This article is a continuation of our article series related to the technological transformation of the phosphorus fertilizer industry aimed at lowering costs and end prices. In our previous work [Ref.1], we discussed reducing the costs via direct sulfidation of phosphorites, instead of their more expensive conventional treatment with sulfuric acid. In the present article we discuss the prospects of providing sulfur for the phosphorus industry for the desired, carbon-free energy sector.
About 90% of the mined sulfur in the world today is being used for the production of sulfuric acid, of which 80% is used for phosphorus fertilizers. Nearly all of the sulfur is a by-product of sour oil and natural gas mining. Because of widespread substitution of carbon energy sources for renewable ones, an extensive decrease in the oil and gas mining is expected, which would result in a significant decrease of sulfur also. Meanwhile, there are no promises that the demand for phosphorus will be diminished. In this situation, the most feasible industrial sources of sulfur appear to be plants for heavy metal production from sulfide ores (copper, zinc, nickel). Nowadays, these plants produce sulfuric acid as an accompanying product, enabling a significant part of the demand of the phosphorus market. The shipping of sulfuric acid is much more expensive than its cost, which also increases fertilizer prices. Assuming that the carbon-free power industry will require a significant enhancement of copper and nickel production, these plants may be able to, at least partially, compensate for the sulfur shortage. However, to maintain the affordability of trans-continental shipping and the phosphorus industry, it is necessary to provide sulfur from metallurgy in its elemental form, rather than sulfuric acid. Thus, we developed a rigorous theoretical basis for a new technological process – AutoCatalytic Sulfurization (ACS). Instead of conventional oxidation of SO2 obtained by the sulfides calcination to sulfuric anhydride SO3, followed by the sulfuric acid obtained, we propose the opposite SO2 reduction to the elemental sulfur. We compare below both the conventional and the new approaches..
The Conventional scheme of the utilization of sulfur-containing gases is shown in Fig.1. If the sulfuric acid is not immediately consumed, it must be neutralized with lime to form gypsum. So, this approach is very expensive:
Fig. 1. Expensive use of sulphureous gases, resulting in sulfuric acid and carbon dioxide emission.
Today, there is a scientific and technological basis to transfer to a more efficient scheme of the sulphureous anhydride utilization, by turning it to the elemental sulfur in Kazakhstan:
SO2 → S
Analogous to the proposed process is the Claus process widely used, for example by Tengizchevroil LLP (TCO) oil company. The Claus process is a catalytic oxidative conversion of hydrogen sulfide used in the oil industry:
H2S + O2 = SO2 + H2O (1)
SO2 + H2S = S + H2O (REACTION ON CATALYST) (2)
A similar process for the sulfides concentrates will be following:
ZnS + O2 = SO2 + ZnO (3)
SO2 + 2 ZnS = 3 S + 2 ZnO – 60,2 kJ (REACTION WITH AUTOCATALYSIS) (4)
Reaction 4 is endothermic, but reactions 3 and 4 are exothermic.
In this case, the utilization is significantly simplified, since seven stages out of eight are not necessary.
Fig. 2. New efficient scheme of the utilization of sulfuric gases, resulting in sulfur.
Major consumers of the sulfuric acid in Kazakhstan, for example, employed by Kazatomprom and Kazphosphate companies, had built powerful plants of sulfuric acid aimed at the elemental sulfur usage. Therefore, it is reasonable to convert copper and zinc calcination plants to the scheme of obtaining sulfur, instead of sulfuric acid. In other countries this transformation would also be economically and ecologically advantageous.
As a compromise solution, a combined scheme with a reduced sulfuric-acid plant might be used, where the zinc plant may flexibly control the release of two products, sulfur and acid, depending on consumer demand. In this case, five out of eight stages may be eliminated. (See Fig.3).
Fig. 3. Combined Scheme.
All of the above mentioned approaches show a high promise of this new technology for reducing carbon footprint(s), improvement in the economics of metallurgy plants, the phosphorus fertilizer industry, and trans-continental shipping of the sulfur products.
The authors are open for collaboration with potential collaborators and relevant companies for joint development of the described technology. Researchers in Kazakhstan, together with international partners, can significantly contribute to preventing a worldwide crisis of phosphorus fertilizers–for the benefit of all countries.
- Baurzhan Duisebayev, Konstantin Polinovsky, Thomas A. Cellucci. Phosphorus Shortage Exacerbates Global Food Crisis in the 21st Century. Homeland Security Today, November, 2022. https://www.hstoday.us/industry/industry-news/phosphorus-shortage-exacerbates-global-food-crisis-in-the-21st-century/