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Based on the results obtained on the mining Industries samples, the following conclusions of ball mill advatages can be drawn:

(1) Unlike earlier mill corrosion studies th at used devices that merely simulated mill motion, the results from the specially designed ball mill more closely resemble those to be expected from the industrial ball mills.

(2) Polarization diagrams indicated th at the current density was higher in nitrogenated solution than in oxygenated solution at the same pH value. This is mainly because the fresh surface of electrode was eas ily oxidized in oxygenated solution than in nitrogenated solution, which passivates metal surface.

(3) The current density of 1018 carbon stee l was considerably higher than that of high-chromium alloy under the same condition. For example, the maximum current density was about 42 mA/cm 2 at pH 4.6 for 1018 carbon st eel in oxygenated buffer solution, while it was 22.5 mA/cm 2 for high-chromium alloy. This is attributed to the fact that the addition of chromium to iron increased the ease of passivation by reducing critical anodic current density. Addition of both chromium and nickel to iron markedly increased the ease of passivation.

(4) The current density wa s higher in buffer solution than that in pond water solution under the same operating conditions. This behavior may be caused by the fact that the pH of a buffer solution resisted chan ges when small amounts of acid or base were added. But the pH of a pond water solutio n was increased by ongoing reactions. The corrosion rated was decreased with increasing the pH value of pond water solution.

(5) The influence of individual variables and their interactions on the wear rate of 1018 carbon steel was in the order of solution pHrotation speedsolid percentagecrop loadrotation speed 2rotation speed × solid percentage. The effects of these variables on the wear rate of high-chromium alloy was in the order of solution pHrotation speedsolid percentagecrop load. The effects of interactions of these variables on the wear rate of high-chromium alloy were not significant. Solution pH had the most significant effect on the wear ra te for both 1018 carbon steel and high-chromium alloy.

(6) The regression equati on for 1018 carbon steel wear rate as coded variables was: Wear rate=530.34–177.50×A+85.72×B–54.58×C–76.53×D+46.41×B 2 –43.95×B×C The regression equation for high-chromium alloy wear rate was: Wear rate=156.24–50.31×A+24.81×B–14.63×C–23.75×D where A represents solution pH, B rotation speed, C crop load, and D stands for solid percentage.

(7) The optimum process parameters for minimum wear rate were: for 1018 carbon steel, solution pH at 7.36, rotation speed at 70.31 RPM, solid percentage at 75.50, and crop load at 71.94%; and for high-chromium alloy, solution pH at 8.69, rotation speed at 61.13 RPM, solid percentage at 64.86, and crop load at 57.63%.

(8) Polarization potential of -1.0 V was sufficient to effectively reduce the wear rate of 1018 carbon stee l and -0.7 V for high-chromium a lloy. The total wear rate was reduced by 42% to 46 % and corrosive wear rate was reduced by 84% to 86% for 1018 carbon steel when potential of -1.0 V was applied. For high-chromium alloy, 36% to 38% was reduced for total wear rate and 80% to 81% was reduced for corrosive wear when polarization potential of -0.7 V was applied.

(9) The required current density to effectively reduce the wear rate of 1018 carbon steel was 210 mA/m 2 in pH 3.1 solution, 180 mA/m 2 in pH 6.8 solution, and 160 mA/m 2 in pH 9.2 solution. The required current density for high-chromium alloy was 150 mA/m 2 in pH 3.1 solution, 125 mA/m 2 in pH 6.8 solution, and 95 mA/m 2 in pH 9.2 solution.

(10) The effect of slurry conductivity on wear rate reduction demonstrated that the wear reduction increased with increasing sodium sulfate concentration. For example, the total wear reduction was 41.1% without sodium sulfate in pH 3.1 solution, while it increased to 47.4% in the same solution after 10 -2 M sodium sulfate was added into the slurry. Similar results were obtained with the high-chromium alloy.

(11) SEM analysis indicated that there were a lot of deep and shallow pits on the surface. This suggests that pitting corrosion was the main corrosion mechanism. Pitting corrosion was caused by the presence of phosphor ic acid, fluosilicic acid, and sulfuric acid that manage to pass through the passive film and initiat e corrosion, resulting in rupture of the passive film. SEM photographs suggested th at corrosion was significantly reduced after polarization potential was applied.

(12) XRD images suggested that Fe 2 O3 was the main corrosion product for 1018 carbon steel in pH 3.1 solution without cathodic protection, while Fe 2 O3 , Fe 3 O4 and Fe(OH)3 were the corrosion products in pH 6.8 and pH 9.2 solutions. For high-chromium alloy, Fe 2 O3 , Cr 2 O3 and FeCr 2 O4 were the main corrosion products in pH 3.1 solution; Cr 2 O3 and FeCr 2 O4 were the corrosion products in pH 6.8 and pH 9.2 solutions. XRD images also suggested that the corrosion was significantly reduced since most of the corrosion products disappeared from the coupon surface when polarization potential was applied.

(13) Economic evaluation indi cates that for the CF fertilizer machine at machine City, Florida where two 14’×24’ ball mills used for size reduction the economic benefit will be $963,993/year if 1018 carbon steel balls are used and $345,842/year when high-chromium alloy balls are employed.

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