Yerubay A.A., Bishimbayev V.K., Shevko V.M.

Ph.D student, Dr of Technical Sciences.

M.Auezov South-Kazakhstan State University

Shymkent, Kazakhstan

Thermodynamic modeling of interaction of calcium sulfate with carbon oxide (II)

For processing large-tonnage production wastes of extraction phosphoric acid that is phosphogypsum some variants of producing phospholime from it have been offered.[1-3] It is shown by semi-industrial tests, that the degree of phosphogypsum decomposition in rotating tube furnaces and fluidized-bed furnaces comprises 94- 97%. Formed by this phospholime contains 58-76% of СаО. Process of phospholime production from phosphogypsum now is not yet fully perfect, as the content of CaS and CaSO₄ in the product comprises accordingly 0,6-1,4 and 0,8-4,9%.

The task of the present work was to determine optimum parameters of CaSO₄ decomposition in presence of CO as applied to agglomeration processing of phosphogypsum, providing incomplete oxidation of natural coal carbon.

Research was carried out by the method of thermodynamic modeling and a method of mathematical planning with further optimization of technological parameters on the computer. For thermodynamic modeling the program complex “Astra” developed in N.E. Bauman Moscow High Technical School was used. Fundamental principle of a maximum entropy is put in the basis of algorithm.[4,5] This method gives a unique opportunity of the generalized description of any high-temperature state with the help of thermodynamics fundamental laws, irrespective of conditions and ways of achieving equilibrium. The database of the program complex Astra contains information on thermodynamic properties of 5547 individual substances systematized in Institute of high temperatures of the Russian Academy of Sciences and the USA National bureau of standards.[6,7] The information received by means of the program complex “Astra” allows to determine the equilibrium degree of initial system elements transition into products of reaction, and also composition of gas and condensed phases. Influence of temperature (Т from 500 to 1800К) was investigated and distinct from [8,9] influence of a number of moles in system CaSO₄ – nCO (n from 0,2 up to 2,5) on the equilibrium degree of Са distribution, including its distribution in СаО was studied.

The carried out research has shown, that the interaction happens with participation of 22 substances: CaSO₄, CaCO₃, CaO, CaS, CO, CO₂, COS, CS₂, S, S₂, S₃, S₄, S₅, S₆, S₇, S₈, SO₂, SO₃, SO, S₂O, O₂, O in system CaSO₄ – nCO depending on temperature and number of CO moles. In figure 1 the information about influence of temperature and number of CO moles on the degree of (α) calcium distribution is given from which it follows that the basic calcium containing compounds in the system are CaSO₄, CaCO₃, CaS and CaO. From (Figure 1) it follows that in examined systems irrespective of CO moles number in a temperature interval 500 – 600K CaCO₃ is being formed. The degree of formation (for example at Т = 500К) CaCO₃ grows with increase of CO moles number in the following sequence:

n	0,2	0,45	1,0	1,55	1,8	2,5	3,0
αCaCO3, %	6,62	14,88	33,30	51,43	58,44	89,97	98,82

Thus dependence looks like:

(1)

for which the criterion of suitability of approach R² (factor of determination) comprises 0,9997.

Figure. 1. Influence of temperature and CO moles number (n) in system CaSO₄ – nCO on the degree of calcium distribution (α) at pressure 0,1MPa. Where: 1 – CaCO₃; 2 – CaSO₄; 3 – CaO; 4 – CaS. In figures a), b), c), d), e), f), g) CO moles number (n) varies as follows: а) – n=2,5; b) – n=1,8; c) – n=1,55; d) – n=1,0; e) – n=0,45; f) – n=0,2

CaS starts to be formed in systems at Т ≥ 600K. Moreover, with the increase of CO moles number the degree of CaS formation and temperature area of its existence grow. So α_maxCaS grows as follows:

n	0,2	0,45	1,0	1,55	1,8	2,5	3,0
αmaxCaS, %	4,99	11,18	24,80	38,56	44,7	63,19	73,6

Dependence looks like:

. (2)

СаО is formed in system at T > 1100K. The temperature of CaO formation start (Т_startСаО) depends on number of CO moles in the system, changing (decreasing) as follows:

n	0,2	0,45	1,0	1,55	1,8	2,5	3,0
ТstartСаО, K	1237,3	1207,5	1166,2	1142,6	1136,5	1126,1	1116,3

Thus dependence looks like:

. (3)

Figure. 2. Influence of temperature and number of carbon oxide moles in the system CaSO₄ – nCO on the degree of СаО formation (α) at Р=0,1МPa,where CO moles number (n) are specified at modeling as follows 1-n=0,2; 2-n=0,45; 3-n=1,0; 4-n=1,55; 5-n=1,8; 6-n=2,5.

Appreciable formation of СаО in systems is being observed at T > 1300K. Moreover, with the deviation of CO moles number from 1 it results in reduction of the degree of СаО formation from CaSO₄. At n = 0,2 ÷ 0,45 the process is being blocked and it does not get any development in a temperature interval 1400 – 1700K (Figure 2). At constant temperature (for example 1600К), close to technological temperature of agglomeration the influence of CO moles number on αСаО has an extreme character:

n	0,2	0,45	1,0	1,55	1,8	2,5	3,0
αCaO, %	19,94	44,85	96,54	91,65	83,69	63,50	32,84

Influence CO on СаО is described by the equation:

. (4)

The research was continued employing rotortable plans of the second order to search optimum parameters of СаО production from CaSO₄ in presence of CO.[10] Temperature (Т,К) and number of CO moles (n) as applied to reaction CaSO₄+CO=CaO+CO₂+SO₂ were used as independent factors. The degree of СаО (αСаО, %) formation was the parameter of optimization. The matrix of conducting the research and results of СаО formation, received by means of program complex Astra, are given in (Table 1) Using the program developed in SKSU (South-Kazakhstan State University) it was defined, that the equations of regression αСаО=f (T, nСО) in the coded and natural scale have accordingly the following types:

(5)

(6)

The received equation is adequate, because the tabular value of Fisher’s criterion (6,59) is bigger than the real value of this criterion (5,59).[11] On the basis of the received equation of regression using the program complex MathCAD – 14 the volumetric picture of the change of response surface (Figure 3) and its horizontal sections (Figure 4) was constructed.^[11] As follows from (Figure 3), the surface of the response has an extreme character. Moreover, the maximum of СаО formation should be expected in the technological area limited by figure ABC (Figure 4), that is, at Т from 1490 up to 1690K and n from 0,1 up to 1,49. The composition of gas phase of the system in this case (at n = 1,1) depends on temperature (Table 2). In technological area (1500 – 1700К) the gas phase contains 49,79 – 49,84% СО₂ and 49,87 – 49,88% SO₂.

Table 1. Matrix of planning research and results of the research of СаО production from CaSO₄ – nCO system at Р = 0,1MPa

№ No.	Independent variables				αCaO, % (researched).	αCaO, % (calculated).
	Coded type		Natural type
	Х1	Х2	n	Т, K
1.	-1	-1	0,45	1446	46,8	48,08
2.	+1	-1	1,55	1446	84,6	85,95
3.	-1	+1	0,45	1654	47,8	49,09
4.	+1	+1	1,55	1654	96,0	97,36
5.	+1,414	0	1,8	1700	78,0	76,63
6.	-1,414	0	0,2	1400	17,2	15,72
7.	0	+1,414	1,0	1700	99,8	98,46
8.	0	-1,414	1,0	1400	91,3	89,68
9.	0	0	1,0	1550	99,4	98,54
10.	0	0	1,0	1550	99,0	98,54
11.	0	0	1,0	1550	98,6	98,54
12.	0	0	1,0	1550	98,7	98,54
13.	0	0	1,0	1550	97,0	98,54

To define the reasons of the low degree of СаО formation at small (0,5) and high (2) CO moles number we also determined the distribution of S, O and C except Ca. On the basis of this the equations of CaSO₄ interaction with various quantity of CO are given in (Tables 3 and 4).

Figure. 3. Influence of CO moles (n) number and temperature (T) on the form of the response surface - αСаО, %

Figure. 4. Horizontal section views of the response surface – αСаО. Figures on lines – αСаО, %.

Table 2. Influence of temperature on composition of the gas phase of CaSO₄ – CO

system at Р = 0,1MPa

Content of form. %	Temperature, K
	1200	1300	1400	1500	1600	1700	1800
CO2	97,28	88,61	50,69	49,79	49,91	49,84	49,67
CO	1,22	1,61	1,27	0,44	0,18	0,15	0,28
SO2	1,47	9,67	47,49	49,72	49,88	49,92	49,87
SO	0,9*10^-3	0,011	0,079	0,03	0,015	0,013	0,028
S2	0,008	0,069	0,42	0,8*10^-3	-	-	-
CaS	0,012	0,021	0,022	0,2*10^-3	-	-	-
S	-	-	0,8*10^-4	0,2*10^-4	-	-	-
SO3	-	-	0,2*10^-3	0,001	0,01	0,017	0,019
O2	-	-	-	0,3*10^-4	0,003	0,053	0,13

Table 3. The chemical equations of interaction in system CaSO₄ – 0,5CO

at Р = 0,1MPa

Temperature, K	Equations
500	CaSO4+0,5CO =
700	Product 1 =
1200	Product 2 =
1400	Product 3 =
1700	Product 4 =

Table 4. The chemical equations of interaction in system CaSO₄ – 2CO

at Р = 0,1MРa

Temperature, К	Equations
500	CaSO4+2CO =
700	Product 1 =
1200	Product 2 =
1400	Product 3 =
1700	Product 4 =

It follows from comparison of given (Tables 3 and 4), that not full CaO formation from system CaSO₄ – nCO at surplus of CO is linked with formation of irreducible CаS. When there is a lack of CO production CaO is restrained by existence of CaSO₄ in wide range.

If to lower pressure in the zone of reaction it is possible to reduce Т_maxСаО in the system CaSO₄ – CO. So, at Р = 0,01МРа Т_maxСаО = 1400K, at Р = 0,001MРa –1300K. At presence of SiO₂ in phosphogypsum up to 17%, Т_maxСаО with formation of CaSiO₃ at Р = 0,001МРa makes 1000K. For phospholime this process is negative, as it reduces СаО activity. However for the subsequent production of ferrumsilicocalcium from agglomerate, presence of SiO₂ positively influences CaSO₄ decomposition.

On the basis of the carried out research of interaction CaSO₄ with CO it is possible to draw the following conclusions:

- originally in system at Т = 500K СаО is formed, and the degree of its formation grows at increase of CO moles number;

- restoration of CaSO₄ to СаО goes through a stage of CaS formation;

- the degree of transition CaSO₄ to CaS grows at increase in CO system;

- Т_startСаО decreases from 1237,3K to 1116,3K at increasing the number of CO moles from 0,2 to 3,0;

- Т_maxСаО (> 99,5%) at Р = 0,1MРa is being observed in a temperature interval 1500 - 1700K number of CO moles = 1,1 – 1,5;

- influence of temperature and number of CO moles on αСаО from CaSO₄ has an extreme character;

- reduction of pressure from 0,1 to 0,001MРa and input in system SiO₂ positively influence CaSO₄ decomposition, allow to reduce Т_maxСаО to 1000K.

References

1. V.V. Ivanitskij, P.V. Klassen, A.L. Novikov Phosphogypsum and its use. Moscow: Chemistry, 1990. 224 p.

2. L.M. Razdorskih, S.V. Hrjashev, N.L. Solodjankina etc. Industrial tests of process of thermochemical decomposition phosphogypsum on calcium oxide and sulphurous gas in the furnace of a boiling layer. II Works of SRIFI (Scientific institute of fertilizers and insect fungicides) «Use of phosphogypsum in a national economy». Moscow 1983. Issue 243. p. 57-67.

3. Technology of phosphoric and complex fertilizers / Under edition of S.D. Evenchika and A.A. Brodskogo. Moscow: Chemistry 1987. 464 p.

4. G.B. Sinjarev, N.A. Vatolin etc. Application of the COMPUTER for thermodynamic calculations of metallurgical processes. Moscow: Nauka, 1982. 263 p.

5. B.G. Trusov, S.A. Barak etc. the Automated system of the thermodynamic data and calculations of equilibrium conditions // Mathematical methods of chemical thermodynamics. Novosibirsk 1982. p. 213-219.

6. JANAF Thermochemical tables, 2^nd edition. NSROS-NBS US Gov Printing Office, Washington, DC, 1971. 1141 p.

7. Thermodynamic properties of individual substances. A reference book by media in L.V. Gurvich, I.V. Vejtsa others in 4 volumes. Moscow: Nauka, 1982. 559 p.

8. V.M. Shevko, E.YA. Kalashnikov, V.A. Каpsaljamov Opportunity reception СаО at interaction СаSО₄ with Н₂, C, CO, СН4 // Works of the international scientific – practical conference «Аuezov readings – 4» and the third scientific conference of high schools of Southern region. Shymkent 2004. p. 99-103.

9. E.YA. Kalashnikov. Development of technology of reception calcium carbide from phosphogypsum. A dissertation of Candidate of technical sciences, Shymkent, 172 p. 2010.

10. S.A. Ahnazarov, B.V. Kafarov Method of optimization of experiment with the chemical industry. Moscow: the higher School, 1978. 319 p.

11. V.F.Ochkov. Mathcad 14 for students and engineers. St.-Petersburg.: BHV-Petersburg, 2007. 368 p.