CZ170394A3 - Process and apparatus for reducing hexavalent chromium contained in a residue from chrome ore dressing process - Google Patents

Abstract

The present invention concerns a new process and the relative plant for reducing hexavalent chromium contained in the resultant earth after the production of chromium, in order to obtain the transformation from water-soluble compounds into insoluble compounds, the process comprising the phases of mixing the said residual products with one or more reducing agents and of indirectly heating the said mixture substantially without the presence of oxygen.

Description

Technical field

The present invention relates to a novel process for reducing hexavalent chromium contained in the soil remaining after the production of chromium to convert water-soluble compounds into water-insoluble compounds.

The invention also relates to an apparatus for carrying out the above method.

BACKGROUND OF THE INVENTION

It is well known that in a process for extracting hexavalent chromium such that the chromate is roasted and subsequently leached, it is not possible to regenerate all hexavalent chromium in order to obtain an alkali chromate.

Indeed, after leaching, a solid is obtained, which is generally called the leaching residue or the resulting soil, which still contains a varied amount of hexavalent chromium.

In the resulting leaching, there are three, very different cases.

Since chromium extraction technology is well known to those skilled in the art, the three cases can be briefly summarized as follows:

A) High calcium oxide (or dolomite) mixtures.

E) Low calcium oxide (or dolomite) mixtures.

C) Calcium oxide-free mixtures (or dolomite).

The problem of eliminating hexavalent chromium, which is soluble in water, is easily solved in a treatment method that does not include calcium oxide, since the relative

the residual products do not contain partially soluble compounds due to the complexes formed between calcium salts and hexavalent chromium.

In this case, it is appropriate to add to the residual products

Il * il il il il V il.

X ί ί 1 ·ι ·! Γ L L Γ L · · ·. .4. ^ 414 wj / ferrous chloride, etc., which reduces chromium from CrO to Cr ^ O ,,

W TL O which is insoluble.

Calcium-containing treatment produces compounds containing hexavalent chromium in a form that is difficult to dissolve in a short time, such as calcium chromate.

Thus, these derivatives cannot be leached immediately during the normal treatment process and remain in the resulting soil. Over time, however, this soil is subjected to washing, for example by the action of rainwater, and hexavalent chromium is leached and thus released.

Thus, it is evident that these solid residual treatment products pose serious problems and are classified and treated as harmful toxic wastes, and it is not easy to dispose of them in suitable landfills according to the applicable rules.

It should also be taken into account that these amounts of residual products tend to be windy, sometimes quite large; they vary from 2.3 to 0.5 times, based on the amount of solid alkaline dichromate produced.

Various methods are proposed for carrying out the reduction of hexavalent chromium present in the resulting soil, with the aid of organic and inorganic reducing agents.

{Η (I /

I. U.S. Patent No. 3,803,032 to Nippon Denko Co., U.S. Pat.

Ltc., Describes a reduction method according to which the resulting wet soil is mixed with carbonizable components (sawdust, bran, lignite and the like) and then heated to a temperature of not more than 700 ° C in an atmosphere, not containing more than 2% oxygen. In this process, it is necessary to control the composition of the atmosphere in the reactor in order to avoid the possibility of burning the material which could occur instead of the desired formation of activated carbon. This also avoids the risk of explosion that can occur when CO is formed.

The second method, which relies on the use of sulfur as a reducing agent, is described in Italian Patent No. 1.055.S53 under the name of the applicant, which corresponds to FR-A-2,342,347. The disadvantage of this method and other analogous methods is that carrying out the desired chromium reduction in an alkaline environment requires boiling an aqueous dispersion of the resulting soils, sulfur and sulfide under pressure.

A disadvantage that is common to the two processes just described is that the reaction does not proceed in a completely complete industrial manner and the costs are very high.

The essence of the invention

It is an object of the present invention to solve the above problems by means of a method and apparatus which makes it possible to carry out the reduction of hexavalent chromium compounds present in the resulting soil in an alkaline medium, which method and apparatus is simple, economical and efficient.

This object was achieved by the present invention, which relates to a process for reducing hexavalent chromium in residual products from the treatment of chromium minerals,

 characterized in that it comprises mixed phases of said residual products with one or more reducing agents, and that said mixture is indirectly heated substantially in the absence of oxygen.

In addition, the invention relates to an apparatus for reducing hexavalent chromium by a method according to any one of claims 1 to 7, characterized in that it comprises: indirect heating in a rotary kiln, as well as methods of sealing against gases collected at the feed and discharge points mixtures.

According to a preferred embodiment of the present invention, sulfur is used as the reducing agent, preferably mixed with residual soils, dried and then heated to a temperature at least equal to the sublimation temperature (445 ° C).

According to another preferred embodiment, the process is carried out continuously, by indirect heating, in a sealed rotary kiln, and without gas circulation.

The advantages of the invention are clearly evident: the use of sulfur greatly simplifies the design of the device in which the risk of explosion is eliminated; moreover, the cost of processing is reduced, and finally, a complete reduction of all the chromium present in the resulting soil is achieved, as will be described in the Examples that follow.

The invention will now be described in more detail in the accompanying drawing, in view of a more detailed illustration thereof, in which the phasing method according to the invention is shown schematically.

According to the attached figure, the residual leach products (or resulting soils) are dried (A) in a manner known per se until they are substantially anhydrous and then mixed in 2. (Phase B) with sulfur, sulfur-containing compounds, reducing compounds which are capable of causing the desired oxidation-reduction reaction described below. however, other types of inorganic reducing agents, such as, for example, sulfites, sulfides, sulfites, ferrous sulfate, ferrous chloride, etc., can be used for the sulfur-based task.

However, sulfur is the preferred reagent, and therefore, references herein in the description that follow will relate to this process without, however, limiting the scope, scope and scope of the invention.

·, Χ · ’\

The residual earth to sulfur ratio is generally between 100: 0.1 to 0.1

100: 5 and usually refers to the percentage of hexavalent amazement and to other reducible compounds present in the residual leaching products.

The anhydrous mixture thus obtained is fed to the reactor 3, which is preferably a rotary furnace with indirect heating, equipped with seals 5 and 5, located at the ends of the furnace, for any case of opening, in order to avoid or minimize the operation was essentially in the absence of oxygen.

The drying, stirring and batching phases to the reactor 3 may be carried out in any time sequence, but the method described above is preferred.

The figure further illustrates schematically an external heating system 6 comprising a series of burners providing the reactor with the heat required for the sublimation of sulfur that can react in this state; oxygen is removed from the compounds present in the residual products and hexavalent chromium is reduced.

It can be assumed, however, that the principle of the following reaction is not fully scientifically explained:

SO ₂ +

Cr ^ Ow + from t

Ka ^ CrO + 3 Ξ

To get good results in a short time is good

OVOLii. \ S · 2 Σ * ς * O J O O 7 3. v »3 · bUO J. iTlci O Ž.

maintain a minimum temperature and the preferred reaction temperature is 500 to 550

As mentioned above, the process is carried out in the absence of oxygen and without gas circulation, since the sulfur-containing atmosphere and SO 2 / generated during the reaction is suitable to obtain the conditions required for the reducing atmosphere.

Thus, the use of sulfur makes it possible to prepare the reactor before the reaction mixture is metered in: it is indeed advisable to initially only add sulfur to the reactor and evaporate it to consume the oxygen present and obtain the necessary atmosphere composition.

At the outlet of the reactor 6, the treated residual products 7 do not contain any detectable amount of hexavalent amazement (see Examples below). In any case, the reacted sulfur becomes bound in the material in the form of reducing salts (sulphites, sulphates, etc.) which have a certain ability to reduce traces of hexavalent chromium which, due to difficult contact, for example

 due to the large granulometry of the residual products, they cannot be reduced during the reaction.

The invention will now be described in more detail in the following Examples, which are intended to illustrate the invention in more detail but are not to be construed as limiting the invention in any way.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Thirty parts of residual chromate treatment products (CrO 4, total solubility - 2.8%; hexavalent chromium extracted with cold water = 7 280 ppm; hexavalent chromium extractable with hot water = 9 880 ppm; Cr ₂ O ₃ - 2.8%; CaO = 26.2%; ^Fe 2 ° 3 ⁼ Prepare for final comminution in the laboratory (93% 0.050 mm) and mix with 0.75 parts sulfur.

The mixture thus obtained is placed in a platinum container in a turbulent muffle furnace and the temperature is maintained at 510 ° C (internal temperature) for two hours. The insertion and extraction phases of the formula are carried out in a stream of nitrogen to avoid oxidation processes that occur in the presence of air.

The amount of hexavalent chromium in the final product is always less than 5 ppm.

Example 2

Thirty parts from the treatment of residual chromate products (Cr ₂ 0 ₃ , total solubility = 3.7%, hexavalent chromium extractable with cold water = 4 160 ppm; hexavalent chromium extractable with hot water = 6 450 ppm; Cr ₂ O ₇ = 2.4 CaO = 2.8%;

^Fs 2 ° 3 ~ ^with final comminution in the laboratory (98% <, 0.090 mm) and mixed with 0.60 parts

The mixture thus obtained is placed in a turbulent muffle furnace at a temperature of 490 ° C (internal temperature).

open.

I can hold up to the platinum stack within two hours

The sample insertion and extraction phase takes place in a stream of nitrogen to prevent oxidation processes that occur in the presence of air.

The amount of hexavalent chromium in the final product is always less than 5 ppm.

Example 3

Thirty parts from treatment of residual chromite products (CrO4, total solubility = 2.8%; hexavalent chromium extractable with cold water = 7 280 ppm; hexavalent chromium extractable with hot water = 9 880 ppm; C ^ Og = 2,8%, CaO = 2.2%;

Fe ₂ C (3 = 17.8%) is subjected to final comminution in the laboratory (98% <0.090 mm) and mixed with 0.75 parts sulfur.

The mixture thus obtained is placed in a platinum container in a turbulent laboratory muffle furnace and the temperature is maintained at 680 ° C (internal temperature) for 30 minutes. The sample insertion and extraction phases are performed in a disk stream to avoid the oxidation processes that occur. in the presence of air.

The amount of hexavalent chromium in the final product is always less than 5 ppm.

Twenty kilograms of residual chromite treatment products (CrO 4, total solubility = 2.8%, hexavalent chromium extractable with cold water - 7 280 ppm; hexavalent chromium extractable with hot water = 9 880 ppm; Cr ₂ O = 26.2%; Fe ₂ O ₂ ⁼ 17.8%) is subjected to final grinding (95% C 0.090 mm) and mixed with 0.4 kg of sulfur.

The mixture thus obtained is fed to a turbulent indirect heating furnace having the following basic dimensions: 0.20 51, heated length 2.8 m. The temperature is then adjusted to 510-520 ° C, the excess air is removed from the furnace and these conditions are maintained for thirty minutes and the furnace rotation is maintained at 3 revolutions per minute.

After cooling, the product is removed and subjected to analysis.

Hexavalent chromium is always less than 5 ppm in the final product.

Example 5

A mixture consisting of 100 parts of residual products from the processing of chromite (CrO ~, soluble total = 2.8%; hexavalent Cr extractable with cold water = 7280 ppm; hexavalent chromium exttrahovatalný hot water = 9880 ppm; c 2 ^O 3 ⁼ CaO ~ = 26.2%; Fe ₂ O ₃ = 17.8%) which are finely ground (95% 0.090 mm) and 2.0 parts of ventilated sulfur are continuously fed, after suitable modification, into the turbulent furnace of Example 4, and at a rate of 80 kg / hour.

The operation is carried out at an internal temperature of 510-520 ° C and is subjected to these conditions for 25-30 minutes.

Testing is continued for several days with no complications.

The amount of hexavalent chromium in the final product is always less than 5 ppm.

Industrial applicability

The method and apparatus for reducing hexavalent chromium contained in the chromium ore treatment residues are important for improved ore processing and treatment.

Claims (5)

1. A process for obtaining water-insoluble tremendous compounds based on hexavalent tremendous derivatives contained in residual products of chromium minerals, characterized in that it comprises a reduction reaction carried out under anhydrous conditions and in a substantial lack of oxygen, derivatives, in &quot; by mixing said derivatives with at least one reducing agent and indirectly heating the mixture thus obtained, at least to the temperature of said reducing reaction. The method of claim 1, wherein from said at least one reducing agent is selected from the following group of chemical compounds: sulfur, disulfites, sulfites, sulfides, ferrous sulfate and ferrous chloride. The method of claim 2, wherein the at least one reducing agent is sulfur, said reduction temperature is the sublimation temperature of sulfur, and the weight ratio of said residual products containing said hexavalent chromium derivatives and sulfur in the reaction mixture is in the range of 10u / 0, and the ratio, in ppm, of hexavalent tremor in the reduction final products and hexavalent chromium contained in said starting derivatives is less than 0.05 / 100. The process according to any one of claims 1 to 3, wherein the reduction reaction products are in the form of powdered water-insoluble anhydrous chromium compounds. A method according to any one of claims 1 to 4, characterized in that it is carried out continuously by indirect heating in a rotary mesh. Process according to any one of claims 1 to 5, characterized in that it is carried out in the absence of gas circulating in the reactor. -/O- The process according to any one of the preceding claims, characterized in that, prior to carrying out the reduction reaction of said hexavalent chromium derivatives, a reducing atmosphere is formed by heating at least one reducing agent alone under anhydrous and substantially oxygen deficient conditions to the temperature of said reducing reaction. . one of claims 1 to 7, characterized in that it comprises: an indirectly heated rotary kiln, gas sealing means positioned so as to correspond to the reactant entry and exit points, and means for supplying said reaction mixture; or separated components into said furnace.