Balance

Southern African Pyrometallurgy 2006, Edited by R.T. Jones, South African Institute of Mining and Metallurgy, Johannesburg, 5-8 March 2006

Sulphuric Acid Manufacture
W.G. Davenport University of Arizona, Tucson, Arizona, USA M.J. King Hatch, Perth, Australia B. Rogers Hatch, Mississauga, Ontario, Canada A. Weissenberger Hatch, Woodmead, South Africa
Keywords: Pyrometallurgy, sulphuric acid,sulphur dioxide Abstract – The raw material for sulphuric acid manufacture is clean SO 2 gas. It comes from (i) burning molten by-product sulphur; (ii) roasting or smelting metal sulphide concentrates, and (iii) decomposing contaminated organic chemical process sulphuric acid catalyst. Efficient gas cleaning is required for metallurgical and contaminated acid decomposition gases, especially theformer. Sulphuric acid is made from SO2 gas by (i) oxidizing the SO2(g) to SO3(g) in contact with supported liquid-phase catalyst then (ii) reacting the resulting SO 3(g) with the water component of 98.5 mass% H 2SO4, 1.5 mass% H2O acid. This paper discusses the reasons for these process steps and indicates how acidmaking can be controlled and optimized. Special emphasis is placed on SO2(g)oxidation efficiency and how it is influenced by feed gas composition, feed gas temperature, catalyst composition, catalyst bed pressure, number of catalyst beds, and double versus single contact acidmaking. In addition, a review of various other treatment methods for SO2-bearing gases is provided. A brief description of each process is included along with commentary on their technical and economicapplicability for use at metallurgical facilities.

PART I: SULPHURIC ACID PRODUCTION FROM STRONG SO2 GAS Around 200 million metric tons of sulphuric acid is manufactured per year – most of it from strong SO2 (~10%) gas.1 South Africa produces and consumes around 3 million tons per year of sulphuric acid.2 A majority of the acid is used for making phosphate fertilizers, but it has a myriad of otheruses. Raw Materials, Products The starting material for sulphuric acid manufacture is clean, dry SO2, 8 to 12% in O2, N2, SO2, CO2, SO3 gas. It is obtained: (a) by burning molten waste elemental sulphur (~70% of world production) (b) from high SO2 strength metallurgical off-gases (~20%) (c) by decomposing spent (used) sulphuric acid catalyst (~10%).

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(a) and (c) beneficially make a usefulproduct from waste. (b) beneficially removes SO2 from smelting and roasting off-gases. The product of sulphuric acid manufacture is liquid acid, 93 to 98% H2SO4, 2 to 7% H2O. Manufacturing Process Sulphuric acid manufacturing consists of: (a) gas cleaning and removal of H2O(g) by condensation (b) gas dehydration with sulphuric acid desiccant, i.e.:
~320 K

H2O(g) + H2SO4(l)
in strong acid

àslightly weakened acid

H2SO4(l) + H2O(l)

[1]

DH = -80 MJ per kg-mole H2O(g) (c) oxidation of SO2 to SO3 in contact with supported liquid phase catalyst, i.e.:
700-900 K in SO2, O2, in feed gas or N2 feed gas added to it in air

SO2(g)

+

½ O2(g)

à

catalyst in SO3, SO2, O2, N2, gas

SO3(g)

[2]

DH = -100 MJ per kg-mole SO2(g) (d) reaction of (c)'s SO3 product withstrong sulphuric acid to make strengthened sulphuric acid, i.e.:
350-380 K in SO3, SO2 O2, N2, gas

SO3(g)

+

in 1.5% H2O, 98.5% H2SO4 sulphuric acid

H2O(l)

à

in strengthened sulphuric acid

H2SO4(l)

[3]

DH = -130 MJ per kg-mole H2SO4(l) (e) mixing of (b)'s and (d)'s liquid products (+ water) to make the acid plant's range of products. Gas Cleaning The gas enteringdehydration must be dust free (0.001 to 0.01 g/Nm3 of gas) to avoid plugging downstream SO2-oxidation catalyst. Dust is removed from the gas by a series of electrostatic precipitation and scrubbing steps. Metallurgical off-gas requires the most attention. Sulphur burning requires the least. Sulphuric acid catalyst decomposition gas is intermediate.

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Condensation and Dehydration Acid plants must...