RU2776054C2 - Method for calcium sulfide decomposition - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method for decomposition of calcium sulfide (CaS) to calcium oxide (CaO) and sulfur dioxide (SO₂). The method includes the provision of the presence of a reactor containing calcium sulfide and a carbon source, oxidation of the carbon source with oxygen to generate carbon dioxide (CO₂). Then, calcium sulfide is brought into the interaction with the specified carbon dioxide to obtain calcium oxide (CaO), sulfur dioxide (SO₂) and carbon oxide (CO) according to the reaction: CaS+3CO₂→CaO+SO₂+3CO. Moreover, the content of oxygen and carbon at the oxidation stage is selected in such a way that the mass ratio of C/CaS is from 0.15 to 0.35, and the mass ratio of O₂/C is from 5 to 25.

EFFECT: generation of energy required for calcium sulfide decomposition is provided in the effective conversion of CaS into CaO and SO₂.

7 cl, 4 dwg, 6 tbl

Description

The technical field to which the invention belongs

The present invention relates to a process for the decomposition of calcium sulfide to calcium oxide and sulfur dioxide by oxidizing carbon under controlled oxygen stoichiometry to control the total carbon dioxide content and energy costs of the decomposition process.

Prior Art

Calcium sulfide (CaS) is one of the main by-products of phosphogypsum (PG) decomposition during its reduction [1, 2] and, accordingly, significant efforts have been made to avoid its presence in solid products formed during the decomposition of phosphogypsum [3, 4 ].

The release of CaS into the environment can lead to the release of H ₂ S as a result of the interaction of CaS with water [5].

One solution to this problem is to convert CaS to CaSO ₄ or CaO under oxidizing conditions, but there are a number of limitations. First, during the oxidation of CaS, two different solid products are formed:

Which reaction will take place, (1) or (2), is determined by the reaction conditions [6]. For example, at a high concentration of O ₂ and a high temperature, CaSO ₄ is formed. If the surface of CaS particles is covered with a CaSO ₄ layer, CaS will not turn into CaSO ₄ until CaSO ₄ decomposes into CaO, SO ₂ and O ₂ at high temperature (above 1200°C) [7]:

It was shown that the use of carbon dioxide as a reacting gas for the decomposition of FG at a ratio of CO ₂ /CO≈5-6 significantly reduces the amount of CaS [8-10].

In other methods, an oxidation step to convert CaS to CaSO ₄ has been considered in conjunction with a reduction step to reconvert CaSO ₄ to CaO and SO ₂ [3, 4]. However, this method has a number of disadvantages. For example, only the outer layer of CaS particles is oxidized to CaSO ₄ and subsequently reduced to CaO, resulting in low CaS conversion. Therefore, a high recirculation rate between the oxidation and reduction reactors is necessary. However, the proposed methods require a very high heating rate to maintain the reactor temperature in the range of 950-1200°C.

Secondly, another limitation that makes CaS decomposition by this method unprofitable is significant energy consumption. All decomposition reactions take place at temperatures above 1100°C, so it is essential to provide the heat needed to keep the reactor at that temperature.

Disclosure of invention

The aim of the present invention is to provide a decomposition method that not only provides the necessary energy, but also efficiently converts CaS to CaO and SO ₂ .

Accordingly, the invention provides a process for decomposing calcium sulfide (CaS) to calcium oxide (CaO) and sulfur dioxide (SO ₂ ). This method is characterized mainly by the fact that it includes:

- ensuring the presence of a reactor containing calcium sulfide and a source of carbon,

- oxidation of the carbon source to generate carbon dioxide (CO ₂ ),

- introduction into the interaction of calcium sulfide with the specified carbon dioxide to obtain calcium oxide (CaO), sulfur dioxide (SO ₂ ) and carbon monoxide (CO) in accordance with the following reaction equation:

CaS+3CO ₂ → CaO+SO ₂ +3CO

in which the amount of oxygen and carbon in the oxidation step is chosen so that:

(i) the C/CaS weight ratio was from 0.15 to 0.35 and

(ii) the mass ratio of O ₂ /C was from 5 to 25.

The calcium oxide can be recycled to the H ₂ S removal process in the bed, while the SO ₂ can be used to produce sulfuric acid.

According to other optional characteristics of the present invention:

the C/CaS ratio is preferably 0.25;

- the ratio of O ₂ /C is preferably equal to 8;

- in the oxidation stage, the reactor is heated to a temperature in the range from 900°C to 1200°C, preferably about 1100°C;

the reactor is preferably heated by induction heating. However, it can also be heated by another external source such as an electrical power source;

- the carbon source includes at least one of: coal, coke, charcoal and shale oil;

the reactor is preferably a continuous reactor, such as a fluidized bed reactor or a rotary kiln, and has an inlet and outlet for passing current O ₂ through calcium sulfide and a carbon source.

Brief description of the drawings

The following features and advantages of the present invention will be apparent from the following detailed description based on the accompanying drawings, in which:

fig. 1 refers to the decomposition of calcium sulfide using CO ₂ : (a) dimensionless output quantities per unit of CaS, and (b) energy costs per unit mass of CaS. The dotted line (b) shows the desired ratio of CO ₂ /CaS at full conversion of CaS. The presented data are derived from equilibrium calculations using FactSage™ simulations;

in fig. 2 shows a diagram of a fluidized bed reactor with induction heating for gypsum decomposition experiments using carbon oxidation;

fig. 3 relates to the decomposition of calcium sulfide by burning carbon at a ratio of C/CaS≈0.2: (a) dimensionless output quantities per unit of CaS, and (b) energy costs per unit mass of CaS. The dotted line (b) shows the optimal O ₂ /C ratio at the maximum yields of CaO and SO ₂ . Data shown is derived from equilibrium calculations using FactSage™ simulations;

in fig. 4 shows SO ₂ production from CaS decomposition by carbon oxidation in a fluidized bed reactor with induction heating at 1100° C. for a total sample weight of 6 g for various C/CaS and O ₂ /CaS ratios.

Implementation of the invention

The present invention relates to the decomposition of CaS by oxidizing (with a stoichiometric excess of O ₂ ) carbon (for example, coal, coke, charcoal or any other carbon source) to remove unwanted CaS formed during the decomposition of FG or any other processes, and converting it into CaO and SO _2.

The decomposition of calcium sulfide can be carried out under oxidizing conditions at a temperature of 1100°C, as indicated in the reaction equations (1) and (2):

However, as a result of this process, CaSO ₄ can be formed, leading to the need for additional processing of the solid.

Another method, carried out at 1100°C, which can be used to decompose CaS, uses CO ₂ according to the reaction equation:

It should be noted that the ratio of CO ₂ /CO≈30 is necessary for complete conversion of CaS (see Fig. 1(a)). Complete removal of CaS with a strong current of CO ₂ directed into the inlet of the reactor can lead to particle entrapment. When using a fluidized bed reactor, a strong current of CO ₂ can cause the emission of particles, which will not only affect the reaction kinetics, but also require large-scale filtration. Approximately 95% of the introduced CO ₂ is withdrawn from the reactor, which can cause serious damage to the environment. In addition, the CO ₂ method requires a significant amount of energy (≈42 kJ/g CaS, Fig. 1(b)). As a result, this method is not recommended for complete CaS conversion.

Under oxidation conditions, carbon, as an energy source, enters into the following two reactions:

where these reactions are exothermic. Calcium sulfide does not directly react with carbon. However, CaS can be converted to CaO and SO ₂ according to the reaction

Of the products of reaction (7), CaO and CO do not interact with CaS, while SO ₂ is able to react with CaS to form CaSO ₄ :

Thereafter, various reactions can take place between CaS and CaSO ₄ , resulting in the desired solid and gaseous products:

With enough carbon and controlled volumes of oxygen, carbon is able to produce the required amount of energy and, at the same time, the required amount of CO ₂ to initiate the CaS decomposition process. Therefore, in continuous flows of CaS and carbon, when oxygen is introduced, all reactions proceed due to the heat generated during the combustion of carbon.

Hereinafter, the present invention is explained in more detail using non-limiting examples and embodiments.

Pure CaS (99.99%, particle diameter 20<d _ρ <60 µm, ρ→2.6 g/cm ³ ) provided by Sigma-Aldrich, USA, and hard coal provided as carbon source by Recommunity Inc., Canada (particle diameter 20<d _ρ <60 μm), having a calorific value of 28289 kJ/kg and characterized by CHNS elemental analysis data (determination of the mass fractions of carbon (C), hydrogen (H), nitrogen (N) and sulfur (S)) and NAA (neutron activation analysis), which are presented below in tables 5 and 6, respectively.

Experiments on the decomposition of CaS by carbon oxidation were carried out in a new fluidized bed reactor with induction heating, which is able to withstand heating up to 1100°C at a rate of 200°C/s.

A schematic of a reaction apparatus 100 including this reactor 10 is shown in FIG. 2.

The reactor 10 includes a gas inlet 20 through which oxygen O ₂ (reacting gas) is fed into the reactor. The oxygen source 21, which is connected to the digital flow controller 22, fed by the power source 23, allows you to control the flow of oxygen O ₂ introduced into the reactor 10.

Similarly, reactor 10 includes a gas inlet 30 through which nitrogen N ₂ (carrier gas) is fed into the reactor. The nitrogen source 31, which is connected to the digital flow controller 32, fed by the power source 33, allows you to control the flow of nitrogen introduced into the reactor 10.

These gases leave the reactor through the outlet 40 provided with thermal and/or electrical insulation 41.

An analyzer 50 such as an FT-IR spectrometer (Fourier transform infrared spectroscopy) can be used to analyze the gases leaving the reactor to collect data from which, for example, infrared spectra, emission spectra, or absorption spectra are obtained. For this purpose, a source of nitrogen 51 is provided, which is connected to a digital flow controller 52 fed by a power source 53.

The reactor 10 consists of a tube, preferably made of aluminum, into which sand 11, phosphogypsum 12 and coal 13 are placed before the reaction takes place. Inside the reactor 10, vertical stainless steel rods 14 are attached to a plate. A metal coil 15, preferably a copper coil, is wound around the reactor tube 10 and is energized by the induction heating power supply 60. The reactor temperature is controlled by a temperature controller 70 . The reaction device 100 is preferably provided with a data acquisition system (DAS or DAQ) 80 for sampling signals of various experimental parameters and converting them into values suitable for computer processing.

Flows of nitrogen N ₂ and oxygen O ₂ at predetermined rates fluidize the material inside the tube so that it provides minimum fluidization conditions, which is achieved by synchronizing the flow rates and the temperature inside the reactor 10, measured using thermocouples. However, the ratio of nitrogen N ₂ and oxygen O ₂ is kept constant by the digital flow controller 22 .

The flow of an electric current (changing its direction at a very high frequency) in the metal coil 15 results in a magnetic field, so that the direction of said magnetic field also changes at a very high frequency. The stainless steel rods 14 are conductors in which an electric current is induced by a magnetic field. As a result, due to the Joule effect, heat is released inside the reactor tube 10, surrounded by a metal spiral 15.

Example. Optimal carbon to O ₂ ratio for CaS decomposition to SO ₂ and CaO

CO₂required for interaction with CaS and formation of SO₂ and CaO, obtained by burning carbon. Therefore, to ensure the production of sufficient CO₂, the amount of oxygen must be greater than stoichiometric. The low oxygen content results in the formation of CO, which does not interact with CaS. As a result, the volume of oxygen introduced into the reactor must at least correspond to the energy costs for all decomposition reactions. Then the resulting CO₂ capable of initiating the initial conversion of CaS to form SO₂ and CaO according to the following reaction equation:

On the other hand, too much oxygen introduced, in spite of the greater amount of energy released, will lead to the formation of large volumes of CO ₂ , which can lead to the production of CaSO ₄ instead of CaO and SO ₂ in accordance with the reaction equation:

Therefore, the amount of oxygen for this method must be optimized. From the FactSage™ simulation results shown in FIG. 3 (a) and (b), it can be seen that the heat released during the combustion of carbon at C/CaS≈2 and O ₂ /C≈6 ratios provides more than enough heat (≈10.5 kJ/g CaS), which can also be used to preheat the solid feedstock and oxidant gas prior to their introduction into the decomposition reactor.

In general, when the process is initiated, the following four series of reactions occur. The first stage includes the combustion of carbon and the oxidation of CaS in the presence of oxygen:

where reaction (6) is more probable than reaction (5) due to the high oxygen concentration in the reactor. Despite the regulation of the oxygen content, a side reaction (13) proceeds, which is unfavorable compared to the reaction (14). At the second stage, after the formation of CaSO ₄ , albeit in a negligibly small amount, the following series of reactions proceeds with the participation of not only CaSO ₄ and CaS, but also carbon, which ultimately contributes to the efficient formation of CaO and SO ₂ :

as well as

Thus, to remove unwanted CaSO ₄ there is no need to add an oxidation step in analogy with previous methods [4], since CaSO ₄ is converted either to CaS or to the desired products by the action of CO formed either as a result of the oxidation of carbon (reaction 5) or solid phase reaction (18) between CaSO ₄ and C. Therefore, CaS can be decomposed to CaO and SO ₂ by carbon oxidation at O ₂ /C≈6 C/CaS≈0.2. The energy required for all decomposition reactions is provided by the oxidation of carbon at 1100°C, so no external energy source is needed.

For experiments carried out in a fluidized bed reactor, the initial conditions of the starting materials are presented below in table 1. In addition, the results of all experiments are summarized in table 2, described in the next two sections.

Table 1. Input properties in fluidized bed experiments in the induction heated fluidized bed reactor shown in FIG. 2, at 1100°C. Input properties C/CaS 0.15 0.25 0.35 CaS (g) 5.22 4.80 4.44 C(g) 0.78 1.20 1.56 About ₂ /FG 2, 3, 4 2, 3, 4 2, 3, 4

It should be noted that coal was used as the carbon source in all experiments.

Table 2. CaS conversion, CaO yield, and CaSO ₄ yield (in each cell, respectively, from left to right) in a fluidized bed reactor at 1100°C under the initial conditions presented in Table 1 and the XPS analysis data presented in Tables 3 and 4. C/CaS O ₂ /CaS 2 3 four 0.15 0.83, 0.55, 0.22 0.25 0.85, 0.75, 0.02 0.93, 0.83, 0.08 0.96, 0.67, 0.28 0.35 0.85, 0.77, 0.03

Table 3 shows the effect of changing the C/CaS ratio at a constant O ₂ /CaS ratio (≈3, considered optimal).

At C/CaS≈0.15, the ratio O ₂ /CaS ≈3 results in the production of more CO ₂ than CO according to the following reaction equations:

In addition, due to an excess of O ₂ , the following two reactions also occur:

which subsequently leads to reactions from (15) to (22) with the formation of CaSO ₄ . As a result, at C/CaS≈0.15, the conversion of CaS is not high enough, because CaS is re-formed in some steps or does not completely react with CO ₂ due to lack of carbon.

A distinctive feature of this series of reactions is that CO is formed as a result of two reactions under any circumstances:

or even in reaction

leading to the production of CaSO ₄ . The formation of CO plays a significant role in achieving a high conversion of CaS, since the interaction of CaS with oxygen inevitably forms CaSO ₄ as a layer on CaS particles. In this case, the CO resulting from the reactions described above reacts with CaSO ₄ and converts it into CaO and SO ₂ . Thus, CaS particles could again come into contact with CO ₂ and O ₂ molecules and participate in decomposition reactions.

With an increase in the proportion of carbon in the feedstock from the ratio C/CaS≈0.15 to 0.25, and since the volume of oxygen is optimal for obtaining the amount of CO ₂ sufficient to interact with CaS, the conversion of CaS increases, while the formation of CaSO ₄ practically stops due to the rapid interaction of carbon and oxygen. The formation of CO ₂ at this stage would most favor the following reaction:

However, if the proportion of carbon in the feedstock was increased to C/CaS≈0.35, the ratio of O ₂ /C introduced would be less than the optimum value, resulting in more CO than CO ₂ being produced. Under such conditions, oxygen partially interacts with CaS and, along with CaSO ₄ , leads to the formation of CaO and SO ₂ . In accordance with the higher yield of CO compared to that at C/CaS≈0.25, in this case the yield of CaSO ₄ is minimal, since the large volume of formed CO contributes to the conversion of CaSO ₄ into other products.

Table 3. Effect of the C/CaS ratio on the composition of the solid residue in the fluidized bed reactor at 1100°C, determined by XPS, under the initial conditions presented in table 1 at the ratio O ₂ /CaS≈3. C/CaS CaS
(mol.%) CaO
(mol.%) _CaSO4
(mol.%) CaS Conversion (≈%) Yield of CaO (≈%) Yield CaSO ₄ (≈%) 0.15 7.2 66.2 26.5 83 55 22 0.25 2.1 89.2 8.6 93 83 eight 0.35 7.8 88.7 3.3 85 77 3

Table 4 shows the effect of changing the O ₂ /CaS ratio at a constant C/CaS ratio (≈0.25, which is considered optimal).

For the C/CaS≈0.25 ratio, the optimal O ₂ /CaS ratio is ≈3.

The lower amount of oxygen in the feed favors the formation of CO rather than CO ₂ .

It should be noted that CaS interacts directly with CO ₂ and not with CO.

As a result, partial reaction of CaS with CO ₂ promotes the formation of CaO and SO ₂ , but CaSO ₄ is also converted to the desired products by reaction with CO.

On the other hand, other amounts of oxygen, exceeding its amount at a ratio of O ₂ /CaS≈3, can lead to partial formation of CaSO ₄ according to the equation CaS+2O ₂ →CaSO ₄ .

However, the presence of a large amount of oxygen also leads to the decomposition of CaS to the desired products.

Thus, the total conversion of CaS at this stage increases, while the yield of CaSO ₄ also increases markedly.

In other words, at a high oxygen concentration, above the required optimal value in the starting material, the overall conversion of CaS increases, although this increase does not lead to the formation of the desired products.

Table 4. Effect of the O ₂ /CaS ratio on the composition of the remaining solids in the fluidized bed reactor at a temperature of 1100°C, determined by XPS, under the initial conditions presented in table 1, at C/CaS≈0.25. O ₂ /CaS CaS (mole %) CaO (mol.%) CaSO ₄ (mol.%) CaS Conversion (≈%) Yield of CaO (≈%) Yield CaSO ₄ (≈%) 2 9.4 88.2 2.4 85 75 2 3 2.2 89.2 8.6 93 83 eight four 1.0 69.8 29.2 96 67 28

The patterns of SO ₂ formation in all experiments carried out in a fluidized bed reactor are shown in FIG. four.

As expected from the experiments described above, the maximum amount of SO ₂ is formed at the ratios O ₂ /CaS≈3 and C/CaS≈0.25 (curve C2).

Higher amounts of either carbon or oxygen inhibit the formation of SO ₂ while lower amounts result in partial CaS conversion and thus less desirable gaseous products.

Table 5. Characterization of coal by CHNS-elemental analysis. These characteristics were used in simulations and in experiments in a fluidized bed. Compound Mass. % min Mass. % max C 71.5 72 H 4.8 5.0 N 1.7 1.8 S 1.3 2.2 O 8.5 8.9 Cl ppm 369 407

It should be noted that only the carbon content in coal was taken into account for the calculations.

Table 6. NAA data for pure coal. Element (ppm) Element (ppm) Element (ppm) Element (ppm) U 1.33 V 39 CD <0.25 Ni 17.4 Ti 633 Cl 341 Au <0.0018 Ag <0.42 sn <53 Al 13980 hf 0.76 sc 3.2 I <3 Ca 1379 Ba 101 Rb fourteen Mn 55 S 17455 Br 3.2 Fe 12982 mg 880 Se 3.5 As 26 An 26 Cu <64 Mo 46.4 Sb 0.61 co 6.3 In <0.01 hg 0.059 W 0.48 K 2326 Si <40000 Th <1 Zr 39 ln eight Na 342 Cr eighteen Cs 3.1

Literature

[1] U.S. Patent 6,024,932.

[2] U.S. Patent 4,503,018.

[3] D. Lynch and J. Elliott, “Analysis of the oxidation reactions of CaS,” Metallurgical Transactions B, vol. 11, no. 3, pp. 415-425, 1980.

[4] US Patent 543,939.

[5] R. Shabou, M. Zairi, and H. Ben Dhia, “Characterisation and environnemental impacts of olive oil wastewater disposal,” Environmental Technol., vol. 26, no. 1, pp. 35-45, 2005.

[6] R. Torres-Ordonez, J. Longwell, and A. Sarofim, “Physical transformations during solid calcium sulfide oxidation,” Energy and Fuels, vol. 3, no. 5, pp. 595-603, 1989.

[7] D. Lynch and J. Elliott, “Analysis of the oxidation reactions of CaS,” Metallurgical Transactions B, vol. 11, no. 3, pp. 415-425, 1980.

[8] US Patent 3,087,790.

[9] C. Cardenas-Escudero, V. Morales-Flarez, R. Parez-Lapez, A. Santos, and L. Esquivias, “Procedure to use phosphogypsum industrial waste for mineral CO ₂ sequestration,” J. Hazard. Mater., vol. 196, pp. 431-435, 2011.

[10] S. Wu, M. Uddin, S. Nagamine, and E. Sasaoka, “Role of water vapor in oxidative decomposition of calcium sulfide,” Fuel, vol. 83, no. 6, pp. 671-677, 2004.

Claims (14)

1. The method of decomposition of calcium sulfide (CaS) to calcium oxide (CaO) and sulfur dioxide (SO ₂ ), characterized in that it includes: - ensuring the presence of a reactor containing calcium sulfide and a source of carbon, - oxidation of the carbon source with oxygen (O ₂ ) to generate carbon dioxide (CO ₂ ), - introduction into the interaction of calcium sulfide with the specified carbon dioxide to obtain calcium oxide (CaO), sulfur dioxide (SO ₂ ) and carbon monoxide (CO) according to the following reaction: CaS + 3CO ₂ → CaO + SO ₂ + 3CO, where the content of oxygen and carbon at the stage of oxidation is chosen so that: (i) the C/CaS weight ratio was from 0.15 to 0.35 and (ii) the weight ratio O ₂ /C was from 5 to 25. 2. The method of claim 1 wherein the C/CaS ratio is 0.25. 3. The method according to claim 1 or 2, wherein the O ₂ /C ratio is 8. 4. The method according to any one of paragraphs. 1-3, in which in the stage of oxidation the reactor is heated to a temperature in the range from 900°C to 1200°C. 5. The method according to claim 4, wherein the reactor is heated by an induction heating method. 6. The method according to any one of paragraphs. 1-5, in which the carbon source includes at least one of: coal, coke, charcoal and shale oil. 7. The method according to any one of paragraphs. 1-6, in which the reactor is a continuous reactor, such as a fluidized bed reactor or a rotary kiln, including an inlet (20) and an outlet (40) for passing an O ₂ flow through calcium sulfide and a carbon source.

Images