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0223/94 17:30 S 702 376 8917 McClelland Lab 21 <br />• 11 <br />1'ilut scale testing of actual waste piles is the most definitive test method fur <br />characterizing acid producing wastes. Ii is atso the most costly. Plans for phut testing <br />must he made early in the project development phase so that the pilot lcsl plot of waste <br />can be constructed on a lined area for wntainmeut and collection of seepage. The test <br />is set to site specific conditions which is a major advantage, but the test duration is <br />measured io years, and costs are extremely high. <br />Humidity cell kinetic tests arc must commonly used for routine evaluation of potential to <br />produce acid from hard rock mine wastes. The test protocol has been modified in recent <br />years (tnodificJ humidity cell test) lu provide more predictable long term acid mine <br />drainage potential. The old test method (standard humidity cell) used 200 grams of <br />solids pulverizxd to '200 mesh in size. Dry and humid air was passed over the surface of <br />the pulverized solids. The sahlration rycle (seventh day of the weekly rycle) duration <br />was excessively long because of poor percolation characteristics of the pulverized solids. <br />Also, to insure reproducible data, usually six replicate tests were performed. Weekly <br />extracts were usually an:tlyrxct for pH, SOS, and acidity. The test duration was only 8 to <br />lU weeks. 1'.xtracts were usually not analyzed frn metals. if acid conditions are <br />established by oxidation of sulfide minerals, metals (some tunic) will be mobilized. <br /> Unc objective fur the standard HC test was to acwlcrate the rate of sulfide mineral <br /> oxidation by increasing particle surface :trey (200 mesh pulverizing). A search w:u made <br /> of arailahle literature end it was determined that a rate effect was somewhat dependent <br /> un particle surface area. However, that rate did not increase upprcciahiy fur feeds <br /> crushed finer than 10 mesh in size. <br />• "fhe modified humidity cell procedure evolved by sequentially incnrpvrating needed <br /> changes to the standard procedure. Thr Mod. 11C yrvcedure uses 1.2 kg of sample <br /> crushed to 100°/n - t/s" (I)DIOM) in size. Because the teed is coarser (more <br /> representative of waste ruck size), dry anJ humid air c;an he pumped upwards through <br /> the charge insuring more intimate air/water/solids contact. Saturation of the solids and <br /> subsequent draining is easier anti of shorter duration because of improved percolation <br /> characteristics of the coarser feed. Elf[raCLt (weekly) arc analyzed fur at least pH, SO„ <br /> acidity, and alkalinity. Meulls analyses arc generally perlormed to determine mohiliry of <br /> toxic metals if acid conditions are established. The Mod. HC procedure and related <br /> antyses un feed solids and IiC extracts used by MLI may differ from other laboratories. <br /> Consequently, costs may be higher. It is our philosophy to emtduct the HC test in a <br /> manner to provide all necessary Jata to predict the lung term hchavior of the waste <br /> amre accurately. Humidity cell tests should be run long enough so that the rate of acid <br /> pruttuclion is predictable. over a long term, Lhcperience has taught us that HC kinetic <br /> tests should he run for a minimum of 2U weeks, We recwnmend that additional analyses <br /> be performed on weekly extracts to obtain additional important data. Weekly extracts <br /> are analyzed for pH, SO,, acidity, alkalinity, emf (rih), total iron, ferrous iron, and ferric <br /> iron. The additive of the alkalinity analysis is helpful in establishing the rate of <br /> alkalinity consumption. Fh, and the three iron analyses :tee very important far <br /> determining if and when strung oxidizing conditions are established during the kinetic <br /> test. Soong oxidizing wnJitiuns arc established when F.h values arc consistently over <br /> SSII mv. lron mobility, especially I'ur pyritic wastes, is another indiuuor of an oxidizing <br />• ' <br />' <br /> Mcl <br />I-IiLI,ANU I.ARC)ltAl <br />C)KII:S, INC. <br />P <br />1 <br />r <br />