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62 <br />the precipitation of Fe hydroxide in mine water and the <br />adsorption of all other cations onto the Fe hydroxide sur- <br />face. For adsorption to be effective, the water should con- <br />tain appreciable Mn if the pH is below 7, or else the pH <br />should be close to 8 to allow precipitation and adsorption <br />by Fe hydroxides. <br />To put what was explained above into practice, reliance <br />on the addition of lime to raise the pH may not be enough to <br />lower the levels of heavy metals to safe concentrations. <br />The concentrations of Fe and Mn in the water are important <br />and the pH of the solid-water system is also a factor. <br />The method of addition of lime may affect the manner in which <br />.-the - predominant -hydroxide precipitates-are formed and-_this <br />- <br />could affect their adsorption capacities. Also, aeration <br />may change the properties of the tailings pond system to <br />promote or hinder heavy metals removal. The tailings pond <br />operation is a place where empirical experiments may appre- <br />ciably change the efficiency of the operation. Parameters <br />that may possibly be varied are the speed and position of <br />where lime is added, the method of aeration, the position <br />of the aerators., the flow of water into the pond, and the <br />size of the pond. <br />Concerning the design and operation of water treatment <br />facilities for mines and mills, Hyatt (38) has analyzed <br />various alternatives quite nicely. He.includes in his review <br />a summary of the EPA limitations for milling operations, <br />internal milling operations to be considered, controls around <br />i_