Carbonate grains precipitation

Calcium carbide is used to produce acetylene. Some of the other chemicals made with lime include calcium hypochlorite, citric acid, and sodium alkalis. Lime is used to produce precipitated calcium carbonate (PCC), which is a fine-grained form of calcium carbonate. To produce PCC, lime is hydrated to produce slaked lime and the slaked lime is combined with water to produce limewater. Carbon dioxide is added to the limewater, causing calcium carbonate to precipitate as PCC. PCC is used widely in plastics production, papermaking, pharmaceuticals, and the petrochemical industry. 

Fig. 6. Percentages of carbonate cement (fringe and late calcite cement) and carbonate grains in lenses and host rocks. Host rocks contain only late calcite cement. Percentages were obtained by point counting thin sections. In host rocks, a statistically significant linear relationship exists between the two variables (expressed by the linear function Y= 1.80 A- 0.87 / = 0.88 n = 18). This suggests that late calcite cementation occurred relatively late after most metastable carbonate grains had been dissolved. Cementation only occurred when carbonate grains were available as nuclei for cement precipitation.

The amounts of calcite cement in the host rocks show a positive correlation with the amount of carbonate grains (Fig. 6). This indicates calcite cement precipitation after dissolution of metastable carbonate grains. Stable carbonate grains (which survived dissolution) served as nuclei for late calcite cement in host rocks. Within lenses, late calcite cement merely filled all available pores, which can be ascribed to either an abundance of nuclei or locally low permeability. The lack of correlation between calcite cement and secondary porosity suggests relatively long-range transport of dissolved CaCOj. Late calcite cement precipitated preferentially within lenses that contained more calcite nuclei than surrounding host rocks. 

Besides mixing different polymers, there are other possible ways to obtain high-impact plastics, for example copolymerization and graft polymerization or use of plasticizers in brittle polymers. These products will not be treated in this section. In the case of PVC-U, addition of ultrafine precipitated calcium carbonate (grain size 75 nm) has excellent impact cmiditioning and surface quality-enhancing effects. 

Titaruum and niobium additions equal to five or ten times the carbon content, respectively, permit the carbon to precipitate as titanium or niobium carbides during a sensitizing heat treatment. The carbon precipitation does not reduce the chromium content of the grain boundaries. 

Precipitation of carbides At the grain boundaries of stainless steels, chromium reacts with carbon to precipitate chromium carbides. Carbides of Ni, molybdenum, titanium and niobium, are well-known. Chromium carbide is represented by Cr23Cg. 

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

On slow cooling the reverse changes occur. Ferrite precipitates, generally at the grain boundaries of the austenite, which becomes progressively richer in carbon. Just above A, the austenite is substantially of eutectoid composition, 0.76% carbon. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

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