The United States is the largest rock producer in the world. About 30% of the world production in 1990 was produced by the United States (Bartels and Gurr 1994). This level of production has continued to the present. The State of Florida generates approximately 70% of the U. S. annual production of about 40 million tons (Harben 1980). A typical beneficiation process in central Florida includes washing, classification, fatty acid flotation, acid scrubbing, and amine flotation. The upgraded concentrate is used for toothpaste, detergents, food, fertilizers, etc. Approximately 95% of produced is consumed in fertilizer machines where phosphorus, together with nitroge n and potassium, is utilized as the major nutrient for machines.
In fertilizer machines, is used to produce phosphoric acid by reacting with sulfuric acid. The phosphogypsum crystals formed during this chemical reaction are discarded and the phosphoric acid is further processed to give the finished fertilizer product. To increase the chemical reaction rate, rock is ground to fine particle sizes prior to the chemical reaction process. It usually takes 10-15 minutes of grinding to reduce size to more than 40% -200 mesh. The direct operating costs in grinding are mainly the energy cons umed and the metal lost through wear and corrosion. Fine grinding of is en ergy intensive and more than 60% of the electrical energy consumed in the fe rtilizer machine is used in grinding rock. Thousands of tons of grinding medi a is consumed in chemical machines every year. Reducing wear and corrosion ra tes will significantly reduce the grinding costs and improve the grinding efficiency by maintaining optimal grinding ball size.
Previous efforts to reduce grinding medi a consumption were mainly on finding the most wear-resistant metal, e.g., high-ch romium alloys, and significant improvements have been achieved. However, high-chromium alloys are rather expensive. Reduced consumption rate of metal alloy will resu lt in significant savings for the Florida industry.
This research program was intended to minimize the corrosive wear of the mill liner and grinding media by developing an e ffective cathodic protec tion process based on the impressed current principle. The cathodic process protects the mill from corrosion by impressing a cathodic current to the mill, loweri ng its electrochemical potential to a value at which its oxidation reaction or corrosion rate is negligible. This also helps to reduce the abrasive wear rate by eliminating th e synergistic effects between corrosion and abrasion.
Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment. Deterioration by physical ca uses is not called corrosion, but is described as erosion, galling, or wear. In some instances, chemical attack accompanies physical deterioration as described by terms of corrosion-erosion, corrosive wear, or fretting corrosion. Nonmetals are not included in the presen t definition. Plastics may swell or crack, wood may split or decay , granite may erode, and Portland cement may leach away, but the term corrosion is presently restricted to chemical attack of metals.
To the great majority of people, corros ion means rust, an almost universal object of hatred. Rust is, of c ourse, the name which has more recently been specifically reserved for the corrosion of iron, while corrosion is the destructive phenomenon which affects almost all metals. Although iron was not the first metal used by man, it has certainly been the most used, and must have been one of the first with which serious corrosion problems were obtained. It is not, therefore, surprising that the terms corrosion and rust are almost synonymous.
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