obtained as oxide, and that the products from this thermal treatment are extracted, after which the solution obtained is separated from the soluble oxide sludge. 2. Method according to claim 1, characterized in that the thermal treatment is carried out in a fluidized bed furnace or in a fluidization reactor. 3. Method according to claims 1 and 2, characterized in that in the event that the sludge contains little or no alkali jarosite, the sludge is supplied with a sufficient amount of alkali compounds, such as e.g. sodium sulfate.