GB1569694A - Process for stabilizing and consolidating residues comprising metal compounds - Google Patents

Description

(54) PROCESS FOR STABILIZING AND CONSOLIDATING RESIDUES COMPRISING METAL COMPOUNDS (71) We, SOCIETY DE PRAYON, a Belgian body corporate of Prayon, Commune de Foret, Belgium, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: This invention relates to a process for treating residues comprising metal compounds and having a consistency such that they are free flowing and cannot be piled up for storage.

In the known technique for preparing pure zinc sulfate solutions, for example formed for the purpose of producing zinc by electrolysis, zinc ore, for example a directly oxidised ore or an ore which has been sulfurized after roasting, is generally attacked under oxidising conditions by sulfuric acid used in such a metered amount that, due to some ore excess, the pH of the resulting solution is regulated to a value between 4.0 and 4.5, whereby the iron is oxidised to the ferric state. By decantation and/or filtration, a zinc sulfate solution which is very poor in iron then separated from a primary residue of undissolved solid particles containing in particular almost all the iron, silica, lead and silver which were present in the original ore this residue possibly also containing a large proportion of the zinc contained initially in the ore.

In some known processes for treating zinc ores which are relatively rich in iron, for example in processes such as are described in Austrian patent 279,188 and US patent 3,434,798, this primary residue is again at elevated temperature with an excess of sulfuric acid. From the resulting slurry, a secondary residue which is rich is lead and silver is separated by decantation and/or filtration. The remaining solution comprises, in addition to an amount of zinc sulfate corresponding to most of the zinc contained in the primary residue and in the sulfuric acid used, mainly sulfuric acid in excess and substantially the whole of the iron - essentially in the ferric state - which was present in the original ore. The solution is then treated at elevated temperature with a metered and progressively added amount either of roasted zinc ore or of zinc oxides or hydroxide, so as to bring the pH of the liquid to a value between 1 and 3, which causes the iron in solution to be precipitated as basic iron sulfate, for example H3O.Fe3.(SO4)2.(OH)6, jarosite hydronium (i.e. Fe4(SO4).(OH)l0) and glockerite, and, if Kt Na+ or NH+4 ions are present, as jarosite [i.e.

KFe3(5O4)2(OH)6 NaFe3(SO4)2(OH)6 NH4Fe3(5O4)2(OH)6].

In the most recent industrial practice. iron is more particularly precipitated as mixed hydronium and sodium or ammonium jarosite [Nax.(oH3)l-x.fe3.(so4)2. (OH)6 or (NH4)x.

(OH3)l X.Fe3.(SO4)2.(OH)6 where 0 S x A1], by maintaining the concentration of Na+ or NH4 ions at an optimum value by adding metered amounts of suitable reactants.

The thus obtained precipitate of basic iron sulfate is separated from the remaining solution by decantation and/or filtration and water-soluble material is removed therefrom by water washing, leaving a tertiary residue containing almost all the iron and arsenic present in the original zinciferous material, as well as other elements, such as lead and silica, and also some residual zinc amount.

A typical analysis of the cake which is discharged after filtration of this latter ferriferous residue is as follows: Moisture content (on wet cake basis) : about 45% by weight Elemental analysis (by weight on dry residue basis) Fe : 26 to 32% Pb : 0.5 to 1.5% S(sulfate) : 10 to 13% Zn : 0.5 to 2% Na : 3 to 4% Cd : 0.01% As : 0.2 to 0.5% Cu : 0.01% A similar process, which is also in current use, precipitates iron from the same impure solution of zinc and iron sulfates remaining after separation of the secondary residue, mainly as hydrated iron hydroxide and/or oxide, such as goethite FeO. OH, in mixture with varying proportions, of basic sulfates, jarosites and other iron compounds. This precipitation is achieved by means of a reduction of the iron to the ferrous state, for example by the addition of sulfurized ore or crude blende to the solution and then its neutralization by the addition of zinc oxide, for example roasted blende, in combination with an oxidation by blowing in air or gaseous oxygen.

This precipitate is also separated, as in the case of the jarosite-based precipitate described above, by decantation and/or filtration and then substantially stripped of its soluble materials by water washing, leaving a tertiary residue having quite similar properties to those of the jarosite-based tertiary residue described above, and having for example the approximate following analysis.

Moisture content (on wet cake) : 4 to 4.5% by weight, Elemental analysis (dry 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% SiO2 1.5 to 2.5% Pb : 0.5 to 2.5% Awl303 . 1 to 3% Zn : 2 to 6% CaO + MgO 1 to 4% Cd : 0.01 to 0.1% In normal industrial practice, such a tertiary residue is reslurried in water and the slurry is drained to a settling tank (basin) from which, after solids have settled, the supernatant water is taken up in order to be reused for the above-mentioned reslurrying. The water involved in these operations is thus continuously recycled and is essentially used for hydraulic conveying of tertiary ferriferous residue to the settling and storage tank.

This residue separated from this water in the decantation tank is particularly difficult to handle and store. At a 45% by weight moisture content, which is the normal mean moisture content of the filtration cake, the residue has a semi-fluid consistency similar to that of a water-soaked clay and it is very difficult or even impossible to handle this residue with conventional mechanical devices.

If the moisture content increases, the residue quickly beomes more and more liquid and when retained between dikes or barriers, it forms more or less horizontally stratified decantation layers.

If, by drying, the moisture content of these layers decreases to substantially below 45% by weight, they crack in the same way as a clay in process of drying-up does but the resulting "dried-up" solid is not firm and hard and under the action of any mechanical strain, e.g. compression, abrasion or erosion, it immediately disintegrates as an impalpable dust which is blown off by even the least wind. Any pile of such dried-up residue would be very quickly spread by the action of the wind and of water, which would thin the wetted material to a flowable consistency.

In settling tanks, deposits stratified in substantially uncompacted layers, which are formed by these residues and remain permeable, give rise to seepages of liquids laden with soluble materials that pollute the water-bearing stratum in a non-impermeable ground.

Thus, the use of settling tanks, which are normally merely basins in the ground, does not completely solve the problems of storing these ferriferous residues and the making of large areas of the basins impervious to seepage, which would be necessary to solve this problem, is not an economical solution to the problem.

It is possible to find a use for these residues after having converted them, hydrothermally or by drying and calcination, into more or less pure iron oxides. These are very expensive treatments which in most cases do not solve the problem because. on the one hand, markets for such iron oxides are not big enough to absorb large amounts produced frm the residues and, on the other hand, the thus obtained iron oxides are also just as difficult to store or keep in stock.

Belgian patent 779,613 and 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 47th street) describe the problem of these ferriferous residues in greater detail.

Belgian patent 779,613 relates to a particular thermal process for treating said residues, this process also having the above-mentioned drawbacks.

Said article more particularly mentions thermal and hydrothermal treatments and draws the attention to the various conditions which are necessary in order that said process should be commercially and economically applicable.

A first condition would be to find a solution to the problem of storing the residues of basic iron sulfates. To this end, an extensive washing of the residues before storage thereof has been mentioned but this washing. however, does not appear as technically practicable according to said article.

Another solution proposed in said article is to form with these residues a slurry at a pH greater than or equal to 10 by combining said washing with a precipitation of soluble metals by addition of excess of lime.

The pH of the slurry thus obtained decreases on exposure to atmospheric conditions due to the presence of CO2 in the air and in rain-water, which again partially redissolves initially precipitated metals.

In order to remedy these drawbacks to some extent, the above-mentioned article proposes that vegetation should be caused to grow on the jarosite deposits, but this would require very particular precautions against the inclusion of toxic materials, and the article draws attention to these precautions.

In the titanium industry, titanium oxide which is used as a pigment for manufacturing paints is produced from ores, very often iron-laden ores (ilmenites), which are solubilized in sulfuric acid. Iron is dissolved with the titanium and is by far the most important impurity of the resulting solution. It is eliminated by crystallization, for example as ferrous sulfate monohydrate or heptahydrate. followed by separation, e.g. by decantation, filtration or centrifugation, of the supernatant liquid leaving as a solid residue very large amounts (several hundred thousands of tons a year from a single production plant) of said ferrous sulfates which are very much soluble in water and very much laden with crystallization water and impregnation solution, these sulfates being left without any possible economical use till now.

Due to the high solubility of said product, it cannot be stored as such in the open air, due to the rapid pollution of the surrounding ground. Discharge into the sea, which has been the adopted solution until now, is likely also to be prohibited soon.

In conclusion, storing of any of these residues gives rise to problems of the amount of space occupied and of water and air pollution, which have no acceptable solution till now.

The present invention seeks to provide a simple and economical solution to these problems.

According to the invention there is provided a process for stabilising and consolidating residues comprising jarosite, goethite. iron oxide, iron hydroxide, iron sulfate and/or other metal compounds reactive with CaO to form a solid water-insoluble material, said residues having a consistency such that they are free flowing or non-coherent and cannot be piled up for storage, wherein these residues are provided in a form in which they contain at least 55% by weight of solids, and are mixed with a stabilizing material containing an amount of active CaO (as hereinafter defined) which is less than the stoichiometric amount for reaction with those constituents of the residues which are reactive with active CaO, the mixing being carried out such that a heterogeneous mixture is obtained which is in the form of particles comprising a firm and hard shell portion with a relatively high content of material derived from active CaO, in which the extend of the reaction with active CaO has been substantial, and a softer and less firm core portion with a relatively low content of material derived from active CaO. in which the extent of the reaction with active CaO has been relatively low or trifling. whereby the consolidated particulate product obtained is mechanically stable when piled up in heaps, and is also substantially insensitive to weathering when exposed to ambient atmosphere in heaps, the shells forming an impervious layer which substantially prevents the extraction of any polluting compounds of the softer portion inside the shells as a result of weathering and thus avoids the risk of pollution from them.

Of the accompanying drawings.

Figure 1 shows a schematic block-diagram of a particular embodiment of the process according to the invention; Figure 2 is a schematic and cross-sectional view of a particle obtained by carrying out the process according to the invention.

Residues which may be stabilized and consolidated according to the invention include (1) free-flowing residues containing at least 3% by weight of sulfur as sulfates, more particularly jarosites or various basic iron sulfates possibly mixed- with iron oxide or hydroxide, particularly as goethite, (2) residues based on iron oxides or hydroxides, such as goethite, having a sulfate sulfur content lower than 3% by weight, and (3) ferrous sulfates which are more or less soaked with their impregnation solution.

These residues may be ferriferous tertiary residues, such as are obtained as a result of the hereinbefore described separation procedures by filtration, or solid residues resulting from decantation of these tertiary residues after their transfer as an aqueous slurry, or ferrous sulfates which are separated when purifying sulfuric titanium solutions.

By means of the process according to the invention, from said residues, which initially are free flowing or non-coherent, particles are produced which are of varying shapes and sizes, although generally rounded and having diameters ranging from about 1 cm, at which size they may be regarded as grains, to about 20 cms, at which size they may be regarded as lumps. These particles are characterized by an inhomogeneous (heterogeneous) structure in that they have a firm and hard shell portion, based on one or more substantially water-insoluble reaction products of active CaO with at least one or the constituents of said residues, enclosing a softer and less firm core portion in which there has been less or only trifling reaction with the active CaO. Frequently, the particles are additionally internally consolidated by a firm and hard skeleton which is also based on at least one of these water-insoluble reaction products like the shell portion.

The firm and hard shell portions and skeleton when present are formed by one or more chemical reactions between the active CaO and reactive constituents of the residues and have a relatively high content of active CaO (in reacted form), indicating that the extend of the reaction has been substantial. The softer and less firm core portions have a relatively low active CaO content (in reacted form) indicating that the extent of the reaction has been relatively low or trifling. This contrast is an essential characterizing feature of the invention and it is necessary that the mixing with active CaO should be carried out such that it is achieved, primarily by using less than the stoichiometric amount of active CaO required for complete reaction with the reactive constituents of the residue but also by preventing the mixing being carried out for long enough and with enough stirring to obtain a homogeneous mixture.

The chemical reactions involved are mainly reactions of sulfate radicals and/or iron oxides with active CaO, said reactions giving rise to a precipitation of a solid network of gypsum and/or calcium ferrite, here sometimes referred to as "ferro-lime". It is known from "Nouveeau Trait due Chimie Minerale," Paul Pascal, Vol. XVII, Paris, (1967) that there are only two true anhydrous calcium ferrites, namely (Fe2O.CaO) and (Fe203.2CaO), of which only the first is unaffected by water and is thus the compound involved in the present invention.

By the term "active CaO" is meant. in the broadest sense, any calcium-oxygen, strontium-oxygen or barium-oxygen compound capable of reacting chemically with jarosite, goethite, iron oxide, iron hydroxide and/or iron sulfate under mixing conditions in the presence of moisture and at ambient temperature to form water-soluble solid compounds capable of forming a firm and hard shell. Primarily, the active CaO is provided by a calcium compound capable of reacting with basic iron sulfates, iron oxide or hydroxide and/or ferrous sulfate under such conditions to form such water-insoluble compounds. The calcium materials principally used are quicklime and slaked lime, although it is also possible to use milk of lime and materials containing lime. It is also possible in principle to replace the calcium compounds by corresponding compounds of other elements having similar properties, namely strontium and barium. MgO, which is present with active CaO in several stabilizing materials, does not produce water-insoluble material by chemical reaction with many of the residues but it does assist in the neutralizing action of the CaO, resulting in the precipitation of iron hydroxides for example, and such precipitated material may contribute to the waterproofing of the shell portions of the particles, particularly in the case of jarosite and goethite residues, thus helping to fix potentially polluting materials.

It is to be noted that quicklime is substantially more effective than slaked lime, and that the amount to be used for best results is a function of the content of sulfate sulfur and/or iron oxide in the residue and of its moisture content.

The advantage of quicklime is that it improves the chemcial stabilization reaction through a substantially greater heat output than in the case of slaked lime.

Advantageously, in the case of ferriferous residues of jarosite with or without goethite and other residues from the precipitation of zinc sulfate solutions, an amount of the stabilizing material is used which is sufficient to provide from 6 to 16%, preferably from 8 to 12%, by weight of active CaO with respect to the dry weight of the residues. When the residues consist essentially of goethite the amount of stabilizing material used is suitably sufficient to provide from 3.5 to 8 parts by weight of active CaO per 100 parts by weight of goethite.

When calculating the amount of "active CaO" to be used it should be borne in mind that not all the Ca content of the stablizing material necessarily takes part in the chemical reactions; in the case of commercial lime for example, the active CaO content corresponds to the total theoretical CaO content less CaO content combined as the carbonate and sulfate.

In the case of jarosites and other residues of relative "high" (5 to 13% by weight) content of sulfate sulfur, the amount of active CaO which is used is generally between 40 and 90% of the amount which is necessary to convert the whole sulfate sulfur into gypsum.

In the case of goethites and analogues residues which are relatively poor in sulfate sulfur (2 to 3% by weight), the amount of active CaO is generally equal to 100% of that which is necessary for reaction with the whole sulfate sulfur, plus 10 to 30% of that necessary for reaction with the whole of the iron oxide content to form calcium ferrites.

It is obviously permissible to use amounts of active CaO which are higher than those indicated hereinabove provided that less than the stoichiometric amount for complete reaction with the residues is used: the stabilization and consolidation of the particles is further increased, although subject to the disadvantage of a higher cost.

During investigations leading to the concept of this invention, our purpose was to achieve the crystallization of gypsum as a result of the reaction of the added lime with SOS radicals present in the residues to be treated, and the amount of lime was therefore selected according to the content of sulfate sulfur.

The amount of lime with respect to the treated dry material decreased as the sulfate sulfur content decreased but more the sulfate sulfur content decreased, more the proportion of lime added relative to the stoichiometric amount for complete reaction of sulfate sulfur increased in order to form a shell portion having a high enough mechanical strength.

For ferriferous residues which are poor in sulfur, about 3% or less, the amount of g-ypsum which could be produced by reaction of lime with SO3, radicals present became insufficient to ensure the necessary strength. Surprisingly. it was then found that by increasing the active CaO amount above that corresponding to the reaction with the whole sulfate sulfur. the consolidation and hardening effect became higher and higher, and it has been found that under these conditions. in addition to gypsum, calcium ferrite formed and precipitated in a similar fashion to gypsum in the reaction zones having the best mixing and best contact between reactants for forming the hardened sheath and skeleton.

On mixing the residues with the stabilizing material containing active CaO, the reaction medium being relatively poor in water, the intentionally imperfectly distributed CaO only partially reacts in a non-homogeneous way according to streaks (striac) and zones in the residues, and the reactions to form gypsum and calcium ferrite mainly occur on the surface of particles which are in course of preparation, where the lime concentration is higher.

The reaction products also form gypsum and/or calcium ferrite shell portions (with or without internal skeletons) which enclose core masses of imperfectly reacted product and of unconsumed active CaO, the reactions then further proceeding slowly inside the cores.

During the mixing operation according to the invention, as soon as the chemical reactions have become sufficiently developed, the medium pH rises above 8, at which point the water-solubility of the compounds of zinc. cadmium, iron and other metal which are then contained in the particles become low enough effectively to slow down any significant diffusion of these elements inside the aggregated material of the particles towards the surfaces thereof. the shell portion and skeleton (if present) additionally providing an effective barrier to migration through the surfaces.

Thus, the general increase of pH resulting from the mixing with lime is sufficient to make the various constituents of said residuces, which are confined inside the particles, substantially water insoluble.

The mixing of residues and stabilizing material containing active CaO, and the agglomeration thereof as particles can be carried out with various types of apparatus, including in particular blenders, granulators, extruders and drum grinders, optionally followed by granulation drums or tables.

The product resulting from this treatment is in the form of particles comprising a firm and hard shell portion, formed of the water-insoluble reaction product of active CaO and material on which the residue is based, usually of gypsum and/or calcium ferrite enclosing a softer and less firm core portion formed of the material on which the residue is based with relatively little interaction with the active CaO and optionally internally consolidated by a skeleton similar in composition to the shell portion.

According to one embodiment of the invention, which is more elaborate and more expensive than the procedure described above but gives better results, the particles are subjected after formation to an additional treatment as a result of which they are made more compact and there is moisture expulsion to the surface thereof. Said additional treatment involves some compression, although this need in general only be the pressure exerted by particles higher in the mass, but will be referred to by the more apt term "compaction treatment" hereinafter. Moisture expulsion causes a reinforcement of the strength of the shell portions by further reaction of the residues with active CaO.

This reinforcing effect is further enhanced if an additional amount of the stabilizing material containing active CaO is added, preferably as a powder, during said compaction treatment.

This compaction treatment advantageously takes place in a granulator wherein the particles are tumbled over and and over, preferably while stabilizing material containing active CaO and also advantageously another material capable of waterproofing the surfaces of the particles more completely are added, examples of suitable waterproofing materials being bentonites, clays, silicas, silicate solutions and ores.

As far as the moisture content of the residues to be mixed with the stabilizing material is concerned, it has been found that favourable results are obtained with moisture contents of 5 to 45%, preferably from 15 to 40%, by weight i.e. using residues containing from 55 to 95%, preferably from 60 to 85%, by weight of solids. Especially in the case of residues derived frm the hydrometallurgical zinc industry and consisting essentially of jarosite, goethite and/or basic iron sulfate with hydrated iron oxide, the solids content is suitably 60 to 70%, preferably 60 to 65%, by weight.

The initial moisture content of tertiary ferriferous residues to be treated, obtained after a normal filtration, is frequently above 45% by weight. Such a residue is advantageously subjected to a pretreatment so as to reduce the moisture content down to 45coo by weight or less, preferably below 40% by weight. To achieve this the residue may be slurried in water, the resulting slurry filtered and the moisture content of the resulting filter cake reduced to a level lower than 45% preferably lower than 40iso, by weight either solely due to the conditions of the filtration or by using in addition mechanical means, e.g. pressure on the filter cake, or pneumatic means, blowing of suction of compressed air through this cake.

Thereafter the cake is ready mixed with the stabilizing material containing active CaO in accordance with the invention.

Although a variety of filters able to produce a very dry cake may be used for this pretreatment, the filter press is particularly suitable. When using a filter press the slurry is filtered through the filter press until it is filled with the cake produced, and compressed air is then blown through this cake until the moisture content has been reduced to 45% or less, preferably below 40%, by weight.

Any other kind of filter having a similar performance may also be used, particularly pressure or vacuum filters, inter alia those which are equipped with mechanical compression systems for the cakes.

The filtered liquid may recycled to a settling tank (basin) or to a process step for slurrying the tertiary residue.

If the ferriferous tertiary residue, such as issuing from this initial filtration, already has a moisture content below 40% by weight, this pretreatment of slurrying and filtration is obviously unnecessary.

Residues containing less than 40% by weight of moisture, either initially or after pretreatment, are mixed with the stabilizing material containing active CaO, preferably at a rate of 60 to 160 kg, more preferably 80 to 120 kg, of active CaO to 1000 kg of dry material in the residues.

Where ferriferous residues essentially consisting of ferrous sulfate, for example as the heptahydrate, are concerned, e.g. residues which are precipitated and separated in the titanium industry as hereinabove mentioned, the stabilization and consolidation treatment according to the invention can be carried out with advantage. The strong heat output of the chemical reactions involving active CaO promotes formation of specially hard and firm particles and it is generally possible to limit the amount of active CaO added to 20 to 100%, preferably 30 to 50%, of the stoichimetric amount as determined by the following reaction: FeSO4 + CaO < CaSO4 + FeO Having regard to the known properties of various iron oxides, the FeO formed oxidises very quickly on exposure to air, more particularly at the surface of the particles, producing an impervious blackish brown layer of basic iron sulfate which reinforces the gypsum barrier to form the shell portion of the particles.

Advantageously, in order to maximise the effect of the active CaO used and thus to reduce the amount thereof as much as possible, the particles which are formed or are in the process of being formed are provided during the treatment with a surface dusting of the stabilizing material, in order to the effect at their surface by a relative excess of active CaO.

Having regard to the distinctly crystalline character of the ferrous sulfate residues from the titanium industry, these can be directly treated at the outlet of the drying machine where their usual moisture content is from 5 to 8% by weight. It is also possible to treat moisture residues of such materials, containing up to 45% by weight, of moisture, successfully, however.

When milk of lime is used as the stabilizing material providing active CaO, it is necessary to ensure that the residue to be treated is relatively dry so that the total moisture content of the residue and the milk of lime is not more than 45% by weight, preferably not more than 40% by weight.

The following examples give additional details on several particular embodiments of the process according to the invention, with reference to Figure 1 of the drawings.

Example 1 Mixed ammonium and hydronium jarosite with a moisture content of 45% by weight, issuing from the outlet of a tilting pan filter 1 with horizontal filtration surface was reslurried in a vat 2 provided with a stirrer (not shown), with recycled liquid so as to form a slurry containing 10% by weight solids, which was pumped to filter-presses 3, the filtered liquid having been returned to the vat 2, as indicated by reference numeral 4.

The feeding to filter-presses 3 was stopped when the pressure therein exceeded 5 kg/cm2, which was an indication of the complete filling of the filter with cake. At that time, compressed air at 6 kg/cm2 was blown according to the arrow 5 through the cake, in the same direction as the filtration, until the filtrate discharge no longer showed any significant flow.

The cake was then discharged from the filter 3 and sent to the storage hopper 6. Its moisture content was about 38% by weight, while the sulfur content as sulfate was about 12% by weight on dry weight basis.

These cakes disintegrated into many pieces whilst falling into said hopper. From the latter, pieces were sent via a screw conveyor 7 to a continuous mixer 8 of conventional type comprising, in a double trough two parallel shafts with inclined blades rotating in opposite directions and interpenetrating. constituent parts of the mixer not being shown. At the same time, according to the arrow 9. quicklime containing 92% by weight active CaO was added at the rate of 80 kg to 1000 kg of residues having a 38% by weight moisture content.

In this mixer, the cake was divided and mixed with the quicklime and the temperature rose to 30"C, following partial neutralization of basic iron sulfates.

The mixing time was about 1 minute.

The material from the mixer consisted of generally rounded particles with diameters varying from 1 to 20 cm. This material was capable of being stored in a pile 10 having an angle of repose of about 45".

Example 2 Mixed ammonium and hydronium jarosite was pretreated in the same way as in Example 1, by reslurrying and filtration in a filter press. The thus obtained cake had a 36% by weight moisture content and a sulfur content as sulfate of 12% by weight, on dry weight basis.

In mixer 8, commercial hydrated lime containing 68% by weight of active CaO was added at a rate of 80 kg of lime to 1000 kg of 36% by weight moisture cake. The duration of the mixing was 80 sec. and the temperature of the material rose by 10 C between the inlet and the outlet of the mixer.

The material from the mixer consisted of particles of the same size as in the preceding example, but much less hard and with a lower crushing strength.

After storage for 1 to 2 hours. it was however possible to take up these particles with a mechanical shovel, to load them on a truck and to discharge them and pile them up.

Example 3 The same jarosite as in both preceding examples, pretreated in the same way, was mixed in mixer 8 with commercial quicklime, containing 92% by weight active CaO, at a rate of 100 kg of lime to 1000 kg of cake. The latter had a 36% by weight moisture content and comprised 12% by weight of sulfur as sulfates on a dry weight basis.

The mixing duration was 1 minute and the material temperature rose to 400C.

The material left the mixer as particles of 1 to 5 cm in diameter, which were much smaller and harder and of higher crushing strength than those in Example 2. This material evolved water and ammonia vapour. It could be piled up immediately.

Example 4 Mixed ammonium and hydronium jarosite, identical to that of example 1, was pretreated, then mixed in mixer 8. as in example 2, with a reduced amount of active CaO (that is to say 80 kg of hydrated lime with 68ass by weight of active CaO to 1000 kg of dry material in the cake).

The material from mixer 8 was sent to a drum granulator (not shown) in which 20 kg of pulverised and dry hydrated lime with 68% by weight of active CaO was added.

The material left this drum granulator as very dry and quite hard particles of 1 to 10 cm in diameter, having substantially rounded shapes.

The grains could be conveyed and piled up immediately.

Example 5 Mixed sodium and hydronium jarosite, also containing iron as goethite [FeO. (OH)], was pretreated as in example 1 by reslurrying and filtration in a filter press. The cake discharged from the latter had a 32% by weight moisture content and its sulfur content as sulfate was 6% on dry material basis.

The material discharged from filter press was mixed with quicklime in mixer 8 at the rate of 50 kg of quicklime comprising 92% by weight of active CaO to 1000 kg of treated dry material. The mixing duration was 2 minutes, during which period the temperature rose to 30"C.

The product leaving the mixer was in the form of particles of 1 to 6 cm in diameter, which were quite dry and hard, and could be piled up immediately.

Example 6 Ferrous sulfate heptahydrate, obtained as a residue from the titanium industry and leaving the drying machine, contained about 8% by weight of moisture (mother liquor). It was mixed in a continuous mixer with fine quicklime at a rate of 14 parts by weight of CaO to 100 parts of sulfate. The products got warm and quickly granulated. forming particles of 1 to 20 mm diameter, which turned brownish when exposed to the air. The particles were then dusted with 0.2 parts of CaO in the granulator drum. A brownish ferric hydroxide layer formed on the external surface of the particles. At the outlet of the drum, the particles were dry and did not stick together. They were immediately taken up by a truck and piled up.

Example 7 Residual goethite from the electrolytic zinc industry containing about 40geo by weight of total iron and 3.0% by weight of sulfate sulfur (based on dry material) and leaving a filter-press as a cake with 40% by weight of moisture was treated in the continuous mixer of Example 1 with fine quicklime in the ratio of 3.5 parts by weight of lime to 100 parts by weight of wet goethite, namely a little more than the stoichiometric amount which is needed to convert the sulfate content into gypsum. When mixed, the product became warm and granulation occurred very imperfectly. The product was passed as very heavy sticky lumps from the mixer to the drum where a portion of fine quicklime for coating was added. The coated lumps remained plastic and hardened only progressively, and several hours were necessary, before it was possible to handle them without rupturing them. Such process conditions cannot be used in commercial practice due to the time needed to obtain acceptable results.

In a parallel test, the amount of quicklime added to the mixer per 100 parts by weight of wet goethite was increased from 3.5 to 8 parts by weight. The product became warm and granulated quickly. The addition of coating lime was not necessary and the product obtained was immediately transportable. The particles obtained had sizes of 5 to 50 mm and were very resistent to crushing. The lime dosage being in excess in relation to the soluble sulfates, the insolubilization of the latter was complete, even inside the particles. This material when piled up and subjected to weathering did not give rise to dust formation or pollution due to solubilized elements.

Figure 2 of the drawings schematically shows a cross section of a typical particle resulting from the use of the process according to this invention. Reference numeral 11 designates the internal gypsum skeleton, 13 designates the softer and less firm core mass of residue and dilute and unconsumed CaO, and 12 designate the firm and hard shell portion, which, like the skeleton, is essentially formed of gypsum and/or calcium ferrite.

The product according to the invention has the form of dry particles (grains or lumps or both), in contrast with the pasty or mud-like form of untreated tertiary residues, and this product can be transported and handled without any particular drawbacks with conventional handling apparatus. The product can also be stored on a storage piece of ground as tall piles, similar to spoil heaps resulting from mining or metallurgical exploitations, on which conventional transport and handling machines with wheels and/or caterpillar tracks can move, particularly on sloping access tracks on the stored material itself.

The possibility of making tall piles allows the use of very much smaller ground areas as compared to the areas required for settling tanks (basins) which have hitherto been used for storing such residues for hydrometallurgical zinc industry.

Moreover, these particles form a valuable hardcore material, for embanking or levelling purposes, which can be an important use in many cases.

The particles produced according to the invention behave remarkably well when exposed to normal atmospheric conditions, particularly rain. The particles of the external layer of the pile, which are reached by rain water, are wetted, but however lose neither their shape nor their strength. Most of the rain water flows between the particles, mainly on the sides of the pile without entering deeply into it. Under these conditions, the pile retains only a little water, which very quickly evaporates after the rain stops, as soon as is allowed by the atmosphere, this evaporation being promoted by the large surface area of the particles.

The rain water acts only on the substantially waterproofed external surface of the particles, while the core mass of the latter is not badly affected. Accordingly, losses of solubilised materials from piles of treated residue due to rain water which is liable to drain into water-bearing strata are very greatly reduced, even to insignificance. The result is that there is no risk of pollution of these strata. In this respect, it is also to be noted that the CO2 present in the atmosphere has a favourable influence on the stabilization of the shell portions of the particles by reinforcing them more completely due to a carbonation of the active CaO which may remain locally in excess on the surface, The process according to the invention thus converts substantially non-storable, free-flowing ferriferous residues, which are sources of air and water pollution, into hard, firm particles which are of suitable mechanical strength and physical properties to allow stable piles of large height to be formed, by means of conventional handling means; these piles may be exposed to weathering without any risk of pollution or slumping, for example by sliding, flowing or creeping.

Although the process according to the invention was originally studied for ferriferrous residues such as produced by iron precipitation from mixed solutions of iron and zinc sulfate in zinc hydrometallurgy and although this process was then successfully applied to ferrous sulfate residues, it is quite obvious that this invention can also be applied on other ferriferrous residues, and other metal residues containing in general iron sulfates, various iron oxides and hydroxides, or compounds of other metals capable of reaction with CaO under the mixing conditions to form a water-insoluble firm and hard shell portion, said residues resulting from any industry, namely from zinc or any other industry.

Claims (24)

WHAT WE CLAIM IS:

1. A process for stabilising and consolidating residues comprising jarosite, goethite, iron oxide, iron hydroxide, iron sulfate and/or other metal compounds reactive with CaO to form a solid water-insoluble material, said residues having a consistency such that they are free flowing or non-coherent and cannot be piled up for storage, wherein these residues are provided in a form in which they contain at least 55% by weight of solids, and are mixed with a stabilizing material containing an amount of active CaO (as hereinbefore defined) which is less than the stoichiometric amount for reaction with those constituents of the residues which are reactive with CaO, the mixing being carried out such that a heterogeneous mixture is obtained which is in the form of particles comprising a firm and hard shell portion with a relatively high content of material derived from active CaO, in which the extent of the reaction with active CaO has been substantial, and a softer and less firm core portion with a relatively low content of material derived from active CaO, in which the extent of the reaction with active CaO has been relatively low or trifling, whereby the consolidated particulate product obtained is mechanically stable when piled up in heaps, and is also substantially insensitive to weathering when exposed to the ambient atmosphere in heaps, the shells forming an impervious layer which substantially prevents the extraction of any polluting compounds of the softer portions inside the shells as a result of weathering and thus avoids the risk of pollution from them.

2. A process as claimed in claim 1, wherein the particles produced are also internally consolidated by a firm and hard skeleton which is similar in composition to the shell portion.

3. A process as claimed in claim 1 or 2, wherein the residues to be stabilized and consolidated contain at least 3% by weight of sulfate sulfur in the form of jarosite and/or other basic iron sulfates, the firm and hard portions of the particles produced having a composition based on gypsum formed by reaction of the basic iron sulfate with active CaO.

4. A process as claimed in any of claims 1 to 3, wherein the particles are subjected to subsequent compaction treatment in which there is moisture expulsion to the surface of the particles, this moisture causing a reinforcing action on the strength of their shell portions by further reaction of the residues with active CaO.

5. A process as claimed in claim 4, wherein an additional amount of the stabilizing material is added as a powder during the compaction treatment, whereby an additional reminforcement of the shell is produced.

6. A process as claimed in claim 4 or 5, wherein a material capable of waterproofing the surfaces of the particles more completely is added during the compaction treatment.

7. A process as claimed in any of claims 1 to 6, wherein the residues used contain 55 to 95% by weight of solids.

8. A process as claimed in claim 7, wherein the residues used contain 60 to 85% by weight of solids.

9. A process as claimed in claim 8, wherein the residues used contain 60 to 70% by weight of solids and are derived from the hydrometallurgical zinc industry consisting essentially of jarosite or goethite and/or basic iron sulfate with hydrated iron oxide.

10. A process as claimed in claim 9, wherein the residues contain 60 to 65% by weight of solids.

11. A process as claimed in any of claims 1 to 10, wherein the residues essentially consist of jarosite, with or without goethite and other residues from the precipitation of zinc sulfate solutions, and the amount of stabilizing material used is sufficient to provide an amount of active CaO of from 6 to 16% by weight of the dry weight of the residues.

12. A process as claimed in claim 11, wherein the amount of active CaO is 8 to 12% by weight of the dry weight of the residues.

13. A process as claimed in any of claims 1, 2 and 4 to 10 wherein the residues essentially consist of goethite and the amount of stabilizing material used is sufficient to provide an amount of active CaO is from 3.5 to 8 parts by weight per 100 parts by weight of goethite.

14. A process as claimed in claim 13, wherein the amount of stabilizing material used is sufficent to provide an amount of active CaO which corresponds to the stoichiometric amount the sulfate sulfur contained in the residue as determined by the reaction CaO + SO3 CaSO4, plus 10 to 30% of the stoichiometric amount reactable with all the iron oxide in the residues.

15. A process as claimed in any of claims 1,2 and 4 to 11 wherein the residues essentially consist of ferrous sulfate heptahydrate and/or monohydrate from the titanium industry and/or a ferrous sulfate from another source and from 20 to 100% of the stoichiometric amount of the active CaO as determined by the reaction FeSO4 + CaO CaSO4 + FeO is used.

16. A process as claimed in any of claims 1 to 15, wherein a ferriferous residue having a moisture content higher than 45% by weight is slurried in water, the resulting slurry is filtered, the moisture content of the resulting filter cake is reduced, either due to the conditions of the filtration or by additional mechanical or pneumatic means, until the moisture content is lower than 45% by weight, and the resulting cakes is then mixed with an amount of stabilizing material containing active CaO until particles are obtained in accordance with the procedure defined in claim 1.

17. A process as claimed in claim 16 wherein the slurry is introduced into a filter press for the filtration and, after the filter press is filled with cake, compressed air is blown through the cake until its moisture content is reduced to 45% by weight or less.

18. A process as claimed in any of claims 1 to 17 wherein quicklime or hydrated lime is used as the stabilizing material containing active CaO.

19. A process as claimed in any of claims 1 to 17, wherein milk of lime is used as the stabilizing material containing active CaO, the total water content of the residues and the milk of lime being not more than 45% by weight.

20. A process for stabilizing and consolidating residues as claimed in claim 1 carried out substantially as hereinbefore described or, illustrated with reference to the accompanying drawings.

21. A stabilized and consolidated material when obtained by a process as claimed in any of claims 1 to 20.

22. A residue based on jarosite, basic iron sulfate, goethite, iron oxide and/or iron hydroxide and consisting of particles comprising a firm and hard shell portion, formed of the water-insoluble reaction product of active CaO (as hereinbefore defined) and material on which the residue is based enclosing a softer and less firm core portion formed of the material on which the residue is based with relatively little interaction with the active CaO.

23. A residue as claimed in claim 22, wherein the particles also have a firm and hard internal skeleton of similar constitution to the firm and hard shell portion.

24. The use of a residue as claimed in any of claims 21 to 23 as hardcore.