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Metal dissolution from tailings collected at a gold-copper mine mill site was examined by researchers from the U.S. Bureau of Mines using column leaching procedures and on-site field monitoring wells. The 3-year laboratory study reviewed effects of column waste depths, dry cycles, waste sampling locations, and metal dissolution abatement techniques. Results of the laboratory testing indicated that the concentrations of metals in the leachate gradually decreased with each leaching and that metal release from unsaturated tailings was enhanced in most, but not all, samples. Sulphide-rich tailings produced leachates with a pH in the range of 2 to 4, sulphate concentrations as high as 40,000 mg/L, and copper concentrations in the range of 0.2 to 2,400 mg/L. The effects of abatement treatments using phosphate, lime, and sodium lauryl sulphate were also examined, but the treatments were demonstrated to be of only marginal value. The interaction between tailings and wood chips, and tailings and a compost-peat moss mixture was also studied.

Researchers from the U.S. Bureau of Mines have been conducting a series of laboratory studies to examine the general characteristics of metal dissolution from mine wastes, as well as possible chemical control strategies. The studies reported upon in this paper involved samples of sulfidic waste rock obtained from a gold-mining operation. Experimental methods included columns, perforated baskets, and Buchner funnels, coupled with humidity and controlled-atmosphere chambers. Results indicated that concentrations of soluble metals were one to two orders of magnitude higher in the leachate obtained during the initial two or three leachings than in leachates collected later. During the initial seven teachings, samples stored and leached under nitrogen produced the same metal leachate concentrations as samples stored and leached in laboratory air. The biocide sodium benzo-ate had little effect on the initial phase of dissolution, although another biocide, sodium lauryl sulfate, did reduce initial metal...

Metal dissolution from tailings from six lead-zinc mine and milling sites in the western United States was examined by researchers from the Bureau of Mines using a column leaching procedure involving a formulated “western rain” leachant. This 2-year laboratory study included the effects of column waste depth, dry cycles with and without oxygen, and pore water evaporation. The results indicated that leachate metal concentrations gradually decreased with each leaching, but that the degree of enhanced metal mobilization from unsaturated tailings varied widely from waste to waste. Further studies on selected samples from this group of tailings demonstrated that the driving force behind enhanced metal dissolution is the oxidation of sulphide minerals by atmospheric oxygen during the evaporation of pore water from the columns of tailings. Leachates from water-saturated columns of these tailings gave pH values from 6.5 to 8.5; acid production and dissolved metal concentrations were enhanced by alternating wet and dry cycles. All studies to date indicate that maintaining tailings at or near saturation and/or excluding atmospheric oxygen produce leachates of nearly constant to slowly decreasing metal concentrations with each subsequent leaching.

The dissolution of metals from various metal-mine tailings has been studied through a series of submerged tailings batch tests at U.S. Bureau of Mines laboratories. Tailings selected for this study included both reactive (as determined by column leaching studies) and benign tailings. Metal mobilization in aerated (97 pct saturated with oxygen) samples appeared to be consistently greater than metal dissolution from stagnant (79 pct or 70 pct saturated with oxygen) waters. In the early weeks of the study, sulfate concentrations increased with solid-liquid contact time, while calcium dissolution remained nearly constant. Metal concentrations varied in relation to solid-liquid contact time and aeration. Leachate pH appeared to be nearly independent of solid-liquid contact time for basic tailings. Reactive tailings having both a submerged and a surface component demonstrated increased metal mobility.

The dissolution of metal species from silver mine mill tailings containing quartz. (70%) and manganiferous siderite was investigated using a column leach procedure. The effects of leachant ionic strength, pH, and buffering ability were examined. The results indicated increased metal transport with increased ionic strength and decreased pH. In another test series, matched sets of columns were compared to determine the effect of leachate residence time on metal concentration. In these tests, leachate metal ion concentrations more than doubled in columns that were allowed to evaporate (wet/dry cycle) about 30% of their pore leachate volume. The pH of the leachate eluted from the dry cycle (unsaturated column) decreased while the conductance increased. Increases of five times were observed for lead and manganese, while increases of two orders of magnitude were seen for zinc and cadmium. The wet/dry cycle effect was enhanced during subsequent cycles, but appeared not to be influenced by the addition of sodium lauryl sulfate, sodium benzoate, or phenol, which are known to inhibit microorganism involvement. Increased sensitivity to further oxidation of those columns that underwent the dry cycle became apparent through increased deviation of the ion yields from duplicate columns. The interrelation of the above factors with the composition of the tailings present adds complexity to determining dissolution mechanisms.

The photolysis of cyclopentene was investigated in a static system using “gettered” xenon (147.0 nm) and krypton (123.6 nm) resonance lamps. Hydrogen, acetylene, ethylene, allene, methylacetylene, vinylacetylene and cyclopentadiene accounted for nearly 90% of the observable decomposition products. Identification of H atoms, vinyl and propargyl radicals was made through the use of numerous additives such as NO, O 2 , H 2 S, D 2 S, and CD 3 I. Seven major primary processes were identified and primary quantum yields were assigned for both 147.0 nm and 123.6 nm photolyses. At 123.6 nm photoionization occurred with an ionization efficiency of 0.16. The formation of cyclopentane at 123.6 nm is attributed to the ion—molecule condensation reaction between the parent ion and a neutral cyclopentene.

The gas-phase photolysis of 2-methyl-1,3-butadiene has been investigated using krypton (123.6 nm) resonance radiation. The observed neutral products of the primary decomposition were vinylacetylene, ethylene, acetylene, methylacetylene, propylene, allene, 2-methy-1-buten-3-yne, pentatriene/1-penten-3-yne, 1,3-butadiene, 2-butyne and butatriene, listed in decreasing order of concentration. There was also evidence of the presence of several radical fragments: CH2/CH3, C2H3, C3H3, and C4H5. Quantum yields for each of the products were determined in the photolysis of 2-methyl-1,3-butadiene, performed both in the presence and the absence of additives. Nitric oxide and oxygen were employed as radical scavengers, while hydrogen sulfide and hydrogen iodide were used as radical interceptors. Twelve primary, neutral molecule, reaction channels were proposed and the quantum efficiency assigned for each. The ionization efficiency of 2-methyl-1,3-butadiene was established as n = 0.55 at 10.03 eV. No products formed exclusively via an ion–molecule pathway were identified and therefore the fate of the C5H8+ ion was not determined.

The gas-phase photolysis of 1,1-dimethylcyclopropane has been investigated using xenon (147.0 nm) and krypton (123.6 nm) resonance radiation. Major products observed in order of decreasing importance were isobutene, ethylene, hydrogen, 1,3-butadiene 2-methyl-1,3-butadiene, propylene, allene, methylacetylene, and acetylene. Radical scavengers, NO and O2, and radical interceptors, H2S/D2S and HI, were used to determine the relative importance of radical and molecular processes. CH3, C2H3, C3H5, and C4H7 radical species were identified and quantified. Ten primary reaction channels were postulated, of which the elimination of methylene was the most predominant, accounting for 34% of the photo-decomposition at 147.0 nm and 39% at 123.6 nm. Although ionization was established at 123.6 nm (η = 0.10) the nature of a charge transfer or other ion-molecule reaction channel leading to the formation of 2-methyl-1-butene and 2-methyl-2-butene could not be determined.

The vacuum-ultra-violet photolysis of methylenecyclobutane was carried out in a static system using “gattered” xenon (147.0 nm) and krypton (123.6 nm) resonance lamps. Hydrogen, acetylene, ethylene, allene, methylacetylene, vinyl-acetylene, and 1,3-butadiene accounted for nearly 90% of the observable decomposition products. Identification of H atoms, vinyl, and propargyl radicals was made through the use of numerous additives such as NO, O2, H2S, D2S and CD3I. Six major primary processes were identified and primary quantum yields assigned for both 147.0 nm and 123.6 nm photolyses. Although photoionization was observed with an ionization efficiency of 0.20 at 123.6 nm, no ion-molecule reactions nor charge transfer processes were established.