ABSTRACT
A laboratory scale fluidized bed reactor was designed and fabricated successfully. The reactor consists of three sections: (a) bed section, (b) freeboard section and (c) conical closure section with inlet cone. The design parameters which affect the performances are identified and discussed. However the objective here is to develop a fluidized bed reactor for the abatement of fluoride from industrial effluent gases. This project focuses on providing a cost effective method for the abatement of Fluoride. Fluorine is a toxic and corrosive chemical with a threshold limit value (TLV) of 1 ppm. The present method utilizes fluidized bed reactor using red-mud as bed material thereby utilizing wastes of aluminium industry. This also provides a simple alternative to Fluoride abatement while producing no hazardous waste. The fluidization characteristics of red-mud were studied. The ratio of red mud and alumina particles required to be used as the bed material was also determined by experimentation. This method of Fluoride abatement is most effective as wastes of Al- industry is used to minimise another problem thereby reducing the operating cost of the Al- industry.
Key words: fluidized bed reactor, red mud, abatement of Fluoride
CHAPTER ONE
INTRODUCTION
Over the past 60 years there has been an enormous expansion in the use of fluorine compounds in agriculture and industry. People are exposed to airborne fluoride ions from aluminum smelting, coal burning and nuclear power plants, glass etching, petroleum refining, plastic manufacturing, phosphate fertilizer production, silicon chip manufacturing and uranium enrichment facilities. Excess exposure to fluorine gas has detrimental effects on health. With fluorine there can be a hazard either from inhalation or from contact with the skin. All organic materials are extremely reactive with fluorine. At as low a concentration as 50 ppm, breathing can be impossible without respiratory equipment, and at 100 ppm skin irritation occurs. Fluorides are released into the environment naturally through the weathering of rocks and through atmospheric emissions from volcanoes and seawater. It is released from the industries when some fluoride containing substances (eg. coal, minerals or clays) are heated at high temperatures. The most relevant inorganic fluorides are hydrogen fluoride (HF), calcium fluoride (CaF2), sodium fluoride (NaF), sulphur-hexafluoride (SF6) and silicofluorides which effects on living organism. These fluorine gases have high global warming potential.
Per-fluorocarbons (PFCs), Hydro-fluorocarbons (HFCs), Sulphur-Hexachloride (SF6) and Nitrogen Trifluoride (NF3) have been identified as potential, global-warming gases because of their characteristics of strong infrared (IR) absorption and very long atmospheric lifetimes. These fluorine (F) saturated species are among the strongest greenhouse gases, with global warming potentials (GWPs) 3 and 4 orders of magnitude higher than carbon dioxide (CO2). Moreover, they are extremely stable molecules with lifetime in the atmosphere of thousands of years.
Another hazardous gas is the molecular Fluorine. Extended exposure to as little as 1 ppm of F2 can be hazardous. Also F2 is difficult to breakdown or reduce to non-toxic forms. Thus, it is desirable to minimize the introduction of such harmful gases and by-products into the environment. There is also a need to minimize the harmful content of the effluent gas released into the atmosphere in an efficient and inexpensive manner to protect vegetation and grazing animals and therefore human health. Recently, the strict restriction from the Ministry of Environment and Forest, govt. of India on the emissions of harmful fluoride containing gases into the atmosphere have increased the need of impurity free effluent gas. Thus, all the industries which emit such fluoride containing gases should adopt modern abatement techniques to reduce their emissions in order to meet the Govt. regulations. The most recent abatement technique for fluorine includes the Fluidized bed method. This study also focuses on fluidized bed disposal of Fluorine using red-mud as the bed material. Fluidized bed method is a commercially viable process for fluorine abatement which
- Generates no pollutants.
- Allows high throughput.
- Avoids clogging.
- efficiently removes Fluorine and
- Allows for recharging of the abatement system for continuous processing.
