Fluorine absorption coefficients in superphosphate den gases

In the manufacture of superphosphate by the acidulation of phosphate rock with sulphuric acid, fluorine is evolved, mainly as silicon tetrafluoride, along with carbon dioxide and steam. This environmentally unacceptable fluoride is required to be removed before being vented to the atmosphere. This is usually done by absorbing the gas in water. However, industrially the absorption is complicated by the presence of entrained phosphate rock dust, mist particles and condensing steam in the gas stream, and by the silica formed from the reaction between silicon tetrafluoride and water. The absorption of dry silicon tetrafluoride in water under laboratory conditions is reasonably well understood. However, information on the absorption under industrial conditions is lacking. In all industrial measurements of the mass transfer coefficient the liquid-gas interfacial area is not known accurately and hence what is usually obtained is a coefficient per unit volume of the absorption equipment. This value is of limited use in exploring the effect of particulates and surface reaction as it is a complex average for different drop sizes and scrubber volumes and the chemical composition of the gas stream. Gas absorption, particulate collection and surface reaction need to be considered separately in comparing industrial scrubbers. Therefore in order to be able to translate the performance of a given scrubber from one works to another, the measurement of the mass transfer coefficient for one standard geometry in industrial conditions is needed. Thus the aim of this work is to establish a method of providing this information. By using a spinning cup sampling apparatus we were not only able to establish a method of determining a mass transfer coefficient for the absorption of the industrial effluent gas stream fluorides, but were also able to establish a method for determining the size of the particulate fluorides present in the gas stream. The main points arising from this work include: (1) The absorption is "gas" phase controlled (under normal conditions surface silica is unimportant) and the particulate fluorides in the gas stream will often be the controlling factor in scrubber performance. Thus the collection of particulate fluorides must be a primary concern in scrubber design, and high liquid to gas ratios in some section of a scrubber will promote this collection. (2) The particulate fluorides are small, of the order of 2 micrometers in diameter. (3) In the spinning cup apparatus used in this study, the mass transfer coefficient for a "gas" stream containing 25% particulate fluorides at 90 to 100°C is approximately 55 m h-¹