re: HSC Chemistry Marathon Archive
The golden question that I love most:
Evaluate the impact on society of the environmental issues associated with THREE of the industrial processes that you have studied in this option. 7 marks
Mineral deposits which contain sulfur are often underground and must be extracted; the process of extraction is called the Frasch process. In short, this process consists of:
- Three concentric pipes forced down to the underground sulfur deposits
- Superheated steam (which is pressurised to 160 degrees) is inserted into the outer-most pipe, directly into the sulfur, causing it to melt.
- Pressurised air is forced down the inner-most tube, which then pushes the molten sulfur and water foam through the middle-tube, back to the surface.
- The superheated temperature causes the sulfur to melt and form an emulsion with water which is easily seperated as it cools down at the surface
While sulfur itself is non-toxic and odourless, it oxidises easily to sulfur dioxide and is reduced easily to hydrogen sulfide, both of which are serious air pollutants; care must be taken to avoid these reactions. The water use to make superheated steam cannot be discharged into the environment, as it contains contaminants, and will cause thermal pollution due to its intense heat so it must be reused.
Extraction of sodium hydroxide is done through three electroylsis methods (Mercury, Diaphragm, and the Membrane process, however the diaphragm process is doing to be discussed). The diaphragm process consists of a carbon anode, within a solution of brine, which is housed in an asbestos diaphragm shell. The asbestos diaphragm is lined by a steel mesh cathode. Positive sodium ions are attracted to the negative cathode, and are able to pass through the asbestos. Steam is passed over the cathode continuously to wash off a solution of sodium hydroxide, which drops to the bottom of the cell, and is collected. Chlorine gas and hydrogen gas are collected separately.
Asbestos fibres are quite dangerous, and inhalation of these fibres can cause a range of lung disease, such as asbestosis and mesothelioma. The depleted brine cannot be released immediately back into the environment, due to the presence of small quantities of basic NaOH, and oxidising ClO- ions, which have to be removed.
Finally, the Solvay process is a major chemical industrial process used to synthesise sodium carbonate (Na2CO3). The Solvay process produces less pollution than previous methods of sodium carbonate. The reactions take place in a tower, designed by Solvay, and by-products such as ammonia, calcium oxide and carbon dioxide are re-used. Australian supplies of sodium carbonate are manufactured at a plant in Osborne, South Australia. Some environmental issues include:
Calcium chloride:
- Discharge of calcium chloride causes an unacceptable increase in calcium and chloride ion concentrations and affects local ecosystems. At Osborne, it has been discharged into the ocean for many years.
Solid waste:
- In 1997, the Solvay plant at Osborne as pumping 200 tonnes per day of waste into the adjacent river, forming huge sludge deposits. Although this is not toxic, it is unsightly and a nuisance as it blocks shipping channels. The company is researching ways to use this waste such as in fertilizers, landfill and brick manufacture.
