This invention relates to methods of desulfurizing fluid materials and, more particularly, to a method of externally desulfurizing fluids such as molten iron and steel, stack gases, coal gases, coal liquification products, and the like using rare earth compounds, including such materials as rare earth oxides, rare earth fluorocarbonates or rare earth oxyfluorides, in an essentially dry process.
The term "rare earth", as used herein, includes the lanthanide rare earth elements having atomic numbers from 57 to 71, inclusive, and the element yttrium, atomic number 39, which is commonly found in rare earth concentrates and acts similarly to the rare earths in chemical separations.
As indicated above, this method is adapted to the desulfurization of essentially any fluid material. We shall, however, discuss the method in connection with two of the most pressing problems of desulfurization which industry presently faces; i.e., the desulfurization of molten iron and steel baths and the desulfurization of stack gases.
External desulfurization of molten iron and steel has been practiced for quite some time. It is a recognized, even necessary, practice in much of the iron and steel produced today. In current practices for the desulfurization of iron and steel it is common to add magnesium metal, magcoke, calcium oxide, calcium carbide or mixtures of calcium oxide and calcium carbide as the desulfurizing agent. Unfortunately, there are serious problems, as well as major cost items involved, in the use of all of these materials for desulfurization. Obviously, both calcium oxide and calcium carbide must be stored under dry conditions, since calcium oxide will hydrate and calcium carbide will liberate acetylene on contact with moisture. Magnesium is, of course, highly incendiary and must be carefully stored and handled. There are also further problems associated with the disposal of spent desulfurization slags containing unreacted calcium carbide.
We have found that these storage, material handling and disposal problems are markedly reduced by using rare earth compounds in a low oxygen content bath of molten iron or steel. The process is adapted to the desulfurization of pig iron or steel where carbon monoxide, evolved by the reaction where carbon is used as a deoxidizer, is diluted, either with an inert gas such as nitrogen or by vacuum degassing the melt, in order to reduce the oxygen potential and thereby increase the efficiency of the reaction by reducing the likelihood of forming oxysulfides. The principle may also be used for desulfurizing stack gases from boilers, etc., as shall be discussed in more detail hereafter.
In desulfurizing molten iron and steel in the practice of this invention, it is preferable to follow the steps of reacting rare earth oxide, rare earth oxyfluorides, rare earth fluorocarbonates and mixtures thereof (including bastnasite concentrates), in the presence of a deoxidizing agent, with the sulfur to be removed, to form one of the group consisting of rare earth sulfide and rare earth oxysulfide and mixtures thereof.
Preferably, hot metal is treated in a ladle or transfer car with rare earth compounds, by the simple addition and mixing of the rare earth compounds, by an injection technique in which the rare earth compounds are injected into the molten bath in a carrier gas such as argon or nitrogen, or by the use of an "active lining"; i.e., a rare earth compound lining in the vessel. In any case, the chemical reactions involved may be shown as follows, where the term RE indicates "rare earth": EQU 2CeO.sub.2(s) +[C]=Ce.sub.2 O.sub.3(s) +CO.sub.(g) ( 1) EQU RE.sub.2 O.sub.3(s) +[C]+[S].sub.1w/o =RE.sub.2 O.sub.2 S.sub.(s) +CO.sub.(g) ( 2)
and EQU RE.sub.2 O.sub.2 S.sub.(s) +2[C]+2[S].sub.1w/o =RE.sub.2 S.sub.3 +2CO.sub.(g) ( 3)
The product sulfide or oxysulfide can either be fixed in an `active` lining or removed by flotation and absorbed into the slag cover and vessel lining, depending upon the process used for introducing the rare earth compound.
The products of desulfurization of carbon saturated iron with rare earth oxides is dependent on the partial pressure of CO, pCO, and the Henrian sulfur activity in the metal, h.sub.S. Using cerium as the representative rare earth, the following standard free energy changes and the equilibrium constants at 1,500.degree. C. for different desulfurization reactions can be calculated from thermodynamic data in the literature:
__________________________________________________________________________ REACTION .DELTA.G.degree. cal. K.sub.1773 __________________________________________________________________________ 2CeO.sub.2(s) + [C] = Ce.sub.2 O.sub.3(s) + CO.sub.(g) 66000 - 53.16T pCO = 3041 Ce.sub.2 O.sub.3(s) + [C] + [S].sub.1w/o = 18220 - 26.43T pCO/h.sub.S = 3395 Ce.sub.2 O.sub.2 S.sub.(s) + CO.sub.(g) Ce.sub.2 O.sub.2 S.sub.(s) + [C] + 2[S].sub.1w/o = 66180 - 39.86T p.sup.2 CO/h.sub.S.sup.2 = 3.6 Ce.sub.2 S.sub.3(s) + 2CO.sub.(g) 3/2Ce.sub.2 O.sub.2 S.sub.(s) + 3[C] + 5/2[S].sub.1w/o = 127050 - 72.1T p.sup.3 CO/h.sub.S.sup.5/2 = 1.25 Ce.sub.3 S.sub.4(s) + 3CO.sub.(g) Ce.sub.2 O.sub.2 S.sub.(s) + 2[C] + [S].sub.1w/o = 120,860 - 61.0T p.sup. 2 CO/h.sub.S = .027 2CeS.sub.(s) + 2CO.sub.(g) C.sub.(s) + 1/2O.sub.2(g) = CO.sub.(g) -28200 - 20.16T pCO/p.sup.1/2 O.sub.2 = 7.6 .times. 10.sup.-7 1/2S.sub.2(g) = [S].sub.1w/o -31520 + 5.27T h.sub.S /1.sup.1/2 S.sub.2 = 5.4 .times. 10.sup.2 __________________________________________________________________________
The thermodynamics of desulfurization with lanthanum oxide, La.sub.2 O.sub.3, are similar although, in this case, LaO.sub.2 is unstable and there will be no conversion corresponding to CeO.sub.2 .fwdarw.Ce.sub.2 O.sub.3.
In the case of desulfurization of gases, such as stack gases, assume the following gas composition at 1,000.degree. C.:
______________________________________ Component Vol. % ______________________________________ CO.sub.2 16 CO 40 H.sub.2 40 N.sub.2 4 H.sub.2 S 0.3 (200 grains/100 ft..sup.3) ______________________________________
This equilibrium gas composition is represented by point A on the diagram illustrated as FIG. 6 where CO/CO.sub.2 =2.5 and H.sub.2 /H.sub.2 S=133. This point lies within the Ce.sub.2 O.sub.2 S phase field and at constant CO/CO.sub.2 desulfurization with Ce.sub.2 O.sub.3 will take place up to point B. At point B, H.sub.2 /H.sub.2 S.perspectiveto.10.sup.4 and the concentration of H.sub.2 S is 0.004 vol.% (.about.3 grains/100 ft..sup.3). Beyond this point, desulfurization is not possible.
The basic theory for this invention is supported by the standard free energies of rare earth compounds likely to be involved. Examples of these appear in Table I which follows:
TABLE I __________________________________________________________________________ Standard Free Energies of Formation of Some Rare Earth Compounds: .DELTA.G.degree. = X - YT cal/g.f.w. Estimated Reaction X Y Temp. (.degree.K.). Error (Kcal) __________________________________________________________________________ CeO.sub.2(s) = Ce.sub.(1) + O.sub.2(g) 259,900 49.5 1071-2000 .+-.3 Ce.sub.2 O.sub.3(s) = 2Ce.sub.(1) + 3/2O.sub.2(g) 425,621 66.0 1071-2000 .+-.3 La.sub.2 O.sub.3(s) = 2La.sub.(1) + 3/2O.sub.2(g) 428,655 68.0 1193-2000 .+-.3 CeS.sub.(s) = Ce.sub.(1) + 1/2S.sub.2(g) 132,480 24.9 1071-2000 .+-.2 Ce.sub.3 S.sub.4(s) = 3Ce.sub.(1) + 2S.sub.2(g) 483,180 98.2.sup.( *.sup.) 1071-2000 .+-.10 Ce.sub.2 S.sub.3(s) = 2Ce.sub.(1) + 3/2S.sub.2(g) 351,160.sup.( *.sup.) 76.0.sup.( *.sup.) 1071-2000 .+-.10 LaS.sub.(s) = La.sub.(1) + 1/2S.sub.2(g) 123,250 25.3 1193-2000 .+-.6 Ce.sub. 2 O.sub.2 S.sub.(s) = 2Ce.sub.(1) + O.sub.2(g) + 1/2S.sub.2(g) 410,730 65.0 1071-2000 .+-.15 La.sub.2 O.sub.2 S.sub.(s) = 2La.sub.(s) + O.sub.2(g) + 1/2S.sub.2(g) 407,700.sup.( *.sup.) 65.0.sup.( *.sup.) 1193-2000 .+-.15 __________________________________________________________________________ .sup.( *.sup.) Estimated
The three phase equilibria at 1273.degree. K. for the Ce--O--S System is set out in Table II as follows:
TABLE II __________________________________________________________________________ Ce--O--S System Three Phase Equilibria at 1273.degree. K. REACTION .DELTA.G.degree. cal K.sub.1273 __________________________________________________________________________ Ce.sub.2 O.sub.3(s) + 1/2S.sub.2(g) = Ce.sub.2 O.sub.2 S.sub.(s) + 1/2O.sub.2(g) 14890 - 1.0T (pO.sub.2 /pS.sub.2).sup.1/2 = 4.6 .times. 10.sup.-3 Ce.sub.2 O.sub.2 S.sub.(s) + 1/2S.sub.2(g) = 2CeS.sub.(s) 145770 - 15.2T pO.sub.2 /p.sup.1/2 S.sub.2 = 2.0 .times. 10.sup.-22 3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2S.sub. 2(g) = 2Ce.sub.3 S.sub.4(s) + 30.sub.2(g) 265830 + 1.4T p.sup.3 O.sub.2 /p.sup.5/2 S.sub.2 = 1.1 .times. 10.sup.-46 Ce.sub.2 O.sub.2 S.sub.(s) + S.sub.2(g) = Ce.sub.2 S.sub.3 59570 + 11.0T pO.sub.2 /pS.sub.2 = 2.3 .times. 10.sup.-13 Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2S.sub.2(g) 85740 - 23.5T p.sup.1/2 S.sub.2 = 2.5 .times. 10.sup.-10 2Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2 S.sub.2(g) 87120 - 31.6T p.sup.1/2 S.sub.2 = 8.9 .times. 10.sup.-8 CO.sub.(g) + 1/2O.sub.2(g) = CO.sub.2(g) -67500 + 20.75T pCO.sub.2 /(pCO.p.sup.1/2 O.sub.2) = 1.1 .times. 10.sup.7 H.sub.2(g) + 1/2S.sub.2(g) = H.sub.2 S.sub.(g) -21580 + 11.80T pH.sub.2 S/(pH.sub.2.p.sup.1/2 S.sub.2) = 13.4 H.sub.2(g) + 1/2O.sub.2(g) = H.sub.2 O.sub.(g) -58900 + 13.1T pH.sub.2 O(pH.sub.2.p.sup.1/2 O.sub.2) = 1.8 .times. 10.sup.7 __________________________________________________________________________
Typical calculations of energy changes involved in the systems involved in this invention are as follows:
______________________________________ S.sub.2(g) + Ce.sub.2 O.sub.2 S.sub.(s) = Ce.sub.2 S.sub.3(s) + O.sub.2(g) Ce.sub.2 S.sub.3(s) = 2Ce.sub.(l) + 3/2S.sub.2(g) : .DELTA.G.degree. = 351160 - 76.0T cal Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + O.sub.2(g) + 1/2S.sub.2(g) : .DELTA.G.degree. = 410730 - 65.0T cal Ce.sub.2 O.sub.2 S.sub.(s) + S.sub.2(g) = Ce.sub.2 S.sub.3(s) + O.sub.2(g) : .DELTA.G.degree. = 59570 + 11.0T cal @ 1273.degree. K. .DELTA.G.degree. = 73573 cal and pO.sub.2 /pS.sub.2 = 2.33 .times. 10.sup.-13 Ce.sub.2 O.sub.3(s) + 1/2S.sub.2(g) = Ce.sub.2 O.sub.2 S + 1/2O.sub.2(g) Ce.sub.2 O.sub.3(s) = 2Ce.sub.(l) + 3/2O.sub. 2(g) : .DELTA.G.degree. = 425621 - 66.0T cal Ce.sub. 2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + O.sub.2(g) + 1/2S.sub.2(g) : .DELTA.G.degree. = 410730 - 65.0T cal Ce.sub.2 O.sub.3(s) + 1/2S.sub.2(g) = Ce.sub.2 O.sub.2 S.sub.(s) + 1/2O.sub.2(g) : G.degree. = 14891 - 1.0T cal @ 1273.degree. K. .DELTA.G.degree. = 13618 cal and (pO.sub.2 /pS.sub.2).su p.1/2 = 4.6 .times. 10.sup.-3 Ce.sub.2 O.sub.2 S.sub.(s) + 1/2S.sub.2(g) = 2CeS.sub.(s) + O.sub.2(g) Ce.sub.2 O.sub.2 S.sub.(s) = 2Ce.sub.(l) + 1/2S.sub.2(g) + O.sub.2(g) : .DELTA.G.degree. = 410730 - 65.0T cal 2CeS.sub.(s) = 2Ce.sub.(l) + S.sub.2(g) : .DELTA.G.degree. = 264960 - 4.98T cal Ce.sub.2 O.sub.2 S.sub.(s) + 1/2S.sub.2(g) = 2CeS.sub.(s) + O.sub.2(g) : .DELTA.G.degree. = 145770 - 15.2T cal @ 1273.degree. K. .DELTA.G.degree. = 126420 cal. and pO.sub.2 /p.sup.1/2 S.sub.2 = 1.96 .times. 10.sup.-22 3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2S.sub.2(g) = 2Ce.sub.3 S.sub.4(s) + 3O.sub. 2(g) 2Ce.sub.3 S.sub.4(s) = 6Ce.sub.(l) + 4S.sub.2(g) : .DELTA.G.degree. = 966360 - 196.4T cal 3Ce.sub.2 O.sub.2 S.sub.(s) = 6Ce.sub.(l) + 3O.sub. 2(g) + 3/2S.sub.2(g) : .DELTA.G.degree. = 1232190 - 195.0T cal 3Ce.sub.2 O.sub.2 S.sub.(s) + 5/2S.sub.2(g) = 2Ce.sub.3 S.sub.4(s) + 3O.sub. 2(g) : .DELTA.G.degree. = 265830 + 1.4T cal @ 1273.degree. K. .DELTA.G.degree. = 267612 cal and p.sup.3 O.sub.2 /p.sup.5/2 S.sub.2 = 1.12 .times. 10.sup.-46 Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2S.sub.2(g) Ce.sub.3 S.sub.4(s) = 3Ce.sub. (l) + 2S.sub.2(g) : .DELTA.G.degree. = 48318 - 98.2T cal 3CeS.sub.(s) = 3Ce.sub.(l) + 3/2S.sub.2(g) : .DELTA.G.degree. = 397,440 - 74.7T cal Ce.sub.3 S.sub.4(s) = 3CeS.sub.(s) + 1/2S.sub.2(g) : .DELTA.G.degree. = 85740 - 23.5T cal @ 1273.degree. K. .DELTA.G.degree. = 55824 cal p.sup.1/2 S.sub.2 = 2.6 .times. 10.sup.-10 3Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2S.sub.2(g) 2Ce.sub.3 S.sub.4(s) = 6Ce.sub.(l) + 4S.sub.2(g) : .DELTA.G.degree. = 966360 - 196.4T cal 3Ce.sub.3 S.sub.3(s) = 6Ce.sub.(l) + 9/2S.sub.2(g) : .DELTA.G.degree. = 1053480 - 228.0T cal 3Ce.sub.2 S.sub.3(s) = 2Ce.sub.3 S.sub.4(s) + 1/2S.sub.2(g) : .DELTA.G.degree. = 87120 - 31.6T cal @ 1273.degree. K. .DELTA.G.degree. = 468893 cal and p.sup.1/2 S.sub.2 = 8.9 .times. 10.sup.-9 ______________________________________ H.sub.2(g) + 1/2S.sub.2(g) = H.sub.2 S.sub.(g) H.sub.2(g) + 1/2S.sub.2(g) = H.sub.2 S.sub.(g) : .DELTA.G.degree. = -21580 + 11.80T cal @ 1273.degree. K. .DELTA.G.degree. = -6559 and pH.sub.2 S/(pH.sub.2.p.sup. 1/2 S.sub.2) = 13.4 ______________________________________ pH.sub.2 /pH.sub.2 S log pS.sub.2 ______________________________________ 1 -2.25 10.sup.2 -6.25 10.sup.4 -10.25 10.sup.6 -14.25 10.sup.8 -18.25 .sup. 10.sup.10 -22.25 .sup. 10.sup.12 -26.25 ______________________________________ H.sub.2(g) + 1/2O.sub.2(g) = H.sub.2 O.sub.(g) H.sub.2(g) + 1/2 O.sub.2(g) = H.sub.2 O.sub.(g) : .DELTA.G.degree. = -58900 + 13.1T cal @ 1273.degree. K. .DELTA.G.degree. = -42223 cal and (pH.sub.2 /pH.sub.2 O) p.sup.1/2 O.sub.2 = 5.6 .times. 10.sup.-8 ______________________________________ pH.sub.2 /pH.sub.2 O log pO.sub.2 ______________________________________ .sup. 10.sup.-4 -6.5 .sup. 10.sup.-2 -10.5 1 -14.5 10.sup.2 -18.5 10.sup.4 -22.5 10.sup.6 -26.5 10.sup.8 -30.5 ______________________________________ Co.sub.(g) + 1/2 O.sub.2(g) = CO.sub.(g) Co.sub.(g) 1/2 O.sub.2(g) = CO.sub.2(g) : .DELTA.G.degree. = -67500 + 20.75T cal @ 1273.degree. K. .DELTA.G.degree. = -41085 and pCO.sub.2 /(pCO.p.sup.1/2 O.sub.2) = 1.1 .times. 10.sup.7 ______________________________________ pCO/pCO.sub.2 log pO.sub.2 ______________________________________ .sup. 10.sup.-4 -6.1 .sup. 10.sup.-2 -10.1 1 -14.1 10.sup.2 -18.1 10.sup.4 -20.1 10.sup.6 -24.1 10.sup.8 -30.1 ______________________________________
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