Thermal Decomposition of the sulfates of Fe(II), Fe(III), Ni, Cu, and Zn.

Transition metal oxides have been utilized as high temperature regenerable sorbents for removal of hydrogen sulfide from fuel gas obtained from coal gasification <1–6}. The performance of these sorbents depends both on their sulfidation ability and their regenerability. Regeneration of the sulfided sorbents is usually performed utilizing oxygen. In addition to SO[sub>2 (desirable product) formation, metal sulfate is also formed during regeneration, which has a negative effect on the sorbent durability (7–10). Thus, it is important to understand the decomposition of the undesirable metal sulfate. Decomposition of transition metal sulfates have been studied by various researchers (11–20). Most of these studies were conducted with thermogravimetric analysis (TGA), gravimetric torsion-effusion, and differential thermal analysis (DTA). Oxy-sulfate has been identified as an intermediate during the decomposition of zinc sulfate and copper sulfate. Kinetic parameters have been determined and decomposition temperatures have also been identified during these studies...

TGA analysis was conducted to understand the bulk decomposition properties of the sulfates. The TGA data for the decomposition of the sulfates are shown in Fig. 1.

The initial decomposition temperature for zinc sulfate was observed around 700C with a sharp decrease in weight being observed around 800C. An additional sharp decrease in weight was observed between 8008C and 862C. Literature values <12–14,18 > for the decomposition temperatures of ZnSO₄ range from 610C to 846C. Several researchers identified the oxysulfate intermediate as the initial decomposition product. Mu and Perimutter <13 > indicated that the anhydrous sulfate begins to convert to oxysulfate, ZnO ¯ 2ZnSO₄ , at 590C, with 4 this process being complete at 712C. The final conversion of the sulfate to ZnO, as reported by Mu and Perimutter <13>, was complete at 837C. The initial decomposition temperature of 600C observed in this study is very close to the value obtained by Mu and Perimutter <13 >. The TGA decomposition profile for ZnSO₄ reached a minimum at 890C and 4 maintained that minimum through the maximum temperature of 950C. The weight loss observed during decomposition was approximately 49%, corresponding to a decomposition product of ZnO. As stated above, in the case of Mu and Perimutter <13 >, complete decomposition of ZnSO₄ to ZnO was 4 achieved at 837C, a temperature difference of approximately 6% below that obtained in these experiments. Platinum sample boats have been shown to have a catalytic effect on decomposition <14>, however in these experiments this catalytic effect was not observed.

Dehydration of nickel sulfate seems to occur between 50C to 450C. The initial decomposition of nickel sulfate seems to be rapid at 700C, which was then followed by slow decomposition up to 750C. Previous researchers have reported temperatures ranging from 675C to 848C for the decomposition of nickel sulfate. Complete decomposition was not observed. The TGA decomposition profile for NiSO₄ reached a minimum at 750C and maintained that minimum through the maximum temperature of 950C. The weight loss observed during decomposition was approximately 40%, corresponding to a stoichiometry of NiSO₄ NiS + O₂

The decomposition temperature ranges for Fe₂(SO₄)₃ and FeSO₄ were similar as shown in Fig. 2. However, the shapes of the TGA decomposition curves were different. Initial dehydration for FeSO₄ 7H₂O was observed at up to 200C, with the 2 decomposition of sulfate initiating around 500C. There was a sharp decrease in weight at 500C for FeSO₄, followed by a slower decomposition at 4 500C – 600C. In contrast to this, Fe₂(SO₄)₂ .started to decompose slowly at 500C with a sharp decrease in weight around 575C. The decomposition was complete at 600C. Other researchers <12,13,19,20> observed a sharp decrease in weight due to decomposition in the temperature range of 550C–625C for FeSO₄, which is similar to the temperature range observed in this work. Formation of intermediates during decomposition of iron sulfates has not been observed by previous researchers. Percent weight loss indicates a decomposition product of Fe O for FeSO₄ and Fe₂O₃ for Fe₂(SO₄)₂.

Initial dehydration of copper sulfate also occurs between 100C to 200C. Decomposition of copper sulfate initiates at 600C, with a rapid change in weight observed around 675C, as shown in Fig. 1. Decomposition proceeds more slowly between 700C and 840C, with the change in weight reaching a minimum at 840C. The decomposition of copper sulfate in previous work has been reported in the range of 598C to 625C.