GB808639A - Improvements in and relating to the oxidation of sulphur dioxide to sulphur trioxide and catalysts therefor - Google Patents

Abstract

A catalyst for the oxidation of sulphur dioxide to trioxide is prepared by impregnating a freshly-prepared silica gel containing sodium and/or potassium ions with freshly-precipitated vanadium pentoxide containing sodium and/or potassium ions, drying, treating with sulphuric acid and calcining, the amounts and compositions of the reactants being chosen to give a product containing 6-9 per cent vanadium pentoxide, 6-10 per cent potassium oxide and 1.5-6 per cent sodium oxide. Calcium or barium ions may also be introduced by washing the silica gel, before impregnation, with lime or baryta water. Freshly prepared silica is defined as silica gel which has not been allowed to dry out, i.e. it is used immediately after precipitation or is stored in an unfiltered state under conditions such that there is no loss of water. Additional quantities of silica, e.g. kieselguhr, up to 10 per cent of the total weight, may be introduced into the catalyst by mixing in after the impregnation step. The silica gel preferably contains sodium ions and is precipitated from sodium silicate solutions by addition of ammonium or aluminium sulphate, ammonium, sodium or magnesium acetate, ammonium bicarbonate, sodium carbonate, ammonium, sodium or calcium chloride, an alkali such as ammonia or an acid such as sulphuric (to a pH not less than 8). The gel is then washed to give the desired sodium content. The vanadium pentoxide preferably contains potassium ions and is prepared by dissolving vanadic acid in aqueous caustic potash or caustic potash and soda and sulphuric acid is added to give a red precipitate which is slurried with the silica and dried by removing supernatant water by evaporation and then heating the paste to 100-120 DEG C. The product is treated with aqueous sulphuric acid, preferably containing 10 per cent by weight of acid, in an amount sufficient to wet the catalyst, e.g. 1 litre per kilogram of catalyst. The mixture is redried and calcined at 300-400 DEG C. and may then be pelleted; alternatively if it is to be used in the fluidized state it may be introduced into the reactor prior to calcination and calcined in situ. The catalyst may be further activated by treatment with air at 550 DEG C. for 6-12 hours and stabilized by treatment with a reaction mixture at 650 DEG C. for one or more periods of 18-24 hours. The advantage of the catalyst over previous catalysts is particularly noticeable at 380-460 DEG C. as is illustrated by the examples. The Provisional Specification (24681/56) also refers to catalysts containing silica and compounds of vanadium, potassium and another alkali metal which need not be treated with sulphuric acid. This subject-matter does not appear in the Complete Specification.