P-Hemihydrate

P-Hemihydrate. The dehydration of gypsum, commonly referred to as calcination in the gypsum industry, is used to prepare hemihydrate, or anhydrite. Hemihydrate is generally called stucco in North America and plaster in many other continents. In North America, plaster is differentiated from hemihydrate or stucco by the inclusion of additives to control intended use properties, eg, rehydration time, density, coverage, strength, and viscosity. 

Calcium sulfate P-hemihydrate, 4 583 manufacture of, 4 588-590 Calcium sulfate dihydrate (gypsum), 4 582 5 479t... 

Calcium. sulfate a- and P-hemihydrates have different crystal habits and specific surface areas... 

Dry dehydration yields calcium sulfate p-hemihydrate and/or anhydrite... 

Type 1 dry processes which lead to calcium sulfate P-hemihydrate and multiphase plaster, and... 

KEY WORDS gypsum, drying, anhydrite. a hemihydrate, P-hemihydrate. phase anal ysis, drying curves, water sorption... 

Figure 2 shows typical drying curves for commercial p-hemihydrate that was preconditioned in three different environments to prepare samples with different amounts of free moisture. The curves exhibit a definite plateau after only a few minutes of exposure to microwaves, which is a strong indication that the weight loss is due to free water removal only. 

It is usual, in -hemihydrate preparation by gypsum dehydration, to do a reversion because the fired products may contain some anhydrite 111. Dehydration products after a certain amount of time are placed in a wet atmosphere. Figure 7 shows variations of t an a function of conservation time in a wet atmosphere (81% relative humidity) for a P-hemihydrate, without any previous drying of the products. 

The only clearly defined crystalline compositions are three forms of phosphoric acid and hemihydrate, pyrophosphoric acid, and crystalline P O q. The phosphoric acids obtained in highly concentrated solutions or by mixing phosphoric acid with phosphoms pentoxide are members of a continuous series of amorphous (excluding [Y OO]) condensed phosphoric acid mixtures. Mixtures having more than 86% P2O5 contain some cyclic metaphosphoric... 

The compound is very soluble in most organic solvents. In order to get a high recovery, it is necessary to complete the crystallization in the deep freeze. From aqueous ethanol the aldehyde crystallized in high yield as the hemihydrate, m.p. 95°. 

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42-35 °C) and there is a crystalline hemihydrate, 2H3PO4.H2O, which melts at 29-35 °C. The acid is tribasic and in aqueous solution has three ionization constants (pA J 2-15,7-1 and 12-4. 

The product of interaction of trinitrobenzene and cone, aqueous potassium hydroxide in methanol is explosive, and analyses as the hemihydrate of a hemiacetal of the aci-p-quinonoid form of picric acid [1], and/or the mesomeric o-forms [2],... 

Figure 1 shows a process diagram for a typical Nissan H installation (3). The slurry produced by the digester contains about 30% solids (mainly hemihydrate with a little undissolved rock) suspended in an acid liquor, typically about 30% by wt P Og with a few percent of sulphuric acid. The crystallizers (threi in this case), each with... 

The adenine radical cation was observed in a single crystal of adenine hydrochloride hemihydrate [43]. In this crystal, the adenine is protonated at Nl. After electron loss, the molecule deprotonates at Nl, giving Ade(Nl -l-H, Nl-H). This produces a radical that is structurally equivalent to the cation of the neutral adenine molecule with spin density on C8 and N6 [p(C8) = 0.17 and p(N6) = 0.25]. The adenine radical cation is strongly acidic (pi a< 1) [22]. This strong driving force makes the reaction independent of environmental conditions. In single crystals of adenosine [42] and anhydrous deoxyadenosine [44], the N6 deprotonated cation [Ade(N6-H) ] is observed which is characterized by p(C8) = 0.16 and p(N6) = 0.42. The experimental isotropic hyperfine couplings are N6-H = 33.9 MHz and C8-H = 12.4 MHz. 

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