Calcium oxalate, thermal decomposition

The sodium formate process is comprised of six steps (/) the manufacture of sodium formate from carbon monoxide and sodium hydroxide, (2) manufacture of sodium oxalate by thermal dehydrogenation of sodium formate at 360°C, (J) manufacture of calcium oxalate (slurry), (4) recovery of sodium hydroxide, (5) decomposition of calcium oxalate where gypsum is produced as a by-product, and (6) purification of cmde oxahc acid. This process is no longer economical in the leading industrial countries. UBE Industries (Japan), for instance, once employed this process, but has been operating the newest diaLkyl oxalate process since 1978. The sodium formate process is, however, still used in China. 

A. The thermal decomposition of calcium oxalate monohydrate. This determination may be carried out on any standard thermobalance. In all cases the manufacturer s handbook should be consulted for full detailed instructions for operating the instrument. 

A popular and useful device is a combined DTA/TG (simultaneous thermal analysis STA) system in which both thermal and mass change effects are measured concurrently on the same sample. An example STA study comprising DTA, TG, and DTG for the decomposition of calcium oxalate is shown in Figure 5.7. 

Calcium salts. The thermal decompositions of calcium oxalate, malonate, maleate and fumarate were studied in significantly higher temperature ranges (above 720, 612 to 653, 733 to 763 and 733 to 803 K, respectively) than those of the same salts of the transition metals. This is evidence of a stabilizing influence on these anions of the strong bond formed with this strongly electropositive cation. 

It is common practice to check the performance of a TGA system by running a sample of calcium oxalate monohydrate. This salt is known to thermally decompose in three stages over well-defined temperature ranges. The first step involves the loss of the single water of hydration molecule followed sequentially by the conversion of anhydrous calcium oxalate to calcium carbonate with the loss of carbon monoxide, and thence the decomposition of calcium carbonate to calcium oxide with the evolution of carbon dioxide. 

Thermogravimetric analysis has been widely used to study the thermal decomposition of oxy-salts, such as metal oxalates and metal sulfates. Dollimore, Griffiths and Nicholson have reported TGA data for a wide range of metal oxalates. In an atmosphere of air, these all decompose in three stages, similar to the thermal decomposition of calcium oxalate monohydrate. 

Other decompositions, which had previously been accepted as simple reactions proceeding in the solid state, have subsequently been shown to be more complicated than was discerned from overall kinetic data. The thermal breakdown of potassium permanganate exhibits almost symmetrical sigmoid curves, now regarded (39) as proceeding with the intermediate formation of K3(Mn04)2 by at least two, possibly consecutive, reactions. Dehydration of calcium oxalate monohydrate proceeds (75) with the loss of H20 molecules from two different types of site by two concurrent reactions that proceed at slightly different rates. 

Few data are available on the concentration of dicarboxylic acid anions in subsurface waters. C2 through C q saturated acid anions have been reported in addition to maleic acid (cz5-butenedioic acid) (5. 15-16L Oxalic acid (ethanedioic) and malonic acid (propanedioic) appear to be the most abundant. Reported concentrations range widely from 0 to 2540 mg/1 but mostly are less than a few 100 mg/1. Concentrations of these species in formation waters are probably limited by several factors, including the very low solubility of calcium oxalate and calcium malonate (5), and the susceptibility of these dicarboxylic acid anions to thermal decomposition (16). This paper will focus on the monocarboxylic acids because they are much more abundant and widespread, and stability constants for their complexes with metals are better known. We do recognize that dicarboxylic acid anions may be locally important, especially for complexing metals. 

Erdey and Paulik (100), in a simultaneous DTA-TG study, investigated the thermal decomposition of barium, strontium, manganese(II), calcium, magnesium, and zinc oxalates in air and nitrogen atmospheres. It was found that the evolved carbon dioxide formed in the reaction played an important part in that it may inhibit the progress of the reaction and shift the peak temperatures to higher values. 

Similar discontinuities in Arrhenius plots are observed in thermal analysis (TA) as well, in particular, in the dehydration of crystalline hydrates performed in humid air. For illustration. Fig. 3.2 reproduces an Arrhenius plot for the dehydration of calcium oxalate monohydrate in an air flow, carried out under non-isothermal conditions by Dollimore et al. [28]. The equilibrium pressure of water vapour Pgqp measured at temperatures of up to 400 K and comparatively moderate decomposition rates turns out to be lower than its partial pressure in air which implies that the decomposition occurs in the isobaric mode. Above 400 K, the equilibrium pressure of H2O becomes higher than p with the process becoming equimolar. The slope of the plot decreases to one half of its former value in full agreement with theory (see Sect. 3.7). 

The desired reactions can be supported in terms of a comparison of calculated and determined values. Calcium oxalate monohydrate (CaC204 H20) is a typical example [47]. As shown in Figure 3.26, the mass-loss values determined by thermogravimetry for three well separated reactions of thermal decompositions are 12, 32 and 62%, which agree well with the following steps ... 

For a simple calibration of weight loss, the weight loss of a standard material can be checked imder reprodncible conditions of sample mass, packing, heating rate, sample holder confignration and atmosphere type, flow, and pressure. The TGA of calcium oxalate monohydrate (CaC204 H2O) is often used as a standard for the calibration of mass loss in thermogravimetry. This is due to three well-resolved steps in its thermal decomposition. 

Fig. 17. TGA and DTGA curves for the thermal decomposition of calcium oxalate (CaC204, H2O) in argon at 20 C/min (3).

Figure 3.25 TGA-DTG-MS curves of the thermal decomposition of calcium oxalate monohydrate measured at 30 K/min in a 70-p.L alumina pan. Purge gas argon, 50 mL/min. The diagram shows that calcium oxalate monohydrate decomposes in three distinct steps. The MS fragment ion curves for water (rrVz 18), CO (rrVz 28) and CO2 Mz44) display peaks that correspond closely to the individual steps in the TGA curve. The first mass loss step relates to the elimination and vaporisation of water of crystallisation (1) the second step to the decomposition of anhydrous calcium oxalate with formation of CO (2) and the third step to the decomposition of calcium carbonate to calcium oxide and CO2 (3). The m/z44 ion curve shows that CO2 is also formed in the second step at 550 C (besides CO). This is a result of the disproportion reaction of CO to CO2 and carbon.

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