Alkalinity concentrations, limestone

Figure 6. Total alkalinity concentrations in limestone DA pilot plant.

The second most common alkalinity control agent is lime [1305-78-8] normally in the form of calcium hydroxide [1303-62-0], used in both water and oH muds. In the latter, the lime reacts with added emulsifiers and fatty acids to stabHi2e water-in-oH emulsions. Lime is used in brine systems containing substantial quantities of soluble calcium and in high pH lime muds. Concentrations are ca 6—57 kg/m (2—20 lb /bbl) (see Lime AND LIMESTONE). 

The Pf and P test results indicate the reserve alkalinity of the suspended solids. As the [OH"] in solution is reduced, the lime and limestone suspended in the mud will go into solution and tend to stabilize the pH. This reserve alkalinity generally is expressed as an equivalent lime concentration, in Ib/bbl of mud. 

Bottom ash from power stations is less of a problem compared with fly ash for the contamination of natural waters firstly because the proportions of fly ash to bottom ash are approximately three to one and a greater proportion of the bottom ash is used (ECOBA 2003). Secondly, the volatile elements are depleted compared with fly ash (Clarke Sloss 1992). Other combustion residues include fluidized-bed boiler ashes and the products from flue gas desulphurization (FGD). The non-regenerable FGD systems commonly use limestone, slaked lime, or a mixture of slaked lime and alkaline fly ash that are sprayed into the flue gases to remove SO2 (Clarke Sloss 1992). Although 90 wt% of the product is used to replace natural gypsum in plasters and wallboards, there is currently a small excess production in Europe of that is disposed of in landfill and equivalent sites (ECOBA 2003). Because the FGD plant treats the cooled flue gases volatile elements are concentrated and there will be similarities with fly ash. 

Elevated arsenic concentrations in oxic aquifers in Arizona (US) were linked to pH-dependent desorption (Robertson, 1989). Similar results exist for metamorphic aquifers in New England (US), where moderately alkaline waters (pH 7.5-9.3) were found to have elevated concentrations of arsenic (Robinson and Ayotte, 2006). Conversely, (BGS (British Geological Survey), 1989) suggested that arsenic concentrations of <4pgF-1 in water of the Lincolnshire Limestone (UK) cannot be explained by pH values of 7.0-9.5. McArthur et al. (2004) commented that the observations of pH increases with arsenic mobilization by Welch, Lico and Hughes (1988) and Robertson (1989) are not by themselves sufficient to prove that arsenic is mobilized by increasing pH. Arsenic may be mobilized by extended residence times, evaporation, and/or weathering, any of which could lead to both increases in pH and dissolved arsenic concentrations. 

Bicarbonate ion is usually the chief anion in freshwaters. In and on silicate rocks, the HCOj concentration is usually 50 to 200 mg/L, whereas in groundwaters that contact a few percent carbonate materials up to pure limestone and dolomite, bicarbonate levels are usually in the range of 200 to 400 ppm. Seawater contains 140 mg/L HCOj. Carbonate alkalinity (CO3 ) rarely exceeds 10 mg/L. Why The presence of caustic alkalinity (free OH ) at pH s above 10 usually indicates artificial contamination of a water by, for example, Ca(OH)2 (portlandite) from the setting of concrete at newly completed wells. Cg concentrations can reach 1000 ppm as HCO3 in sodium carbonate-bicarbonate brines found in evaporative, closed basin lakes. 

The buffering activity of adipic acid limits the drop in pH that normally occurs at the gas-liquid interface during SO2 absorption, and the resultant higher concentration of SO2 at the interface significantly accelerates the liquid-phase mass transfer. The capacity of the bulk liquor for reaction with SO2 is also increased by the presence of calcium adipate in solution. Thus, the SO2 absorption becomes less dependent on the dissolution rate of limestone or calcium sulfite in the absorber to provide the necessary alkalinity. 

The dissolution increases alkalinity (HCOa ) with the added benefit of increasing an equivalent amount ofhardness (Ca ). Limestone is a sedimentary rock primarily composed of calcite, aragonite and vaterite. Calcite can be pure in composition (CaCOa) or can contain low concentrations of magnesium-forming magnesium calcite. Natural limestone can contain minerals or impurities such as dolomite [CaMg(C03)2] and quartz (Si02). 

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