Leaching cement

Fujita, T Sugiyama, D., Swanton, S. W. Myatt, B. J. 2003. Observation and characterization of colloids derived from leached cement hydrates. Journal of Contaminant Hydrology, 61, 3-16. 

M. RothweU, H.M. Anstice, G.J. Pearson, The uptake and release of fluoride by ion-leaching cements after exposure to toothpaste, J. Dent. 26 (1998) 591-597. 

The corrosion process consists in migration of chloride ions into the concrete with simultaneous OH ions diffusion in an opposite direction, from the leached cement hydrates. Therefore the porosity, and consequently the permeability of concrete is of fundamental importance. Cement matrix is a part of concrete which is subjected to the destructive action as a first one. The attack of chloride ions on cement paste will be then discussed first. However, some information concerning the effect of aggregate on the chloride ions diftusion is needed for comprehensive presentation of phenomena occurring in concrete. 

Figure 2 Process flowsheet of FeCfi leaching-cementation with iron powder...

Syneresis of sodium silicate gels may occur under some conditions, eg, in pure gels or coarse formations. Cement grouting should then precede chemical grouting. Leaching that results from dissolution under water-saturated conditions may be eliminated by use of proper reagent proportions. 

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). 

Aeration must be avoided since it can oxidize and resolubiUze the cemented (precipitated) impurities. Filter presses are used after each step and the cakes are leached to recover various values. For example, cadmium is dissolved, recemented with zinc, and recovered on site either electrolyticaHy or by distillation. A copper residue of 25—60% copper is sold for recovery elsewhere. The other impurities cannot be recovered economically with the exception of cobalt in some plants. 

Cement plants in the United States are now carehiUy monitored for compliance with Environmental Protection Agency (EPA) standards for emissions of particulates, SO, NO, and hydrocarbons. AH plants incorporate particulate collection devices such as baghouses and electrostatic precipitators (see Air POLLUTION CONTROL methods). The particulates removed from stack emissions are called cement kiln dust (CKD). It has been shown that CKD is characterized by low concentrations of metals which leach from the CKD at levels far below regulatory limits (63,64). Environmental issues continue to be of concern as the use of waste fuel in cement kilns becomes more widespread. 

Hydrometallurigcal Processes. In hydrometaHurgical processes, metal values and by-products are recovered from aqueous solution by chemical or electrolytic processes. Values are solubilized by treating waste, ore, or concentrates. Leaching of copper ores in place by rain or natural streams and the subsequent recovery of copper from mnoff mine water as impure cement copper have been practiced since Roman times. Most hydrometaHurgical treatments have been appHed to ores or overburden in which the copper was present as oxide, mixed oxide—sulfide, or native copper. PyrometaHurgical and hydrometaHurgical processes are compared in Reference 34. 

Cementation. Cementation is the precipitation of copper from copper leach solutions by replacement with iron. It was formerly the most commonly used method of recovering copper from leach solutions but has been replaced by solvent extraction—electro winning. The type of iron used ia cementation is important, and the most widely used material is detinned, light-gauge, shredded scrap iron. This operation can be performed by the scrap iron cone (Keimecott Precipitation Cone) or a vibrating cementation mill that combines high copper precipitation efficiency and reduced iron consumption (41). 

Polyphosphoric acid supported on diatomaceous earth (p. 342) is a petrochemicals catalyst for the polymerization, alkylation, dehydrogenation, and low-temperature isomerization of hydrocarbons. Phosphoric acid is also used in the production of activated carbon (p. 274). In addition to its massive use in the fertilizer industry (p. 524) free phosphoric acid can be used as a stabilizer for clay soils small additions of H3PO4 under moist conditions gradually leach out A1 and Fe from the clay and these form polymeric phosphates which bind the clay particles together. An allied though more refined use is in the setting of dental cements. 

Although fluoride is added as the tin salt, fluoride release is accompanied by the release of aluminium and not tin (de Freitas, 1973). There is little leaching out of tin apart from an initial wash-out. Of course, aluminium is not released from the normal cement (Wilson, 1976 Wilson, Abel Lewis,... 

For the various AB cements used in clinical dentistry, erosion and/or leaching of components have been considered important in assessing durability (Wilson McLean, 1988). In fact, the two aspects are not... 

Recovery of copper from solutions generated in heap/dump leaching with sulfuric acid by cementation with iron. 

Purification of leach liquors - as an example, mention may be made of the removal of cadmium and thallium from a zinc sulfate solution. In this case it is very convenient to use metallic zinc as a cementing metal, since the zinc that enters into solution is recovered subsequently. 

Separation of two metals from a leach solution - as examples, mention may be made of solutions containing cobalt and copper, and nickel and copper. The former solution is treated by metallic cobalt to precipitate copper, and the latter by metallic nickel to precipitate copper. In both cases, the metal added to cement the copper is recovered afterwards. 

---