Corrosion protection of phosphate grinding mills using impressed current technology

Grinding mills are commonly used in mineral beneficiation to reduce particle size. The corrosion of metallic grinding media and mill liner is a very serious problem, particularly in acidic conditions as encountered in the Florida phosphate fertilizer industry. Cathodic protection using impressed current was investigated to reduce the wear rate during ball milling.; A statistical Box-Behnken Design (BBD) of experiments was performed to evaluate effects of individual variables and their interactions on wear rate of grinding ball mill. The optimum process parameters for minimum wear rate for 1018 carbon steel were solution pH at 7.4, rotation speed at 70 RPM, solid percentage at 76% by weight, and crop load at 72% by volume. For high chromium alloy, optimum parameters were solution pH at 8.7, rotation speed at 61 RPM, solid percentage at 65% by weight, and crop load at 58% by volume.; The effects of polarization potential and impressed current density on corrosion rate were studied using a specially designed ball mill whose electrochemical potential was controlled. The required polarization potential to effectively reduce the wear rate of 1018 carbon steel was -1.0 V in pH 3.1 solution, -0.9 V in pH 6.8 solution, and -0.85 V in pH 9.2 solution, while the required current density for 1018 carbon steel was 210 mA/m2 at pH 3.1, 180 mA/m2 at pH 6.8, and 160 mA/m2 at pH 9.2. Experimental results indicate that the total wear rate was reduced by 41% to 46% and corrosive wear rate was reduced by 83% to 85% for 1018 carbon steel when a potential of -1.0 V was applied at pH's of 3.1 to 9.2. Similar results were obtained with the high chromium alloy.; The cathodic protection mechanisms of grinding mill were investigated in extensive fundamental studies. SEM images suggested that pitting corrosion was the main corrosion type. The corrosion products were deduced from the element ratio on the coupon surface by EDS. XRD images showed that Fe 2O3 was the main corrosion product for 1018 carbon steel in pH 3.1 solution without cathodic protection, while Fe 2O3, Fe3O4 and Fe(OH)3 were the corrosion products in neutral and alkaline solutions. XRD images also indicated that the corrosion was significantly reduced since most of the corrosion products disappeared from the coupon surfaces when polarization potential was applied. These conclusions are consistent with those obtained from Eh-pH diagrams.