Calcium sulfide decomposition process

The present invention relates to a process for decomposing calcium sulfide (CaS) into calcium oxide (CaO) and sulfur dioxide (SO2), comprising:—providing a reactor containing calcium sulfide and a source of carbon,—oxidizing the source of carbon so as to generate carbon dioxide (CO2),—reacting the calcium sulfide with said carbon dioxide so as to produce carbon oxide (CaO), sulfur dioxide (SO2) and carbon monoxide (CO) according to the following reaction: CaS+3CO2˜CaO+SO2+3CO wherein the oxygen and carbon contents in the oxidation step are chosen such that: (i) the mass ratio C/CaS is comprised between 0.15 and 0.35 and (ii) the mass ratio O2/C is comprised between 5 and 25.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C § 371 of International Application No. PCT/MA2018/000018 filed Nov. 22, 2018, which claims priority from European Application No. 17203183.3 filed Nov. 22, 2017, all of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for decomposing calcium sulfide into calcium oxide and sulfur dioxide using carbon oxidation under the controlled oxygen stoichiometry for the total carbon dioxide and energy requirements for the decomposition process.

BACKGROUND OF THE INVENTION

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

Disposing of CaS into the environment can result in H2S release from the reaction between CaS and water [5].

Converting CaS to CaSO4or CaO under oxidizing conditions is one of the solutions, however, there are some limitations. First, under oxidizing circumstances, CaS yields two different solid products:
CaS+1.5O2→CaO+SO2(1)
CaS+2O2→CaSO4(2)

Whether reaction (1) or (2) occurs depends on the reaction conditions [6]. For example, at high O2concentration and high temperature, CaSO4is formed. If a layer of CaSO4covers the outside of CaS particles, then, CaS is not converted to CaSO4until CaSO4is decomposed into CaO, SO2and O2at high temperature (above 1200° C.) [7]:
CaSO4→CaO+SO2+0.5O2(3)

Using carbon dioxide for PG decomposition as the reacting gas with CO2/CO≈5−6 has been shown to remarkably reduce the amounts of CaS [8-10].

Other methods considered an oxidation step for CaS conversion to CaSO4while it was combined with a reduction step to reconvert CaSO4to CaO and SO2[3, 4]. This process, however, has some drawbacks. For example, only an outer layer of CaS particles is oxidized to CaSO4and subsequently reduced to CaO, yielding a low CaS conversion. Therefore, a large particle recirculation rate between the oxidation and reduction reactors is required. Nevertheless, the proposed processes require a very high heating rate to maintain the reactor temperature between 950-1200° C.

Secondly, another limitation making CaS decomposition not economically viable by the above mentioned processes is the considerably high energy demands. All decomposition reactions occur above 1100° C., hence providing the heat required to keep the reactor at this temperature is remarkably significant.

BRIEF DESCRIPTION OF THE INVENTION

A goal of the invention is to provide a process for decomposing CaS in which not only the required energy is provided, but also favorable CaS conversion to CaO and SO2occurs.

Accordingly, the invention provides a process for decomposing calcium sulfide (CaS) into calcium oxide (CaO) and sulfur dioxide (SO2). The process is primarily characterized in that it comprises:providing a reactor containing calcium sulfide and a source of carbon,oxidizing the source of carbon so as to generate carbon dioxide (CO2),reacting the calcium sulfide with said carbon dioxide so as to produce calcium oxide (CaO), sulfur dioxide (SO2) and carbon monoxide (CO) according to the following reaction:
CaS+3CO2→CaO+SO2+3CO
wherein the oxygen and carbon contents in the oxidation step are chosen such that:(i) the mass ratio C/CaS is comprised between 0.15 and 0.35 and(ii) the mass ratio O2/C is comprised between 5 and 25.

Calcium oxide can be recycled to the in-bed removal of the H2S process while SO2can be used for sulfuric acid production.

The oxidation reaction of carbon (C) into carbon monoxide (CO) then carbon dioxide C02is exothermic.

The process of the invention uses the carbon dioxide CO2and the energy produced by this exothermic reaction to carry out the reaction of decomposition of the calcium sulfide CaS, which is an endothermic reaction.

An essential condition is to have over-stoichiometric conditions of O2and sub-stoichiometric conditions of C, so as to favor the oxidation of C into CO2and to limit the reaction of CaS with other species than carbon dioxide CO2. This condition is achieved with mass ratios C/CaS comprised between 0.15 and 0.35 and O2/C comprised between 5 and 25.

According to other optional characteristics of the invention:the ratio C/CaS is preferably equal to 0.25;the ratio O2/C is preferably equal to 8;in the oxidation step, the reactor is heated to a temperature comprised between 900° C. and 1200° C., preferably around 1100° C.;the reactor is advantageously heated by induction. However, it can also be heated by another external source such as a source of electricity;the source of carbon comprises 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 comprises an inlet and an outlet for flowing O2through the calcium sulfide and the source of carbon.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention relates to CaS decomposition by carbon (e.g., coal, coke, charcoal, or any other source of carbon) oxidation (O2above stoichiometry conditions) to remove any undesired CaS produced from PG decomposition reactions or any other processes and convert it to CaO and SO2.

Calcium sulfide decomposition can be performed in an oxidative environment at 1100° C. as mentioned in reactions (1) and (2):
CaS+1.5O2→CaO+SO2(1)
CaS+2O2→CaSO4(2)

However, it could lead to CaSO4formation requiring further treatment of the solid.

Another process at 1100° C. that can be used for CaS decomposition employs CO2according to:
CaS+3CO2→CaO+3CO+SO2(4)

Note that this requires CO2/CO≈30 for a complete CaS conversion (seeFIG. 1(a)). A complete CaS removal by high CO2flow in the inlet of the reactor can cause particle entrainment. When using a fluidized bed reactor, high CO2flow can cause the particle discharge, which not only affects the reaction kinetics but also requires massive filtration. About 95% of the injected CO2is discharged from the reactor, which can yield serious environmental impacts. Moreover, the process of using CO2requires considerable amounts of energy (≈42 kJ/g CaS,FIG. 1(b)). As a result, this method is not recommended for a complete CaS conversion.

Carbon as the source of energy under oxidizing conditions undergoes the two following reactions:
C+0.5O2→CO  (5)
C+O2→CO2(6)

where these reactions are exothermic. Calcium sulfide does not directly react with carbon. However, CaS can be converted to CaO and SO2according to
CaS+3CO2→CaO+SO2+3CO  (7)

Among the products of reaction (7), CaO and CO do not react with CaS while SO2can react with CaS and produce CaSO4:
CaS+2SO2→CaSO4+2S  (8)

Then, various reactions can occur between CaS and CaSO4, thereby leading to desired solid and gas products:
3CaSO4+CaS→4CaO+4SO2(9)
CaSO4+3CaS→4CaO+4S  (10)
CaS+3CaSO4+4CO2→4CaCO3+4SO2(11)
CaS+3SO3→CaO+4SO2(12)

With sufficient amounts of carbon and controlled amounts of oxygen, carbon can produce the required amounts of energy and, simultaneously, the required amount of CO2to initiate the CaS decomposition process. Therefore, in continuous streams of CaS and carbon, while oxygen is injected, all reactions occur with the heat provided by carbon combustion.

The present invention is illustrated in further detail below by non-limiting examples and embodiments.

Pure CaS (%99.99, particle diameter 20<dp<60 μm, ρ≈2.6 g/cm3) was provided by Sigma-Aldrich, USA, and coal (particle diameter 20<dp<60 μm) as a source of carbon was provided by Recommunity Inc., Canada, with the heating value of 28,280 kJ/Kg and characterized by CHNS (determination of the mass fractions of carbon (C), hydrogen (H), nitrogen (N) and sulfur (S)) and NAA (Neutron Activation Analysis) presented in tables 5 and 6 below, respectively.

CaS decomposition experiments by carbon oxidation were performed in a novel, heat-induced fluidized bed reactor that was able to be heated up to 1100° C. at 200° C./s.

The scheme of a reaction device100comprising this reactor10is illustrated inFIG. 2.

The reactor10comprises a gas inlet20from which oxygen O2(reacting gas) is supplied to the reactor. An oxygen source21in communication with a digital flow controller22powered by a power supply23allows to control the flow of oxygen O2being introduced into the reactor10.

Similarly, the reactor10comprises a gas inlet30from which nitrogen N2(carrier gas) is supplied to the reactor. A nitrogen source31in communication with a digital flow controller32powered by a power supply33allows to control the flow of nitrogen being introduced into the reactor10.

The gases leave the reactor via the gas outlet40which comprises a thermal and/or electrical insulation41.

An analysis device50, such as an FTIR spectrometer (Fourier Transform InfraRed) can be used to analyze the gases leaving the reactor, in order to collect data from which infrared spectrum, emission spectrum or absorption spectrum, for example, are obtained. A nitrogen source51in communication with a digital flow controller52powered by a power supply53are provided for this purpose.

The reactor10is formed of a tube, preferably made of alumina, loaded with sand11, and phosphogypsum12and coal13, prior to the reaction. Within the reactor10, stainless steel vertical rods14are fixed to a plate. A metal coil15, preferably a copper coil, is wrapped around the tube of the reactor10and supplied with energy by an induction heating power supply60. The temperature is the reactor is controlled by a temperature controller70. The reaction device100is advantageously provided with a Data Acquisition System80(DAS or DAQ) for sampling signals of various experimental parameters and converting them into computer processable values.

The flow of nitrogen N2and oxygen O2with predetermined rates fluidize the material inside the tube so that it provides the minimum fluidization conditions, which is achieved by synchronizing the flow rates and the temperature inside the reactor10measured by thermocouples. However, the ratio between nitrogen N2and oxygen O2is kept constant by the digital flow controller22

The current flow (which changes direction with a very high frequency) in the metal coil15induces a magnetic field so that the direction of said magnetic field also changes with a very high frequency. The stainless steel rods14act as conductors where the current is induced by the magnetic field. As a consequence, heat is released by Joule effect inside the tube of the reactor10surrounded by the metal coil15.

Example—Optimum carbon and O2for CaS decomposition to SO2and CaO

The CO2required to react with CaS and produce SO2and CaO is provided by carbon combustion. Therefore, oxygen should be above stoichiometry to assure enough amounts of CO2production. Low oxygen content yields CO formation, which does not react with CaS. As a result, the oxygen amount injected into the reactor should at least fulfill energy requirements for all decomposition reactions. Then, the produced CO2can initiate CaS initial conversion to SO2and CaO as follows:
CaS+3CO2→CaO+SO2+3CO  (7)

On the other hand, if the injected oxygen is too high, although the released energy would be higher, it yields large amounts of CO2production, which can lead to CaSO4formation instead of CaO and SO2according to the following reaction:
CaS+2O2→CaSO4(13)

Therefore, oxygen should be optimized for this process. FactSage™ simulation results provided inFIG. 3(a) and (b)show that the heat provided by carbon combustion when C/CaS≈0.2 and O2/C≈6 provides ample heat (≈10.5 kJ/g CaS), which also can be used for preheating the solid feed and the oxidizing gas before the decomposition reactor.

In general, the following four series of reactions take place in order as the process starts. Step one includes carbon combustion and CaS oxidation in the presence of oxygen:
C+0.5O2→CO  (5)
C+O2→CO2(6)
CaS+1.5O2→CaO+SO2(14)
CaS+2O2→CaSO4(13)

in which reaction (6) with respect to reaction (5) is the most probable phenomenon due to the high concentration of oxygen in the reactor. Although the oxygen is under control, the side reaction (13) occurs unfavorably compared to reaction (14). In step two after CaSO4production, however negligible, the following series of reactions occur not only between CaSO4and CaS, but also with carbon, which ultimately helps the favorable production of CaO and SO2:
CaSO4+CO→CaO+SO2+CO2(15)
CaSO4+4CO→CaS+CO2(16)
2CaSO4+C→2CaO+2SO2+CO2(17)
CaSO4+4C→CaS+4CO  (18)

Thus, for unfavorable CaSO4removal, it is not necessary to add an oxidizing step similar to old processes [4] because CaSO4is converted to either CaS or desired products by the CO produced from either carbon oxidation (reaction 5) or CaSO4—C solid-solid reaction (18). Consequently, CaS can be decomposed to CaO and SO2by a carbon oxidation process where O2/C≈6 and C/CaS≈0.2. The energy required for all the decomposition reactions is provided with carbon oxidation at 1100° C., thereby requiring no external energy.

For the experiments performed in the fluidized bed reactor, initial conditions of the feed are summarized in Table 1 below. The results of all experiments are also summarized in Table 2, which is described in the next two sections.

Note that coal was used as a source of carbon in all experiments.

Table 3 shows the effect of C/CaS variation with constant O2/CaS (≈3, which is considered to be optimum).

When C/CaS≈0.15, O2/CaS≈3 yields more CO2production than CO according to the following reactions:
C+0.5O2→CO  (5)
C+O2→CO2(6)

Moreover, according to the O2abundance, the two following reactions also occur:
CaS+1.5O2→CaO+SO2(14)
CaS+2O2→CaSO4(13)

which result in CaSO4formation undergoing reactions (15) to (22) afterwards. As a result, CaS conversion is not high enough when C/CaS≈0.15 since CaS is reproduced at some stages or does not fully react with CO2because of a carbon shortage.

The remarkable feature of this series of reactions is that CO is produced from two reactions under any circumstances:
CaS+3CO2→CaO+SO2+3CO  (7)
C+0.5O2→CO  (5)

or even from
CaSO4+4C→CaS+4CO  (18)

providing the formation of CaSO4. The CO formation plays a significant role in obtaining high CaS conversion, because when CaS reacts with oxygen, CaSO4formation is inevitable as a layer around CaS particles. The CO production according to the above mentioned reactions, in this case, reacts with CaSO4and converts it to CaO and SO2. Therefore, CaS particles would again find the opportunity to be in contact with CO2and O2molecules and undergo decomposition reactions.

By increasing the carbon in the feed from C/CaS≈0.15 to 0.25 and since oxygen is at the optimum value required to produce a sufficient amount of CO2to react with CaS, the conversion of CaS increases while CaSO4formation is almost halted by the fast reaction of carbon and oxygen. The CO2formation at this stage would favor the following reaction to its maximum extent:
CaS+3CO2→CaO+SO2+3CO  (7)

However, when carbon in the feed increases to C/CaS≈0.35, the injected O2/C would be under the optimum value, thereby producing more CO instead of CO2. Under these circumstances, the oxygen partially reacts with CaS and produces CaO and SO2along with CaSO4. According to the higher CO formation compared to that of C/CaS≈0.25, CaSO4yield is lowest in this case because a high amount of CO formation favors CaSO4conversion to the other products.

TABLE 3Effects of C/CaS ratio on the remaining solid composition ina fluidized bed reactor at 1100° C. by XPS analysis with theinitial conditions summarized in Table 1 when O2/CaS ≈ 3.CaSCaOCaSO4CaSCaOCaS04conversionyieldyieldC/CaS(mol. %)(mol. %)(mol. %)(≈%)(≈%)(≈%)0.157.266.226.58355220.252.189.28.6938380.357.888.73.385773

Table 4 shows the effects of O2/CaS variations with constant C/CaS (≈0.25, which is considered to be optimum).

For C/CaS≈0.25, the optimum O2/CaS is ≈3.

Any lower amounts of oxygen in the feed favor CO production rather than CO2.

Note that CaS does not react with CO directly but with CO2.

As a result, partial CaS reaction with CO2favors CaO and SO2production, however, CaSO4is also converted to the desired products as a result of a reaction with CO.

On the other hand, any other amounts of oxygen larger than that of O2/CaS≈3 would result in the partial production of CaSO4according to CaS+2O2→CaSO4.

However, a large oxygen presence yields CaS decomposition to desired products as well.

Thus, CaS total conversion increases at this stage while CaSO4yield also noticeably increases.

In other words, at high oxygen values more than the optimum required in the feed, CaS total conversion increases, although this increase does not culminate into the formation of desired products.

TABLE 4Effects of O2/CaS ratio on the remaining solid composition ina fluidized bed reactor at 1100° C. by XPS analysis withthe initial conditions summarized in Table 1 when C/CaS ≈ 0.25.CaSCaOCaSO4CaSCaOCaSO4conversionyieldyieldO2/CaS(mol. %)(mol. %)(mol. %)(≈%)(≈%)(≈%)29.488.22.48575232.289.28.69383841.069.829.2966728

The SO2formation patterns from all the performed experiments in the fluidized bed reactor are shown inFIG. 4.

As expected based on the above experiments, the maximum SO2production occurs when O2/CaS≈3 and C/CaS≈0.25 (curve C2).

Higher amounts of either carbon or oxygen hinder SO2formation while lower amounts result in partial CaS conversion and, thus, less desired gaseous products.

Note that in simulations, only the carbon content of coal was considered for calculations.

REFERENCES