Silicates Specification Requirements

Table 2.10 gives the relative share of the various core-making processes in German automobile foundries in 1991. This shows that the amine cold-box and hot-box systems dominate the market. Over 90 % of the automobile foundries use the amine cold-box system. The other processes (croning, C02-silicate) are mainly used for supplementary use, i.e. making cores with specific requirements (size, thickness,...). 

Typical primary mixtures contain some of the following lead styphnate, tetrazene, aluminum, antimony sulfide, calcium silicate, lead peroxide, boron, metals, barium nitrate, secondary explosive, binder, sensitizer, etc. (Fig. 1.3). Variations in the ingredients and their relative amounts result in compositions which possess sensitivity and ignition properties tailored to specific requirements. 

Each mineral raw material has a specific influence on the rheology of the paste, the development of the microstmctme, the phases formation during the heat treatment and the properties of the finished product. The manufacture of all silicate ceramics requires such a large number of raw materials, which cannot be discussed here. Only those most commonly used, i.e. clays, feldspars and silica, will therefore be described. 

An understanding of much of aqueous geochemistry requires an accurate description of the water-mineral interface. Water molecules in contact with> or close to, the silicate surface are in a different environment than molecules in bulk water, and it is generally agreed that these adsorbed water molecules have different properties than bulk water. Because this interfacial contact is so important, the adsorbed water has been extensively studied. Specifically, two major questions have been examined 1) how do the properties of surface adsorbed water differ from bulk water, and 2) to what distance is water perturbed by the silicate surface These are difficult questions to answer because the interfacial region normally is a very small portion of the water-mineral system. To increase the proportion of surface to bulk, the expanding clay minerals, with their large specific surface areas, have proved to be useful experimental materials. 

The fibrous glass used for insulation materials does not require starting materials of high purity. Beach sand, which consists largely of the mineral quartz, is easily obtainable and quite inexpensive. Alternatively, silicate slags or waste rock from mining can be melted and processed to create the products called rock-, slag-, and mineral-wool. Such materials are used for insulation. Where specific properties are desired, such as resistance to ultra-... 

These micellar cubic mesophases require large surface curvature and low charge density. Their formation is thus favored by the use of surfactant molecules with large polar head group, and acidic conditions under which the charge density at the silicate/surfactant is always limited. The fact that this phase can be prepared with CTAB when PTES is present, suggests the existence of specific interactions between the phenyl groups and the polar head of the surfactant molecules. It was indeed reported that benzene molecules are preferably located at the hydrophilic-hydrophobic interface [29]. 

The synthesis of crystalline microporous titanium silicate requires conditions that are specific to each product, such as the chemical nature of the structure directing agent, reactant concentrations, temperatures, and times of crystallization. For brevity, only a short outline of the basic rules that apply to the synthesis of all titanium silicates will be given, with reference to a few selected papers in which clear, detailed descriptions of the syntheses can be found. 

The flotation process is applied on a large scale in the concentration of a wide variety of the ores of copper, lead, zinc, cobalt, nickel, tin, molybdenum, antimony, etc., which can be in the form of oxides, silicates, sulfides, or carbonates. It is also used to concentrate the so-called non-metallic minerals that are required in the chemical industry, such as CaF2, BaS04, sulfur, Ca3(P03)2, coal, etc. Flotation relies upon the selective conversion of water-wetted (hydrophilic) solids to non-wetted (hydrophobic) ones. This enables the latter to be separated if they are allowed to contact air bubbles in a flotation froth. If the surface of the solids to be floated does not possess the requisite hydrophobic characteristic, it must be made to acquire the required hydrophobicity by the interaction with, and adsorption of, specific chemical compounds known as collectors. In separations from complex mineral mixtures, additions of various modifying agents may be required, such as depressants, which help to keep selected minerals hydrophilic, or activators, which are used to reinforce the action of the collector. Each of these functions will be discussed in relation to the coordination chemistry involved in the interactions between the mineral surface and the chemical compound. 

The US military specification (Ref 9a) contains the following chem and physical requirements for Ca silicate used as an absorbant in expls... 

The US military specifications embodying the requirements for Na silicate are presented in 0-S-604 C (1) for Sodium Metasilicate, Technical (Ref 12a), P-S-651E (1) for Sodium Orthosilicate, Technical (Ref 12b) and O-S-605D for Sodium Silicate Solution (Ref 6a)... 

Quaternary amines, such as tetraalkylammonium bromides and hydroxides (the alkyl group being Q to C4) are the typical zeolite templates. Quaternary amines fulfill the above-mentioned requirements of stability, specific interaction with the precursor (electrostatic interaction between quaternary amines and silicate), and easy removal (by calcination). 

Starch can be enzymically converted in the presence of pigment. The conversion follows a similar time-temperature cycle as in neat starch conversion. The pigment will adsorb a portion of the enzyme adsorption can be minimized by the addition of sodium silicate to the mixture prior to the addition of the enzyme (Vanderbilt process). Even with silicate treatment, a higher quantity of enzyme will be required to reach a specific viscosity target. Other coating components, such as latex and lubricants, have to be added after the conversion. The Vanderbilt process is now rarely used for the preparation of coating binder. 

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