Conventional sulfate process

The first commercial process, the sulfate process, uses concentrated sulfuric acid as reagent. After use, the acid cannot readily be recycled because of its content of metals and water taken up during the process. Therefore, this acid (so-called spent acid) was typically discarded as waste. Because of the large acid quantities involved, the (conventional) sulfate process became increasingly obsolete and had to be improved to a modern version [191]. 

Ecological Aspects of Modern Production Processes. Sulfate Process. The conventional sulfate process is characterized by a linear flow of sulfuric acid through the process. Some H2SO4 ends up in the copperas by-product, but the main part remains separate from the pigment end product, as a used reagent with deteriorated quality in terms of concentration and purity. Formerly, this large stream was discarded as waste (see Fig. 80). Now, the spent acid is recovered, and complex acid concentration and filter salt treatment plants are added to recycle the acid entirely. Hence, the modern sulfate process plant for titanium dioxide pigment manufacture is characterized by a closed sulfuric acid cycle that completely withholds spent acid from the environment (see Fig. 81). 

Figure 80. Mass flow in the conventional sulfate process...

Allied-Signal Process. Cyclohexanone [108-94-1] is produced in 98% yield at 95% conversion by liquid-phase catal57tic hydrogenation of phenol. Hydroxylamine sulfate is produced in aqueous solution by the conventional Raschig process, wherein NO from the catalytic air oxidation of ammonia is absorbed in ammonium carbonate solution as ammonium nitrite (eq. 1). The latter is reduced with sulfur dioxide to hydroxylamine disulfonate (eq. 2), which is hydrolyzed to acidic hydroxylamine sulfate solution (eq. 3). 

Dutch State Mines (Stamicarbon). Vapor-phase, catalytic hydrogenation of phenol to cyclohexanone over palladium on alumina, Hcensed by Stamicarbon, the engineering subsidiary of DSM, gives a 95% yield at high conversion plus an additional 3% by dehydrogenation of coproduct cyclohexanol over a copper catalyst. Cyclohexane oxidation, an alternative route to cyclohexanone, is used in the United States and in Asia by DSM. A cyclohexane vapor-cloud explosion occurred in 1975 at a co-owned DSM plant in Flixborough, UK (12) the plant was rebuilt but later closed. In addition to the conventional Raschig process for hydroxylamine, DSM has developed a hydroxylamine phosphate—oxime (HPO) process for cyclohexanone oxime no by-product ammonium sulfate is produced. Catalytic ammonia oxidation is followed by absorption of NO in a buffered aqueous phosphoric acid... 

Traditionally, the cellular biomass is separated by centrifugation. Ammonium sulfate is then added to the supernatant to precipitate the protein product from the media. This is followed by further centrifugation and dialysis to remove the residual ammonium sulfate from the protein product. Cross-flow filtration (CFF) can replace all of these steps with a significant improvement in recovery and yield. Indeed, CFF appears to offer many advantages over conventional separation processes like centrifugation, vacuum filtration and precipitation/dialysis for this application. 

Conventional (SulfiM ) Process. Raw material is either an ilmenite (FeTiO ) concentrate from benefidation of naturally occurring ilmenite or a slag obtained from ilmenite by electrosmelting and tapping off the major part of the iron as pig iron for further use. The titaniferous material is digested with concentrated sulfuric acid, leading to a solution of sulfates of most of the metals contained in the ore, e.g. ... 

The final processing in tire production of high-purity metals is often carried out electrolytically and is referred to as electrorefining. In this process the metal to be refined, such as copper or silver, has a typical initid purity of 95 to 99% and the aim is to reduce the impurity level to less than 0.1%. Conventional purification processes are often either inadequate or too expensive for this purpose. In electrorefining, the impure metal, e.g., copper, is placed in an electrolytic bath as an anodic plate that is paired with a cathode on which the purified metal is deposited electrolytically. The electrolyte typically consists of an aqueous solution of a salt of the metal to be purified, for example, copper sulfate, and the electrolytic cell is composed of an array of closely spaced alternating cathodes and anodes. A sample electrode pair and the configuration of the electrolytic cell are shown in Figure 3.2a. 

The reaction is mn for several hours at temperatures typically below 100°C under a pressure of carbon monoxide to minimise formamide decomposition (73). Conversions of a-hydroxyisobutyramide are near 65% with selectivities to methyl a-hydroxyisobutyrate and formamide in excess of 99%. It is this step that is responsible for the elimination of the acid sludge stream characteristic of the conventional H2SO4—ACH processes. Because methyl formate, and not methanol, is used as the methylating agent, formamide is the co-product instead of ammonium sulfate. Formamide can be dehydrated to recover HCN for recycle to ACH generation. 

The formation of oxime and rearrangement to caprolactam are conventional. The rearrangement produces 1.5 kg of the total 2.4 kg by-product ammonium sulfate per kilogram of caprolactam. Purification is accompHshed by vacuum distillation. A similar caprolactam process is offered by Inventa (11). 

Toray. The photonitrosation of cyclohexane or PNC process results in the direct conversion of cyclohexane to cyclohexanone oxime hydrochloride by reaction with nitrosyl chloride in the presence of uv light (15) (see Photochemical technology). Beckmann rearrangement of the cyclohexanone oxime hydrochloride in oleum results in the evolution of HCl, which is recycled to form NOCl by reaction with nitrosylsulfuric acid. The latter is produced by conventional absorption of NO from ammonia oxidation in oleum. Neutralization of the rearrangement mass with ammonia yields 1.7 kg ammonium sulfate per kilogram of caprolactam. Purification is by vacuum distillation. The novel chemistry is as follows ... 

Snia Viscosa. Catalytic air oxidation of toluene gives benzoic acid (qv) in ca 90% yield. The benzoic acid is hydrogenated over a palladium catalyst to cyclohexanecarboxyhc acid [98-89-5]. This is converted directiy to cmde caprolactam by nitrosation with nitrosylsulfuric acid, which is produced by conventional absorption of NO in oleum. Normally, the reaction mass is neutralized with ammonia to form 4 kg ammonium sulfate per kilogram of caprolactam (16). In a no-sulfate version of the process, the reaction mass is diluted with water and is extracted with an alkylphenol solvent. The aqueous phase is decomposed by thermal means for recovery of sulfur dioxide, which is recycled (17). The basic process chemistry is as follows ... 

Symmetrical and unsymmetrical benzoins have been rapidly oxidized to benzils in high yields using solid reagent systems, copper(II) sulfate-alumina [105] or Oxone-wet alumina [105, 106] under the influence of microwaves (Scheme 6.32). Conventionally, the oxidative transformation of a-hydroxy ketones to 1,2-diketones is accomplished by reagents such as nitric acid, Fehling s solution, thallium(III) nitrate (TTN), ytterbium(III) nitrate, ammonium chlorochromate-alumina and dayfen. In addition to the extended reaction time, most of these processes suffer from drawbacks such as the use of corrosive acids and toxic metals that generate undesirable waste products. 

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