SK10762001A3 - Calcium and / or lime carbide desulphurization mixture - Google Patents

Abstract

The invention concerns a desulphurizing mixture in powder form for treating liquid cast iron by injection, consisting (by weight) at more than 50 % of technical calcium carbide and/or lime, and optionally magnesium up to 40 %, aluminium up to 20 % and other additives such as carbon-containing products and/or coating agents in a total amount of not more than 10 %. The invention is characterised in that it contains sulphur at a content ranging between 1 % and 3.5 %. Said desulphurizing mixture results in improved desulphurizing performance.

Description

CALCIUM CARBIDE BASED MIXTURE AND / OR CALCIUM o

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Technical field

The invention relates to a desulphurization mixture based on technical calcium carbide, lime or a mixture of the two products for desulphurisation of pig iron and in particular of liquid pig iron obtained in a blast furnace prior to introduction into the converter.

Technical calcium carbide refers to an industrial product derived from the reduction of lime by carbon in an electric furnace. This product by weight contains 70 to 88% pure calcium carbide, 12 to 25 calcium oxide and at most 5% of various impurities.

BACKGROUND OF THE INVENTION

The treatment of liquid pig iron with calcium carbide or lime to reduce the sulfur content has been known for a very long time; for example, the lime desulfurization process implemented by IRSID and published in May 1956 in the Revue de Létallurgie (pp. 338 and 339) by B. Trentini, L.Wahl and M.SAlardlard.

Since that time, the desulfurization mixtures used have been constantly improved; In particular, it is necessary to mention the addition of processing agents, most often organic products and strongly reducing metals, such as aluminum and magnesium, which have specific desulfurizing properties themselves, to said mixtures. These mixtures have also been subject to gradual improvements in the granulometry of their constituents, the role of said granulometry being particularly studied by Professor G. Irons. An example of this type of blend is described in patent document FR 2747132, which relates to a calcium carbide blend containing by weight 1 to 20% magnesium and 5 to 15%

The aluminum and optionally other additives such as lime, calcium carbonate, carbon products and / or grain encapsulating agents in a total amount of at most 30%.

Desulphurization of pig iron using a mixture of calcium and / or lime leads to debris containing unreacted proportions of calcium or lime carbide; From an environmental and operational cost perspective, it is very important to achieve the greatest possible desulfurization yield in order to reduce the costly and contaminating waste. The aim of the calcium and / or lime desulfurization desulfurization mixtures according to the invention is therefore to achieve an improvement in desulfurization yield.

SUMMARY OF THE INVENTION

The present invention provides a powder desulfurization composition (preferably having a particle size less than 250 microns) comprising at least 50 wt. % of industrial calcium carbide and / or lime, optionally magnesium up to 40 wt.%, aluminum up to 20 wt. and other additives such as carbon products and / or encapsulating agents in a total amount of at most 10% by weight, the sulfur content of the mixture being 1 to 3.5% by weight, preferably 1.5 to 2% by weight.

Said sulfur content is understood to mean the total sulfur content of said mixture, irrespective of the nature of the chemical compound (s) in which the sulfur is present.

The invention is based on a priori paradoxical finding by the Applicant that a desulfurization composition based on micronized calcium and / or lime carbide powder will provide better yields when a small amount of sulfur-containing product is added to the mixture.

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In order to determine the efficiency of the desulfurization mixture, the Applicant has developed a test method which has been reproduced as many times as necessary to evaluate and compare the results provided by the various existing products and products realized within the scope of the invention.

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This method is described below with the working conditions modified to achieve good reproducibility of the results:

processing of 400 kg of pig iron with an initial sulfur content

0.040%, in a 800 kW blast furnace;

the start temperature is 1500 ° C;

injecting 400 g of the test product at a rate of 40 + 2 g per minute and 2 kg of slag (10% SiO ₂ , 45% CaO and 45% Al ₂ O ₃ ) while maintaining the furnace performance until reaching 1450 ° C at the end of processing;

taking a sample of liquid pig iron after processing.

The reliability of this test was determined by repeating this type of treatment using a constant composition desulfurization mixture; the average deviation obtained to reduce the measured sulfur content is 0.002% (20 ppm), ie. 5% of the initial sulfur content. For this reason, each product was tested in triplicate, allowing the mean deviation to be divided by the square root of three, resulting in an average deviation of 0.0012% (12 ppm). If a control means is thus available, different desulfurization mixtures can be evaluated. The results obtained show significantly improved efficacy when the sulfur compound product is added to the desulfurization mixture. Particularly good results are obtained when either sulfur supplement or lime substitute, either calcium sulphate or calcium sulphide or fly ash from coal combustion, such as lignite, is used as a sulfur supplying product,

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having a high sulfur content, containing 5 to 50%, preferably 8 to 15%, of calcium sulfate, which content is expressed as anhydrous CaSO ₄ . The addition of calcium sulfide can be accomplished by recycling the slag resulting from the desulfurization operation, the slag being ground prior to said use to achieve a desulphurisation granulometry.

In the context of the invention, the total sulfur content of the desulfurization mixture, which is given both by the sulfur contained in the raw materials used, in particular calcium carbide in the form of CaS and by the added sulfur, is between 1 and 3.5%. 1.5 and 2%. Magnesium (up to 40% by weight) or aluminum (up to 20% by weight) in powder form and carbonaceous products with a high content of volatiles and / or encapsulating agents (up to a total of 10%) can be added to the calcium and / or lime carbide in a known manner. Conversely, the addition of CaCO ₃ proves to be ineffective and the addition of CaF _{2 is} rather harmful.

The desulfurization mixtures according to the invention lead to a significant improvement in the desulfurization yield for the same processing times. It is likely that the addition of sulfur contributes to the intensification of desulphurisation kinetics, which is industrially beneficial, since the desulphurization of pig iron is the operation that most often hinders the operation of steel mills.

In the following, the invention will be explained in more detail by way of examples of its specific implementation, and these examples are illustrative only and are not intended to limit the scope of the invention, which is clearly defined by the definition of the claims and the description.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Prepare 10 kg of powdered desulphurisation mixture with a particle size of 160 micrometers containing:

31761 hrr kg of technical calcium carbide corresponding to 310 l of acetylene / kg and containing by weight: 88,5% CaCl2, 10,4% CaO and 1,1% CaS, 2,25 kg quicklime containing 0,005% sulfur and 0,75 kg black Coal containing 38% volatile matter and 0.85% sulfur. '

Thus, the mixture contains 61.9% CaCl 2, 29.8% CaO, and 0.41% sulfur, which is essentially in the form of CaS. This mixture was divided into four equal parts weighing 2.5 kg.

The first part was carried out as described above for the desulfurization test using pig iron having a sulfur content of 0.40%. Three desulfurization tests were conducted which resulted in the following final sulfur contents: 0.0124%, 0.0142% and 0.014%, respectively, representing an average sulfur content of 0.0137%.

The average desulphurization yield is expressed as the number of moles of sulfur bound to the total number of moles of pure CaCl ₂ and CaO mol pure, which means that the yield is given by the following formula:

yield R = (M _f / M _o) (S _{0 f} S) / (% CaCl _2/64% CaO + / 56) 32, in which formula

So means the initial sulfur content of pig iron (%),

Sf means the final sulfur content after processing (in%),

Mf means weight of pig iron treated (in kg),

Mo means the mass of the desulphurisation mixture used (in kg).

Thus, in the case of said test, the ratio Mf / Mo = 1000.

% CaC ₂ refers to the weight content of calcium carbide in the composition,% CaO refers to the weight content of free lime in the composition (ie, unbound lime, such as in sulfur or calcium silicate).

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Thus, when the above formula is used for processing carried out using the first portion of the mixture, an R value of 54.8% is obtained.

Example 2

To the second portion of the mixture of Example 1, 150 g of anhydrous calcium sulfate was added. Analysis of the 2.65 kg sample obtained gave the following results:

CaC ₂ = 58.44%

CaO. = 28.09%,

S = 1.72% (essentially in the form of CaS and CaSO ₄ ).

Three desulfurization operations were performed to provide the following final sulfur contents: 0.0105%, 0.0095% and 0.0110%, respectively, representing an average sulfur content of 0.0102%. Thus, the average desulfurization yield is 65.8%.

Example 3

To each part of the mixture of Example 1 was added 275 g of anhydrous calcium sulfate. Analysis of the mixture obtained weighing 2.775 kg gave the following results:

CaC ₂ = 55.81%,

CaO = 26.83%, and

S = 2.70%.

Three desulfurization operations were performed to give the following final sulfur contents: 0.0094%, 0.0113% and 0.0105%, respectively, representing an average sulfur content of 0.0103%. Thus, the average desulfurization yield is 68.7%.

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Example 4

To the fourth part of the mixture of Example 1 was added 1 kg of fly ash with the following weight composition:

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CaSO ₄ = 20.5%

CaCO ₃ = 1.8%

CaO = 6.4%,

2CaO, SiO ₂ = 60.2%,

Al ₂ O ₃ = 9.3%,

MgO = 1.7%.

Analysis of this 3.5 kg mixture gave the following results:

CaCl ₂ = 44.25%;

CaO = 23.10%,

S = 1.67%.

Three desulfurization operations were performed to provide the following final sulfur contents: 0.0148%, 0.0132% and 0.0142%, respectively, representing an average sulfur content of 0.0140%. Thus, the average desulfurization yield is 73.6%.

Claims (7)

A desulphurizing powder composition for the injection molding of liquid pig iron consisting of more than 50 wt. industrial calcium carbide and / or lime and optionally magnesium up to 40 wt.%, aluminum up to 20 wt.%. and other additives, such as carbon products and / or encapsulating agents, in a total amount of not more than 10% by weight, characterized in that it contains sulfur in an amount of 1 to 3.5% by weight. The desulphurization mixture according to claim 1, characterized in that the sulfur content is 1.5 to 2% by weight. Desulphurization mixture according to either of Claims 1 and 2, characterized in that the sulfur is supplied at least in part in the form of sulphide or calcium sulphate. Desulphurization mixture according to claim 3, characterized in that the addition of sulfur is carried out as a supplement or replacement of lime in the form of flue ash containing 5 to 50% by weight of calcium sulphate, said content being expressed as anhydrous CaSO ₄ . A desulfurization mixture according to claim 4, characterized in that the fly ash contains 8 to 15% by weight of calcium sulfate. Desulphurisation mixture according to claim 3, characterized in that the sulfur supply is in the form of a desulphurisation slag. Desulphurization mixture according to any one of claims 1 to 6, characterized in that its particle size is less than 250 micrometers.