Thermochemical reduction process

At a concentration of 22.7 g kg sodium is among the most abundant elements in the Earth s crust, and is found in relatively pure form in extensive deposits of chloride, sulfate, and other salts. With this concentration it occupies seventh place in the frequency list of elements. Of all species dissolved in ocean water, sodium is that with the highest concentration, about 11 gkg . Sodium chloride, occurring as rock salt or halite, is by far the most common natural source of sodium other important sodium salts found widely in nature are sodium borate (kernite), sodium carbonate (trona), sodium nitrate (Chile saltpeter), and sodium sulfate (mirabilite) (Klemm et al. 2000). The history of the industrial production of sodium, which extends over more than 100 years, can be divided into four periods. Thermochemical reduction processes were used in two factories between 1854 and 1890. The annual production of sodium was 5-6 tons in 1854, and > 150 tons between 1888 and 1890. 

In vacuum thermochemical reduction process, aluminum and silicon are suitable reduction agents [5, 6]. Vacuum aluminothermic reduction lithium is from a US patent about aluminum reduction of lithium oxide. Aluminum reduction of spodumene has been reported by Stauffer [7]. Lithium is difficultly reduced if not adding calcium oxide into spodumene. When the mass ratio of calcium oxide and spodumene is 3 2, the maximum productivity was 92.2% under the conditions of 1050 1150"C for 2 hours. Fedorov and Shamrai used aluminum to reduce lithium aluminate, and pointed out that the lithium productivity could reach 95% when the reduction temperature was 1200 C and the system pressure was below 0.0013 Pa [4]. The previous researches were focused on the production of lithium. But the recovery of reduction residue was not investigated. In present work, a novel vacuum aluminofliermic reduction lithium process is developed which used lithium carbonate, alumina and calcium oxide as raw materials. The products were metal lithium and high-whiteness aluminum hydroxide. 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Rubidium does not exist in its elemental metallic form in nature. However, in compound forms it is the 22nd most abundant element on Earth and, widespread over most land areas in mineral forms, is found in 310 ppm. Seawater contains only about 0.2 ppm of rubidium, which is a similar concentration to lithium. Rubidium is found in complex minerals and until recently was thought to be a rare metal. Rubidium is usually found combined with other Earth metals in several ores. The lepidolite (an ore of potassium-lithium-aluminum, with traces of rubidium) is treated with hydrochloric acid (HCl) at a high temperature, resulting in lithium chloride that is removed, leaving a residue containing about 25% rubidium. Another process uses thermochemical reductions of lithium and cesium ores that contain small amounts of rubidium chloride and then separate the metals by fractional distillation. 

One problem in refining cesium is that it is usually found along with rubidium therefore, the two elements must be separated after they are extracted from their sources. The main process to produce cesium is to finely grind its ores and then heat the mix to about 600°C along with liquid sodium, which produces an alloy of Na, Cs, and Ru, which are separated by fractional distillation. Cesium can also be produced by the thermochemical reduction of a mixture of cesium chloride (CsCl) and calcium (Ca). 

Characteristics often ascribed to MVT deposits include temperatures generally <200°C and deposition from externally derived fluids, possibly basinal brines. Sulfur isotope valnes from MVT deposits suggest two major sulfide reservoirs, one between -5 and +15%c and one greater than +20%c (Seal 2006). Both sulfide reservoirs can be related, however, to a common sea water sulfate source that has undergone different sulfur fractionation processes. Reduction of sulfate occurs either bacterially or by abiotic thermochemical reduction. High 5 S-values should reflect minimal fractionations associated with thermochemical reduction of sea water sulfate (Jones et al. 1996). 

Different methods used to prepare titanium diboride have been reviewed by Samsonov et al. (1975). At present, it is mainly produced as a powder by thermochemical reduction of boron and titanium oxides followed by hot pressing and sintering to process the final product. The less costly alternative appears to be to coat suitable substrate materials with TiB2 or TiB2-based composites by hot pressing, plasma spraying, chemical vapor deposition, etc. 

Plutonium trifluoride. Plutonium trifluoride can be converted directly to plutonium metal, or it is an intermediate in the formation of PUF4 or PUF4 -PUO2 mixtures for thermochemical reduction, as described in Sec. 4.8. The stabilized Pu(III) solution, produced by cation exchange in one of the Purex process options for fuel reprocessing, is a natural feed for the formation of plutonium trifluoride, as is shown in the flow sheet of Fig. 9.9 [03]. A typical eluent solution from cation exchange consists of 30 to 70 g plutonium/liter, 4 to 5 Af nitric acid, 0.2 Af sulfamic acid, and 03 Af hydroxylamine nitrate. The sulfamic acid reacts rapidly with nitrous acid to reduce the rate of oxidation of Pu(III) to about 4 to 6 percent per day. Addition of ascorbic acid to the plutonium solution just before fluoride precipitation reduces Pu(IV) rapidly and completely to Pu(III). 

G. G. Hoffmann and I. Steinfatt. Thermochemical sulfate reduction at steam flooding processes— chemical approach. In ACS Petrol Chem Div Preprints, volume 38, pages 181-184. 205th ACS Nat Mtg Enhanced Oil Recovery Symp (Denver, CO, 3/28-4/2), February 1993. 

Figure 16.2 Thermochemical cycles involving the heterolytic and homolytic cleavages of the R-X bond, and reduction or oxidation processes. The reaction numbers are the same as in figure 16.1.

Chapters 6 to 12 address specific groups of processes and methods employed for converting biomass to energy and fuels. In this chapter, the physical processes employed to prepare biomass for use as fuel or as a feedstock for a conversion process are discussed. The processes examined are dewatering and drying, size reduction, densification, and separation. The physical process, a few specific examples of the process, and its relationship to the thermochemical or microbial process that may be used for subsequent conversion are described. 

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