Sulfur leaching rates

Sulfur Polymer Cement. SPC has been proven effective in reducing leach rates of reactive heavy metals to the extent that some wastes can be managed solely as low level waste (LLW). When SPC is combined with mercury and lead oxides (both toxic metals), it interacts chemically to form mercury sulfide, HgS, and lead sulfide, PbS, both of which are insoluble in water. A dried sulfur residue from petroleum refining that contained 600-ppm vanadium (a carcinogen) was chemically modified using dicyclopentadiene and oligomer of cyclopentadiene and used to make SC (58). This material was examined by the California Department of Health Services (Cal EPA) and the leachable level of vanadium had been reduced to 8.3 ppm, well below the soluble threshold limit concentration of 24 ppm (59). 

Figure 6. A comparison of leaching rates of sulfur-infiltrated and -uninfiltrated concrete cylinders by acids of different concentrations...

A second area of concern is that of reduced tree growth in forests. The leaching of nutrients from the soil by acid deposition may cause a reduction in future growth rates or changes in the type of trees to those able to survive in the altered environment. In addition to the change in soil composition, there are the direct effects on the trees from sulfur and nitrogen oxides as well as ozone. 

Similar rate equations can be developed for the parallel leaching of other minerals (see Eq. 3). The oxidation of intermediate sulfur compounds to sulfates such as Eq. 2 is a consecutive reaction to mineral leaching. Other reactions are virtually instantaneous, e.g., precipitation of insoluble ferric arsenates from soluble NH4H2ASO4 which is consecutive to Eq. 3. On the other hand, the instantaneous shift in the amonia -ammonium equilibrium,... 

The inherent instability of sulfur-infiltrated concrete in aqueous media illustrated in this study may be the most important factor in utilization, because it will affect long-term durability of the concrete in many natural settings. The Ca(OH)2 produced by the hydration of portland cement is a principal reactant in the leaching process, and while it remains sulfur could be extracted, leaving the matrix vulnerable to other destructive processes. The removal rate of sulfur will vary greatly, depending mostly upon the pH of the immersion medium thus, the concrete deteriorates in alkaline sulfatic soils but is relatively stable in the corrosive neutral sulfatic solutions from the sodium sulfate plant. 

Between 45 and 90°C, the reaction of cubanite with acidic ferric sulfate solutions followed linear kinetics, indicating that the rate-controlling step was some reaction occurring on the surface of the cubanite. The dissolution rate increased with ferric ion concentration and decreased with increasing concentration of sulfuric acid and ferrous sulfate. The naturally slow reaction was accelerated with the addition of NaCl or HCl. The addition of salt in a dump leaching operation would be a relatively easy and cheap procedure to attain increased reaction rates. 

Vizsoljd et al. (V9) proposed a scheme to produce Pb from galena concentrates without producing elemental sulfur via aqueous oxidation of the lead sulfide in ammonia solutions at temperatures below 100°C and in the presence of 20 psi of oxygen. A 3-to-l molar ratio of ammonium sulfate to galena is maintained in the presence of associated iron in the concentrate to assure rapid reaction rate. Lead sulfate and ammonium sulfate are produced and the lead can be recovered following a procedure similar to the amine leaching process proposed by the same authors (F12). 

The rate-determining step in an ion-exchange process is the diffusion of the adsorbable ions into the resin matrix. Retention times of 2-10 min are used in the uranium industry to attain full equilibrium. The metal ion to be recovered must almost completely occupy the resin fimctional sites to attain a very high degree of selectivity. Resins should be useful for at least two years if clean clarified leach liquors containing no poisonous ions are used. A drastic reduction in the usefulness of the resins is observed in the presence of such ions. Everest et al. (Ell) studied in detail the deleterious effects of thiocyanates, polythionates and sulfur, cobalt... 

XPS study by Buckley and Woods (1984b) showed that freshly fractured chalcopyrite surfaces exposed to air formed a ferric oxyhydroxide overlayer with an iron-deficient region composed of CuSi. Acid-treated surfaces of fractured chalcopyrite showed an increase in the thickness of the CuS2 layer and the presence of elemental sulfur. Hackl et al. (1995) suggested that dissolution of chalcopyrite is passivated by a thin (< 1 pm) copper-rich surface layer that forms as a result of solid-state changes. The passivating surface layer consists of copper polysulhde, CuS , where n > 2. Hackl et al. (1995) described the dissolution kinetics as a mixed diffusion and chemical reaction whose rate is controlled by the rate at which the copper polysulhde is leached. The oxidation of chalcopyrite in the presence of ferric ions under acidic conditions can be expressed as... 

Nesbitt et al. (1995) conducted a detailed study of the oxidation of arsenopyrite in oxygenated solutions. Arsenic and sulfur were observed to exist in multiple oxidation states near the pristine surface. After reaction with air-saturated distilled water, Fe(III) oxyhydroxides formed the dominant iron surface species, and As(V), As(III), and As(I) were as abundant as As(—I) surface species. An appreciable amount of sulfate was observed on the mineral surface. Arsenic was more readily oxidized than sulfur, and similar rates of the oxidation of As(—I) and Fe(II)" surface species were observed. Nesbitt et al. (1995) concluded that continued dilfusion of arsenic to the surface under these conditions can produce large amounts of As " " and As, promoting rapid selective leaching of arsenites and arsenates. 

Attempts to increase pyrite removal by increasing the reaction time met with limited success under our standard conditions because reaction of the ferric ion with the coal matrix depleted the ferric ion that was needed for extraction of the pyrite. Thus, for example, increasing the coal reaction time from 2 to 12 hrs only increased pyritic sulfur removal from 60 to 80% for Pittsburgh coal. Similar results were obtained for the other three coals. The only alternatives were to increase the amount of leach solution or to use a continuous or semi-continuous (multiple-batch) reactor. A multiple-batch mode was chosen because it was a simple laboratory procedure and at the same time it could approximate conditions encountered in a commercial plant. A 1-hr-per-batch leach time was used because our 2 hr results indicated that in the early stages of removal the rate begins to decrease after 1 hr, and six leaches (or batches) per run were used to assure that any pyrite that could be removed in a reasonable amount of time was removed. The progress of removal was monitored by analyzing the sulfate content in each spent leach solution elemental sulfur was not removed until all the leaches were completed. Table VII shows pyrite extraction as a function of successive leaches as followed by sulfate analysis of the leach solution. Note that the major portion of pyritic sulfur is removed in the first two leaches or 2 hrs, followed by lesser amounts in... 

As mentioned earlier (see p. 374), organisms other than T. ferrooxidans have been demonstrated to catalyse the oxidation of ferrous iron at low pH. The Sulfolobus-like organism isolated by Brierley and Brierley (1973) had a temperature optimum of 70 C and oxidized both sulfur and ferrous iron the rates of oxidation are considerably lower than those recorded for T. ferrooxidans. The isolate, however, was able to oxidize molybdenite, M0S2, at 60°C and the rate was increased by addition of ferrous sulfate. The organism showed a unique tolerance to molybdenum (2 g l ) (Brierley and Murr, 1073). Whether organisms of this type play a significant role in the oxidation of sulfide minerals under the conditions of elevated temperature known to exist in leaching heaps, remains to be demonstrated. 

Bacterial leaching of uranium ores is the subject of a subsequent section of this chapter. Here it suffices to say that the bacterial activity in such situations is that of enhancing the rate of oxidation of sulfides, through which ferric sulfate and sulfuric acid are liberated to speed up the process of solution of uraninite. 

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