Necrotizing periodontal diseases is one of the seven categories of periodontitis as defined by the American Academy of Periodontology 1999 classification system and is one of the three classifications of periodontal diseases and conditions within the 2017 classification. Necrotizing periodontal diseases are a type of inflammatory periodontal (gum) disease caused by bacteria (notably fusobacteria and spirochaete species). The diseases appear to represent different severities or stages of the same disease process, although this is not completely certain. These diseases usually have a sudden onset, and so the term acute is often added to the diagnosis. The mildest on the spectrum is necrotizing ulcerative gingivitis (NUG), followed by the successively more severe conditions necrotizing ulcerative periodontitis (NUP), necrotizing stomatitis and finally cancrum oris (noma), which is frequently fatal. Necrotizing ulcerative gingivitis
Necrotizing ulcerative gingivitis, (NUG), or simply necrotizing gingivitis (NG), is a common, non-contagious infection of the gums. Acute necrotizing ulcerative gingivitis (ANUG) is the acute presentation of NUG, which is the usual course the disease takes. If improperly treated NUG may become chronic and/or recurrent. In developed countries, ANUG occurs mostly in young adults with predisposing factors such as psychological stress, sleep deprivation, poor oral hygiene, smoking, immunosuppression and/or malnutrition. In developing countries, ANUG occurs mostly in malnourished children. Due to shared predisposing factors in a population (e.g. students during a period of examinations, armed forces recruits) ANUG is known to occur in epidemic-type patterns. This has led to the popular belief that ANUG is contagious, but this is not the case.
The organic stream is recycled through the solvent extraction process while the aqueous streams cycle through leaching and electrowinning processes respectively. Ion exchange
Chelating agents, natural zeolite, activated carbon, resins, and liquid organics impregnated with chelating agents are all used to exchange cations or anions with the solution. Selectivity and recovery are a function of the reagents used and the contaminants present. Metal recovery
Metal recovery is the final step in a hydrometallurgical process. Metals suitable for sale as raw materials are often directly produced in the metal recovery step. Sometimes, however, further refining is required if ultra-high purity metals are to be produced. The primary types of metal recovery processes are electrolysis, gaseous reduction, and precipitation. For example, a major target of hydrometallurgy is copper, which is conveniently obtained by electrolysis. Cu2+ ions reduce at mild potentials, leaving behind other contaminating metals such as Fe2+ and Zn2+. Electrolysis
Electrowinning and electrorefining respectively involve the recovery and purification of metals using electrodeposition of metals at the cathode, and either metal dissolution or a competing oxidation reaction at the anode. Precipitation
Precipitation in hydrometallurgy involves the chemical precipitation of either metals and their compounds or of the contaminants from aqueous solutions. Precipitation will proceed when, through reagent addition, evaporation, pH change or temperature manipulation, any given species exceeds its limit of solubility. References
External links
Hydrometallurgy, BioMineWiki