NO763961L - - Google Patents

Abstract

Stabilization procedure and. consolidation of metal residues.

Description

The present invention relates to a method for treating residues comprising metal compounds that are physically unstable, especially against weathering, which method results in a restructuring of the residue by consolidating and stabilizing it, which enables storage in piles, without it being necessary with some protection against. weather and wind.

It is a known technique to produce zinc by electrolysis from pure zinc sulfate solutions. Such a zinc sulfate solution can be obtained by treating zinc ore, for example directly oxidized ores or ores sulfurized after roasting, under oxidizing conditions with sulfuric acid in such an adjusted amount that the pH of the obtained resolution will be in the range 4.0 - 4.5, as a result of a certain . surplus of ore, as iron is oxidized to the 3-valent state. By decanting and/or filtering, a zinc sulphate solution with a very low iron content is obtained, as well as a primary residue of undissolved solid particles essentially containing all the iron, silicon oxide, lead and solv, which were originally present in the ore used. Furthermore, the residue may also contain a larger proportion of the zinc that was originally present in the ore.

According to certain known processes for treating zinc ores with a relatively high iron content, for example processes as described in Australian patent no. 279,188 and US patent no. 3,434,798, the said primary residue is re-treated with an excess of hot sulfuric acid. From the suspension thus obtained, a secondary residue with a high content of lead and solvate is separated by decantation and/or filtration. The resulting solution also contains zinc sulphate, corresponding to most of the zinc contained in the primary residue and the

<1> turned sulfuric acid., essentially a sulfuric acid eov is shot ,

and most of the originally present iron, 13-valent state, which was present in the used ore. The resulting solution is then heat-treated with a measured and successively added quantity of either roasted zinc ore or zinc oxides or hydrated oxide, so that. the pH of the liquid is brought to a value in the range 1 - 3, which causes the iron present in the solution to be precipitated as basic sulphate, for example H^O.Fe^. (S04) 2, (OH) g j arrosite hydrate, or Fe4(S04) , (OH)-^0) glockerite, and with regard to the K+, Na+ or NH4<+>ions present as jarosites:

|CFe3(S04)2(OH)6 NaFe3(S04) 2(OH)6 NH4Fe3 (S04) 2(OH) g) ,

According to recent industrial practice is precipitated

iron as a mixed hydrate and sodium or ammonium jarosite (Nax.(OH3)1_x.Fe3.(S04)2, (QH)6or .(NH4)x.(<0B>|)1_x.Fe3.(S04)2. (OH)g where 0 ^ x ^1), by keeping the concentration of Na<+>or NH4<+>ions at an optimal value by adding adjusted amounts of suitable reactants.

The thus obtained precipitate of basic iron sulphates is separated from the remaining solution by decantation and/or filtration, and soluble materials are removed from this by washing with water, which gives a tertiary residue containing almost all of the iron and arsenic present in the zinc-containing material, as well as other elements such as lead and silicon and zinc in residual amounts.

A typical analysis of a cake that is thrown away after filtering the last-mentioned iron-containing residue can be, for example: Water content (calculated on the basis of the wet cake): approx. 45% Elements (on the basis of dry residue): Fe: 26 to 32% Pb: 0.5 to 1.5%

S (sulphate) : 10 to. 13% Zn: 0.5 to 2%

Na:3 to 4% Cd:0.01%

As : 0.2 to 0.5% Cu : 0.01%

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According to a corresponding currently used method, iron is precipitated from the corresponding impure solution of zinc and iron sulphates, essentially in the form of hydrated iron hydroxides and/or oxides, such as goethite, FeO.OH, in mixture with varying proportions of basic sulphates, such as jarosites

and other iron compounds. This precipitation is obtained by reducing the iron to a 2-valent state, for example by adding a sulfur ore or roasted mixture, ("crude blend"?-)

followed by a neutralization by adding zinc oxide, for example in the form of a roasted ore mixture, combined by an air oxidation or oxidation by blowing in oxygen.

This precipitate is also separated, as is the case for the jarosite-based precipitate, as previously described, by decantation and/or filtration, after which the precipitate is freed of soluble constituents by washing with water, giving a tertiary residue with properties quite similar to those of the jarosite-based tertiary residue described above, and for example the residue can have the following composition: Moisture (on the basis of the wet cake): 4 - 4.5% by weight Elements (on a dry basis residue): % by weight

Fe: 38 to 42% Cu : 0.01 to 0.1% S(total) : 2.5 to 5% As : 0.2 to 0.6% S(sulfate): 2 to 4% Si02: 1, 5 to 2.5% Pb : 0.5 to 2.5% Al2O3 : 1 to 3%

Zn : 2 to 6% CaO + MgO : 1 to 4%

Cd : 0.01 to 0.1%

According to the usual industrial practice, such tertiary residues are resuspended in water, and the suspension is transferred to a decantation tank in which the solid is settled, and the supernatant water is reused for the above-mentioned resuspension. The water included in these operations is thus continuously recycled and is essentially used for a hydraulic transport of the tertiary iron-containing residue to the decantation and storage tank.

This residue, which is separated from the dilution water, is particularly difficult to handle and store. At a moisture content of j 45%, which is the normal moisture of the filter cake, the residue has a semi-liquid consistency, such as a watery clay and it is very difficult, if not impossible, to handle this residue with ordinary mechanical devices.

If the moisture content increases, the residue will quickly become more liquid, and when it is held in channels or barriers, more or less horizontal stratified decantation layers are formed.

If the moisture content of these layers, as a result of drying, is significantly lowered below 45%, these layers will pop up as a

clay during the drying process and such a dried solid has no consistency and under the influence of mechanical stress, compression, abrasion or erosion it will immediately disintegrate into an unmanageable dust which is blown away by the slightest breath of wind. Any such pile of dried residue will very quickly spread as a result of wind and rainwater which will dilute the moist material to a liquid state.

In settling tanks, stratified, lightly packed layers formed by these residues will remain permeable, resulting in leaching of liquids saturated with soluble elements that will contaminate water-bearing layers in a water-permeable soil.

Thus, the use of decantation tanks will not completely solve the problems with regard to storing these iron-containing residues and large areas with closed tanks, which are necessary to prevent pollution, generally do not constitute a particularly economical solution to the problem.

It is possible to find a use for these residues after a hydrothermal conversion or by dry calcination to more or less pure iron oxides. Such treatments are very expensive and in most cases the problems will not be solved because on the one hand the market for such iron oxides is not large enough to absorb the large quantities of produced residues and on the other hand such iron oxides will give a material which is relatively difficult to store.

In Belgian Patent No. 779,613 and in the article "Treatment of Iron Residue in the Electrolytic Zinc Process", pp. 18-27 in TMS

pages No. A73-11 of "The Metallurgical Society of AIME" (New York NY 10017, East 47 Street) provides a more detailed description of the problem concerning these ferrous residues.

Belgian patent no. 779,613 relates more particularly to a thermal treatment of such residues, but this method is also burdened with the above-mentioned disadvantages.

In the aforementioned article, thermal and hydrothermal treatment is mentioned in particular and refers in particular to the various conditions that are necessary for such a process to be commercially and economically applicable.

A first condition would be to find a solution to the storage of residues based on iron sulphates. For this purpose, a careful washing of the residues for storage is mentioned, but according to the article, such washing is not practically feasible on a technical scale.

Another solution proposed in the article is to form a suspension of these residues at a pH higher than or equal to 10 by combining the washing with a precipitation of soluble metals by adding an excess of lime.

The pH of a suspension thus obtained which is exposed to atmospheric conditions decreases due to the presence of carbon dioxide in the air and in rainwater, resulting in a partial leaching of the originally precipitated metals.

In order to remedy these disadvantages to a certain extent, it is proposed in the aforementioned article to form a vegetation on the jarosite deposits, which would necessitate special precautions against the presence of toxic products, and in the article these precautions are pointed out.

Within the titanium industry, titanium dioxide for paint pigment is produced from ores, which often contain iron (ilmenites), which are dissolved in sulfuric acid. Iron dissolves together with titanium and forms the most significant impurity in the resulting solution. The iron is removed by crystallization, for example as iron(II) sulfate mono- or heptahydrate, followed by separation, decantation, filtration or centrifugation, which gives large amounts of a solid residue (several hundred thousand tons in the course of a year. from just one production plant ) of such sulphates, which are very soluble in water and which contain large quantities of crystallization water or impregnation solution, and these sulphates have up to now had no economic application.

Due to the high solubility of such sulphates, they cannot be stored in the open as a result of the rapid contamination of the surrounding soil. Discharges into the sea, which have been used until now, can be expected to be banned.

It can be concluded that the storage of these residues, which presents problems with regard to space consumption and water and air pollution, has until now no technically valuable solution.

The present invention is more particularly intended to propose a simple and economical solution to this problem.

For this purpose and in accordance with the invention, the original free-flowing residues, containing at least 55% dry matter, are mixed with a stabilizing material containing active CaO, in an amount sufficient to form a solid product which is mechanically stable and essentially inert to the water's dissolving effect, which enables the product obtained to be stored in piles exposed to the weather and wind without any risk of contamination.

By agglomerating the aforementioned residues with the aid of a stabilizing material, grains such as lumps can advantageously be produced consisting of a mass which is coated with a hard, protective layer consisting essentially of water-insoluble reaction products of active CaO with at least one of the components in the aforementioned residues.

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The invention also includes products obtained by reproducing '

the method, as well as the use of this product as a filling material.

In fig. 1 schematically shows a block diagram for a particular embodiment of the method according to the invention.

rich. 2 schematically shows a cross-section of a lump or grain obtained by carrying out the method.

The invention relates to a method which allows a transformation into products which can be stored by means of conventional mechanical devices in stable piles without any danger arising

for disintegration or water or air pollution. The products that can be treated are (1) either free-flowing residues containing at least 3% by weight of sulfur as sulphates, more particularly in the form of jarosites or various iron sulphates possibly mixed with iron oxides or hydroxides, particularly in the form of geotite, (2) or residues based on iron oxides or hydroxides, such as geotite, with a sulfate sulfur content of less than 3% by weight, (3) or iron sulfates to which are bound larger or smaller amounts of the solution from which they were separated.

These materials are in particular ferrous, tertiary residues, obtained as described above by separation, by filtration or solid deposits as a result of decantation of the same residues after their hydraulic transfer, or they may include iron(II) sulphates, which are separated by purification of titanium sulfate solutions.

According to the invention, grains and lumps of a generally round shape with a diameter in the range of 1 - 20 cm are produced from the aforementioned residues, which are initially very free-flowing, by mixing a stabilizing material containing active CaO, and this mixture is carried out so that the residues is restructured into grains and lumps which are coated with a hardened, protective coating, consisting of at least one substantially water-soluble

reaction product of active CaO, and at least one of the constituents in the residue, which grains and lumps are initially consolidated by a

skeleton which is also based on at least one of these reaction products.

By one or more physico-chemical reactions, lumps and grains are obtained from these original powdery and free-flowing residues, each of which is made up of a mass which is coated with a hardened, protective coating and which is internally consolidated by a skeleton, as both the coating and the skeleton constitute zones in the materials, where the physico-chemical reaction rates are very high, while these reactions occur slowly or not at all in the unhardened or slightly unhardened mass. These reactions can, for example, be reactions with sulfate radicals and/

or iron oxides in the residue treated with active CaO, where these reactions result in the precipitation of a solid latticework of gypsum and/or ferro-calcium compounds, which is hereinafter referred to as "ferro-lime".

Advantageously, the aforementioned grains and lumps can be obtained by an agglomerating treatment comprising a mixture of the residues with a quantity of stabilizing material containing active CaO, more particularly slaked lime or unslaked lime, milk of lime or materials containing lime, until the above-mentioned grains or lumps are obtained.

In this connection, it should be noted that slaked lime is significantly more effective than slaked lime, and that the exact dose is a function of the sulphate sulfur content and/or the iron oxide content of the residue, as well as its moisture content.

The advantages of using slaked lime arise as a result of an improved chemical stabilization reaction as a result of a significantly greater heat release than when slaked lime is used.

Advantageously in the cases of iron-containing residues of jarosite, geotite and other precipitation residues from zinc sulfate solutions, a neutralizing material is used in an amount that constitutes 8-16% by weight, preferably 8-12% by weight, of the active CaO in relation to the dry weight of the treated residue. It is important to note that, according to the invention, it is specifically aimed at carrying out a stabilization reaction with part of the residue with active CaO, which results in the formation of gypsum or the aforementioned "ferro-lime" on the basis of the residue's content of sulfates and iron oxides .

The term "active CaO" in the above-mentioned material means the part of CaO that takes part in the reaction, for example for commercially available lime, so the active CaO content corresponds to the total CaO content minus the CaO content in the form of carbonate or sulphate.

When applying the conditions according to the invention applied to the residue from the zinc industry, only a part of the iron sulphates and oxides will be converted into gypsum and/or iron lime.

For the case of jarosite and other residues with a relative

With a "high" (5-13%) content of sulphate sulphur, the active amount of CaO used generally comprises 40-90% of the amount necessary to convert all sulphate sulfur into gypsum.

For the case of geotite and analogous residues which have a relatively low sulfate sulfur content (2 - 3%) the amount of active CaO is usually equal to 100% of the amount required to convert the entire sulfate sulfur content, plus 10 - 30% of the amount necessary to convert the entire content of iron oxide. It is of course also possible to use CaO amounts that exceed those indicated above, which results in an increased stabilization and consolidation of the lumps and grains, however, this will entail greater costs.

During the work that led to the present invention, it was attempted to carry out crystallization of gypsum as a result of the reaction between lime and SO3 radicals in the treated material and consequently the amount of lime was used in relation to the content of sulphate sulphur.

The quantity in relation to the treated dry material decreases with this content, but the more the sulphate<y>ovel content decreases, the greater will be the necessary added quantity, in relation to the sul-j fatsulphur content, for a complete conversion. This is necessary in order to achieve the formation of a skeleton and a coating that has sufficiently high strength.

For ferrous residues with a low sulfur content, of approx. 3% or less, the amount of gypsum that can be formed will be insufficient to ensure this strength. However, it has surprisingly been shown that by increasing the amount of CaO well above what corresponds to the reaction with the total sulphate sulphur, the consolidation bg the hardening effect will still be higher, and in this connection it has been noted that in addition to gypsum, one or more are precipitated lime and iron compounds in a similar way to gypsum in the reactions that mix best and have. the best contact between the reactants to form a cured coating and skeleton.

By mixing the material containing active CaO, where the reaction medium has a relatively low water content, the intentionally intended imperfect distribution of CaO will cause a partial iriho-. mature reaction as a result of cracks and zones in the material, and consequently the formation of calcium sulphate and iron lime will essentially develop on the surfaces of the lumps and grains during production, where the lime concentration is highest.

The reaction products thus include gypsum bg/or iron-lime coatings and skeletons, but which also contain components of incompletely converted product and unconsumed active CaO, which is why the reaction inside the product will progress slowly.

Under the conditions according to the present invention, as soon as the physico-chemical reactions are sufficiently developed, the pH of the product will be higher than 8, at

what value compounds of zinc, cadmium, iron and other metals contained in the grains or lumps, be relative

slightly water-soluble, and the solubilities are sufficiently low to effectively dampen the diffusion of these elements from the interior of the aggregated material of lumps and grains to the surface thereof, forming skeletons and coatings

act as effective barriers against such migration.

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As a consequence of the foregoing, the general increase in pH as a result of the lime mixture will be sufficient for the various components of the residues, which are found inside the grains and lumps, to become essentially water insoluble.

The mixing of the residues and the stabilizing material, containing unslaked lime, and the agglomeration thereof into grains and lumps, as previously described, can be achieved with the help of various devices, such as mixers, granulators, extruders, roller mills and the like, possibly followed by granulation drums and tables .

The product obtained by this treatment is shaped like grain

or lumps which are coated with a hardened surface skin, shell or layer of gypsum and/or iron lime, and which are further consolidated in their interior by skeletons based on the same materials.

According to a more extensive and expensive embodiment of the invention, but which gives better results, the coated and internally consolidated grains and lumps are subjected to a further treatment in which the grains and lumps are made compact by squeezing moisture from their surface. The further

treatment includes a special compression treatment which will be referred to as compaction treatment in the following. Moisture perspiration will cause a more prominent strengthening and hardening of the outer shell.

This effect is further enhanced by the addition of additional quantities containing active CaO during the compaction treatment.

This compaction treatment preferably takes place in a granulator in which the grains and lumps are repeatedly turned while materials containing active CaO are added, as well as advantageously other components that contribute to a more complete sealing of the grains and the surface of the lumps, for example such as bentonite, clay, silicates, silicate solutions, ores and such.

With regard to the moisture content of the residue to be mixed

with the stabilizing material, which is an important factor, it has been found that favorable results are obtained when the moisture content is 5 - 45%, preferably from 15 - 40%, i.e. residues containing 55 - 95% solids, preferably 60 - 85% solids.

As previously mentioned, the original moisture content of the residue to be treated, obtained after normal filtration, will in most cases be in the region of 45%.

Such a residue can advantageously be subjected to a pre-treatment to lower the moisture content down to 40%.

For this purpose, according to the invention, the residue is suspended in water, and the suspension is filtered, and the moisture content in a cake thus obtained is lowered to a level lower than 40%, possibly by using mechanical measures such as to press the cake or by means of pneumatic means such as air blowing or air suction of the cake, where this is mixed with the stabilizing material containing active CaO, as previously described.

Although various filters capable of producing a "very dry" cake may be suitable for this pretreatment, filter presses are particularly suitable. The above-mentioned suspension is filtered through the filter press until the filter is packed maximally with the resulting cake, . after which compressed air is blown through the cake.

According to the invention, any filter with a similar effect can be used, especially certain types of pressure or vacuum filters, and i.a. those provided with mechanical compression systems for the cakes.

The filtered liquid is recycled to a decanting tank

or to a process step for suspension of the tertiary residue. j

If the ferrous tertiary residue obtained from the initial filtration already has a moisture content of less than 40%, this pretreatment and resuspension and second filtration will obviously be useless..

The residues containing less than 40% moisture, possibly after pretreatment, are mixed with the stabilizing material containing CaO, preferably in an amount of 50 - 160 kg, or more preferably 80 120 kg of active CaO per 1000 kg of the residue's dry matter.

Especially with regard to iron-containing residues essentially formed by iron sulfate, for example the heptahydrate, such as that precipitated and separated during titanium production, as previously described, is particularly suitable for stabilization and aggregation treatment according to the invention. The strong development of heat during the reaction promotes the formation of particularly hard and strong grains and lumps, and it is possible to limit the addition of CaO to 20 - 50% of the stoichiometric, necessary amount indicated by the following reaction:

Once the known properties of the various iron oxides have been given, the FeO formed and exposed to air will oxidize very quickly, and more especially on the surface of the grains and lumps as they are obtained, which in addition forms an impenetrable blackish, brown layer of basic iron sulphate which strengthens the plaster barrier coating. Advantageously, in order to achieve the maximum effect when using CaO and thus reduce its quantity as much as possible, a surface dusting of the grains or lumps that are formed or are in the process of being formed with the stabilizing material is provided during the treatment for that purpose to enhance the effect of the surface films with the relative excess of active CaO.

As a result of the pronounced crystalline nature of the iron sulphate residues from the titanium industry, this can be treated directly

In at the outlet of the tumble dryer at their usual moisture in-! hold on 5 - 854. That. It is also possible to treat such residue as is. very humid, for example up to 45% humidity.

In cases where milk of lime is used as a stabilizing material, it is obviously necessary to take into account that the residue to be treated must be relatively dry so that the total moisture content does not become too high to enable obtaining on one side a product which is mechanically stable and which, on the other hand, is essentially insensitive to the dissolving effect of water, so that storage in piles exposed to weather and wind can be made possible without risk of contamination of the surrounding areas.

In this connection, it should be noted that in certain cases the required quantity of the stabilizing material may exceed that required to achieve the simple mechanical stabilization. This depends more particularly on the distribution of lime with respect to the residue, which will be apparent from the foregoing.

The following examples provide further details and several special embodiments of the method according to the invention with reference to fig. 1.

EXAMPLE 1

A mixture of ammonium and "hydronium" jarosite with a moisture content of 45%, which is fed from the outlet of a vibrating cell filter 1 with a horizontal filtering surface, was re-suspended in the vessel 2 provided with a stirrer not shown, with re-circulating liquid to to give a suspension containing 10% dry matter, which was pumped to the filter press 3, the filtered liquid being returned to the vessel 2, as indicated by the reference numeral 4.

Supply to the filter press 3 was stopped and at this point the pressure in the press exceeded 5 kg/cm 2 , which indicates a complete filling of the filter with formed filter cake. Then, compressed air at a pressure of 6 kg/cm was introduced as indicated by arrow 5 and blown through the filter cake, in the same direction as the filtration was carried out, until filter liquid indicates no significant outflow.

The cake then quickly left the filter 3 and entered the storage container 6. The material's total moisture content was approx. 38% and the sulfur content, calculated as sulphate, was approx. 12%, calculated on the dry matter of the material.

The filter cake disintegrates into a number of smaller pieces as it falls into the storage container 6 and from this the material is transported by means of the screw conveyor 7 to a continuous mixing device 8 of conventional type comprising a double funnel, two parallel axes provided with inclined blades which rotate in the opposite direction, and where the individual components of the mixing device are not shown. At the same time, it is introduced,

as indicated by arrow 9, quicklime containing 92% active CaO in a quantity of 80 kg per 1000 kg of residue with a moisture content of 38%.

In the mixing device, the cake was further divided and mixed with lime and the temperature rose to 30°C as a result of a partial neutralization of the basic iron sulphates.

The mixing time was approx. 1 minute. IN

The material discharged from the mixer was generally rounded grains and lumps with a diameter in the range of 1-20 cm. This material was then stored in a pile 10, the side of which had an angle of inclination of approx. 45°.

EXE MPEL 2

Mixed ammonium and "hydronium" jarosite was pretreated in a similar manner as stated in example 1 by resuspension and filtration in a filter press. The cake thus obtained had a moisture content of 36% and a sulfur content calculated as

sulphate, of 12%, calculated on the dry weight of the material.

In mixer 8, commercially slaked lime, containing 68% active CaO, was added in an amount of 80 kg of lime per 1000 kg filter cake with a moisture content of 36%. The mixing time was 80 sec. and the temperature of the material rose 10°C from the inlet to the outlet of the mixer.

The material that quickly came out of the mixer was grains and lumps of the same size as indicated in the previous example, but these were less hard and had less resistance to crushing.

After storage for 12 hours, however, it was possible to load these grains and lumps with a mechanical shovel and transport the material on a lorry and place it in a pile.

EXAMPLE 3

The same jarosite mixture, pretreated as indicated in the preceding examples, was mixed in the mixer 8 with commercially slaked lime containing 92% active CaO in an amount of 100 kg of lime per 1000 kg of the cake. The latter had a moisture content of 86% and contained 12% sulfur as sulphate, calculated on a dry basis.

During the mixing operation, which lasted 1 minute, the temperature of the material rose. 40°C.

I The material that quickly left the mixer was grains and lumps

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i with a diameter of 1 - 5 cm, which was much smaller and harder and exhibited a higher crushing resistance. This material gave off water and ammonia vapors and could be piled immediately.

EXAMPLE 4

A mixture of ammonium and "hydronium" jarosite, identical to that according to Example 1, was pretreated and mixed in the mixer 8, as indicated in Example 2, with a smaller amount of active CaO, i.e. 80 kg of slaked lime (Containing 68 % active CaO) to 1000 kg of the dry material of the cake.

The material obtained from the mixer 8 was quickly fed into a drum granulator (not shown) to which 20 kg of powdered and dry slaked lime containing 68% active CaO was added.

The material that quickly came out of the drum granulator was very dry and fairly hard grains with a diameter of 1-10 cm, and whose shape was significantly rounder and could be transported and stored in piles immediately.

EXAMPLE 5

Mixed sodium and "hydronium" jarosite also containing iron in the form of geotite [.FeO.(OH)] was pretreated according to Example 1 by resuspension and filtration in a filter press. The cake extracted from the latter had a moisture content of 32% and its sulfur content, as sulphur, was 6% on a dry basis.

The material that came out of the filter press was mixed in the mixer 8 with 50 kg of quicklime containing 92% active CaO per 1000 kg of processed dry material. The mixing time was 2 min. and during the mixing the temperature rose to 30°C.

The product that quickly came out of the mixer was grains and lumps

with a diameter of 1 - 6 cm which were quite, dry and hard and could be stored in piles immediately.

1

EXAMPLE 6

Iron (II) sulphate heptahydrate of the kind obtained from the production of titanium dioxide and which, when discharged from the dryer, contained approx. 8% mother liquor. This material was mixed in a continuous mixer with finely divided quicklime in a quantity

of 14 parts by weight of CaO to 100 parts by weight of the sulphate. The product was hot and <1>rapidly granulated to give a granule with a diameter of 1 - 20 mm, which turned brown in air.

The granulate was then dusted with 0.2 g parts by weight of CaO in the granulator drum. A brownish iron(III) hydroxide layer was formed on the outer surface of the grains. At the outlet of the drum, the grains were dry and did not stick together and were immediately transported by car and stored in piles.

EXAMPLE 7

The geotite residue obtained as a by-product during the electrolytic production of zinc, and which contained approx. 40% total iron and 3.0% sulphate sulfur (dry material) and the discharge from a filter press in the form of a cake with a moisture content of 40% was treated in a continuous mixer as indicated in Example 1 with finely powdered quicklime in a ratio of 3.5 parts by weight of lime to 100 parts by weight of wet geotite, which corresponded to slightly more than the stoichiometric amount necessary to convert the sulphate present into gypsum. During mixing, the product became hot and a very imperfect granulation occurred. The product quickly came out as heavy, sticky lumps to the drum, where a quantity of finely powdered quicklime for coating was added. The coated lumps remained plastic and hardened only slowly, and it took several hours for it to be possible to handle the material without it collapsing. The aforementioned processing conditions cannot be applied in practice as a result of the necessary time which

included in order to achieve favorable results.

In a parallel trial, the quantity of quicklime to the mixer was increased from 3.5 to 8 parts by weight. In this case, the product became hot and quickly formed a granule. A further addition

of lime for coating was not necessary, and the product obtained was immediately transportable. The grains thus obtained

had a size of 5 - 50 mm and were very resistant to crushing. When the lime addition was in excess, the water-soluble materials were made completely insoluble even inside the grains. This material was stored in heaps and exposed to the weather and wind, which did not cause dust formation or pollution as a result of the dissolution of the constituents present.

Fig. 2 schematically shows a cross-section of a grain or lump obtained using the present method. The reference number 11 corresponds to the internal gypsum skeleton, 13 denotes the rest of the residue and unconsumed CaO 9 and 12 denotes the hardened shell, the latter as well as the skeleton being formed of gypsum and/or iron lime.

The product according to the invention is grains and lumps with a dry appearance in contrast to the suspension formed by untreated tertiary residues, and this product can be transported and handled without particular disadvantages when using conventional handling equipment. This product can also be stored on the ground in the form of high piles, similar to slag piles from the metallurgical industry, on which transport and handling with wheels or tracked vehicles can be carried out, especially on sloping access roads arranged on the stored material itself.

The possibility of storage in high piles offers significant advantages in relation to the areas required for large decantation tanks which have until now been used to store residues obtained from the hydro-metallurgical zinc industry.

Furthermore, these grains and lumps can form a useful material for the production of dams due to embankments, which can be an important application in many cases.

Residues treated according to the invention exhibit one remarkable behavior under atmospheric conditions, especially when exposed to rain. When lumps and grains of the outer layer

exposed to rain, it will be moistened, however, without them

! 1 loses s, in form or strength. Most of the rain flows j i

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between the clods and the grains, preferably on the outside of the pile without penetrating deeply into it. Under these conditions, the main mass of the heap will retain very little water, as the water will quickly evaporate after a rain shower as soon as the atmospheric conditions permit, and this evaporation is promoted by the large area formed by the grains and lumps.

In the event of rain, the water stream will only affect the essentially waterproof surface of the grains and lumps, while the inner part of these will not be adversely affected. Consequently, loss of soluble material from the residue piles, as a result of rainwater, will be significantly reduced and consequently such soluble components will not be carried down into a water-bearing soil layer. This results in there being no risk of contamination of the soil's water content. In this respect, it should also be noted that carbon dioxide, which is present in the atmosphere, has a beneficial effect on the stabilization of the grains and lumps by strengthening the outer layer of these more completely as a result of carbonation of the active CaO which is found in excess on the outside of the lumps or grains.

The method according to the invention therefore enables the transformation of essentially non-storable, non-free-flowing powdered, suspended or sludge-like ferrous residues, which are sources of pollution for water and air, into hard, non-adherent grains or lumps, which exhibit suitable mechanical strength and physical properties that allow storage in piles at great heights. The material obtained can be handled with conventional means and stored piles can be exposed to weather and wind without risk of contamination or spreading, for example by erosion, runoff or currents.

Although the method according to the invention was originally investigated with regard to iron-containing residues, such as obtained by iron precipitation from a mixed solution of iron and zinc sulphate, such as occurs in the zinc industry,

and although the method was successfully applied to iron sulfate residues, it is quite obvious that the invention can also be applied! des on other iron-containing residues, as well as other metal residues

j

which generally contain iron sulphates, various iron oxides and hydroxides, or compounds of other metals with similar properties as regards reaction with lime.

It will also be obvious that lime, slaked or unslaked, can be replaced as a stabilizing agent by metal oxides with similar properties, such as corresponding oxides of barium, strontium and the like.

With regard to MgO, which is generally present in active CaO in many stabilization materials, this will not form insoluble materials, however it will contribute to the neutralizing effect of the lime by e.g. precipitating iron hydroxides, which helps to make the protective film waterproof, especially for jarosite and geotite materials, and thus fix polluting metals, such as insoluble compounds when the pH is raised.

Claims (10)

1. Procedure for the stabilization and consolidation of residues based on metal compounds, in particular residues containing jarosites, geotites, iron oxides, iron hydroxides or iron sulphates in order to enable such residues to be stored in the form of piles exposed to weather and wind, characterized by initially free-flowing residues comprising at least 55% solids are mixed with a stabilizing material containing CaO in an amount sufficient to form a solid product that is mechanically stable and essentially unaffected by water, which enables the product obtained to be stored without risk of contamination in piles exposed for weather and wind. IN 2. Method according to claim 1, characterized in that the residue is agglomerated using the stabilizing material to form grains or lumps which consist of an inner mass surrounded by a hard protective shell based on an essentially water-insoluble reaction product of active CaO with at least one of the constituents of the residue. 3. Method according to claim 2, characterized in that grains or lumps are produced which are internally consolidated by a skeleton on the basis of the aforementioned reaction product. 4. Method according to claims 2 or 3, characterized in that the grains and lumps' are subjected to a further compacting treatment which causes water to be squeezed out to the surfaces of the grains or lumps and thereby helps to strengthen their outer shell. 5. Method according to claim 4, characterized in that a further amount of stabilizing material in the form of a powder is added for the compaction treatment. 6. Method according to claims 4 and 5, characterized in that a material which further promotes the formation of a waterproof surface of the treated grains and lumps is added for the compaction treatment. 7. Method according to claims 1-6, characterized in that a residue containing 55 - 95% solids is used. 8. Method according to claim 7, characterized in that a residue containing 60 - 85% dry matter is used. 9. Method according to claims 1-8, characterized in that when the residue essentially contains jarosite, the active CaO is used in an amount of 6 - 26% by weight calculated on the dry residue. 10. Method according to claim 9, characterized in that the active CaO is used in an amount of 8-12% by weight calculated on the dry residue. llu Method according to claims 1-8, characterized in that when the ire residue essentially contains geotite, the stabilizing material is added in such an amount that the CaO content is higher than the stoichiometric amount determined by the following reaction CaO + C2 O3 ^ CaSO^ . 12. Method according to claim 11, characterized in that the stabilizing material is added in such an amount that 3.5 - 8 parts by weight of CaO are added per 100 parts by weight geotite. 13. Method according to claims 1-8, characterized in that when the residue essentially contains iron (II) sulphate, in particular iron (II) sulphate heptahydrate or -monohydrate obtained from the titanium industry, the active CaO is added in an amount which make up 30 - 100% of the stoichiometric amount determined by the reaction FeSO^ + CaO CaS04 + FeO. 14. Method according to claims 2-13, characterized in that ferrous residues with a moisture content higher than 45% are suspended in water, after which the suspension is filtered and the moisture in the resulting filter cake is reduced, either as a result of the new filtration conditions or by means of mechanical or pneumatic means until the moisture content is lower than 45%, and that the cake is mixed with a quantity of a stabilizing material containing active CaO until grains and lumps are obtained, which are coated and which contain a skeleton based on the same products.. 15. Method according to claim 14, characterized in that the suspension is introduced into a filter press, j and that when this is filled with a filter cake, compressed air <!> is blown into ! through the cake until a moisture content of 4.5% is achieved. 16. Method according to claims 1 - 15., characterized in that the material containing active CaO is used quicklime, slaked lime or milk of lime.