Разработана рациональная технология комплексной переработки отработанных ванадиевых катализаторов. Изучены физико-химические закономерности фазообразования в системе V2O5 – V2O4 – SO2 – SO3, структуре и свойствах образующихся компонентов. Установлены причины деградации активного компонента ОВК, одна из которых состоит в том, что окисление диоксида серы носит колебательный характер.
Мақаланың мақсаты – қалдық ванадилі катализаторды кешенді рационалдық технологиясын жасау. Түзілетін компонентердің V2O5 – V2O4 – SO2 – SO3, жүйесінде құрылымы мен қасиеттерін, фазалық негіздерін, физико-химиялық заңдылықтары зерттеу
The purpose of this article is to develop a rational technology of complex recycling of dead vanadium catalysts and produce all its components as commercial products. The research of interaction of the system V2O5 – V2O4 – SO2 – SO3 with water is also valuable for understanding characteristics of chemical processes at a hydrochemical stage of synthesis and regeneration of vanadium sulfuric acid catalysts. The results of this research give new data which allow to reason about phase composition of the active component at the hydrochemical stage of synthesis and recycling of vanadium catalysts.
Keywords: vanadium catalyst, complex recycling, phase composition
Nowadays the manufacturers of H2SO4 bury dead vanadium catalysts (DVC) with frequent violation of rules or even throw them out in a dump, which causes not only significant losses of valuable components (vanadium and potassium) but also poisons the environment with toxic chemicals. The life cycle of the catalysts used in sulfuric acid manufacture is 1-2 years on average. Inactivation of the catalysts is mainly happen because of decrease in catalytic activity as a result of chemical and phase changes of the catalysts’ base components (a carrier and an active component) and poisoning with contact toxins, such as alkali metals, iron, arsenic and others.
Sulfuric acid holds one of the first places among other chemicals in terms of production and utilization volumes. More than 200 million tons of sulfuric acid is produced in the world annually. This consumes about 40 thousand tons of vanadium catalysts which contain 6-8% of pure oxide of vanadium . The vanadium catalysts of sulphur dioxide conversion are the multicomponent systems which contain a porous paste - the diatomite or silica gel, covered by an active component - a mix of sulfa-vanadate oxide of potassium. The following catalysts are used in the sulfuric acid manufacture:
Sulfa-vanadate on diatomite (SVD) is a highly active at high temperature catalyst which is loaded into first layers of a contact device. The catalyst is produced with dry blending of natural diatomite, unrefined oxide of vanadium (V), and the solution of potassium bisulphate. SVD contains 6-7% V2O5, 10% К2О, 55-65% SiO2.
The aim to increase activity of the catalyst at low temperatures has resulted in the development of sulfa-vanadate on silica gel (SVS) and IК 1-6. Compare to SVD, these catalysts are produced using silica gel as a carrier which is saturated with mixture of vanadium sulphate and potassium bisulphate. SVS contains 12% V2O5, 15% К2О, 50-60% SiO2. Catalyst IК 1-6 contains 9-9,5% V2O5, 30% К2SО4, 55-60% SiO2.
In some cases catalytic activity of DVC can be recovered by using pyro- and hydrometallurgical methods [2-6]. However, they do not provide complex component recycling of catalysts. Therefore, creation of the efficient technology of DVC recycling can solve urgent ecological and economic problems.
Full understanding of chemical processes of contact paste formation and the influence of reactionary gas mixture on chemical composition of the active component  is necessary for the development of a complex rational technology of DVC recycling and manufacturing of catalysts with predefined set of properties.
RESULTS AND DISCUSSION
Steady transformation of SO2 into SO3 on a vanadium catalyst at temperatures of 400-5800C is convenient to consider in a prism diagram (Figure 1.) The diagram takes into account oxidation-reduction, acid-base balance and phase relations of systems K2O – V2O5 – SO3 and K2O – V2O4 –SO2. It is plotted in the system of triangular plane coordinates VO2+ , SO22 -, O2- and VO2+, SO2+, O2- with percent ratios of the components on each axis. Six corners represent K2O, V2O5, SO3 and K2O, V2O4, SO2 respectively. The central attention is devoted to the system of K2O - V2O5 – SO3 which models the active component of the vanadium catalyst.
Figure 1. Phase equilibrium of system K2O – V2O5 – V2O4 – SO2 – SO3
The initial chemical composition of the vanadium catalyst K4V2O3(SO4)4 is shown at the point A of the bottom triangle. However, this composition is not the active component of a vanadium catalyst yet. It is known that in operational conditions the active component represents a melt of sulphate of potassium and oxide of vanadium (V) and (IV). This composition enters into the system K2O – V2O5 – V2O4 – SO3. To achieve the stationary condition, practically all known catalysts originally work in a phase of saturation, duration of which depends on the content ratio of connected SO3. According to the chemical analysis and nuclear magnetic resonance of 51V, the concentration of connected SO3 in the active component of functional catalyst does not fall outside the boundary of the system K3VO2+(SO4)2 – K2(VO2+)2(SO4)3. Saturation of the catalyst is carried out using a mix of 30 % of SO2 and 70 % of air. Therefore, the changes in composition of the catalyst should go along the line that connects initial composition of the catalyst with a point of gas composition. The maximal saturation of the catalyst is 55%. To simplify the representation of mechanisms of work of the catalyst the first stage is represented as chemisorption of SO2 (point B). In the process of reduction of V5+ to V4+ the system stays on a point C, where the proportion of V4+ : V5+ equals to 2:1. The chemical composition at this point is V6O14 with only one atom of vanadium that has a delocalized 3d-electron. Only such ratio of VO2+- VO2+ - VO3- - O2 allows simultaneous absorption of SO2 and extraction of SO3 in the catalyst. The dual oxidation level of vanadium with delocalized electron in a vanadium-oxygen framework allows effective oxidation of SO2 during oxidation V4+ with oxygen. The increase in concentration of V4+ reduces efficiency of the catalyst. At 100 % of V4+ the catalyst stops working.
The presented diagram is assumptive and requires further research and refinement. However it is clear that the obtained data of the active component interconversion can be used for several purposes: 1) to update the conditions of synthesis and operation of vanadium catalysts; 2) to select the carriers which meet the requirements of sulfuric acid manufacture such as chemical inactivity and advanced cellular structure which is stable in varied external conditions; 3) to develop the regeneration techniques of the waste contact pastes and reduction of their activity to the required level.
The research of interaction of the system K2O – V2O5 – V2O4 – SO2 – SO3 with water is also valuable for understanding characteristics of chemical processes at a hydrochemical stage of synthesis and regeneration of vanadium sulfuric acid catalysts. The results of this research give new data which allow to reason about phase composition of the active component at the hydrochemical stage of synthesis and recycling of vanadium catalysts.
The purpose of this article is to develop a rational technology of complex recycling of DVC and produce all its components as commercial products.
The base manufacturing methods include:
• combined leaching of vanadium and potassium with subsequent sorptive production of vanadium as the finished product;
• hydrochemical activation of the carrier and production of silicon oxide with required physico-chemical properties;
• production of complex liquid fertilizers.
The leaching process is carried at solid/liquid ratio (S:L) of 1:3 and рН 1,0-2,0 by water combined with recirculated sorptive mother solution. The chemical reaction of pyrosulphate of potassium with water produces acid solution of vanadium (V and IV), potassium, iron, ions of SO4-2, other impurities, and insoluble residue - the carrier (SiO2). The utilization of water and recycled mother solutions at the leaching stage allows transferring about 90 % of vanadium, 98 % of potassium, and 99 % of all impurities (Аs, Fe) to the solution of vanadium. After this stage the solutions is processed with manganiferous reagents (Mn2O3) to increase the percent of vanadium extraction until it achieves рН level of 2,0-4,5. At this level and the potential of 1,0-1,2V the vanadium stays in the solution as deca-vanadium acid which has significant solubility and stability . It prevents precipitation of vanadium from the solution which increases percent of vanadium extraction.
The insoluble carrier, obtained at the stage of leaching, is hydrochemically activated to increase specific surface to 120-150 g/m2 which allows reusing it in the manufacture of catalysts.
The proposed technological process provides a new sorption method for separation of vanadium and potassium from their solutions. Comparing to extraction, the sorption method prevents fire and explosion hazard during manufacturing. Ionite is not evaporated and dissolved. This prevents pollution of the environment. The decision to use ionite is determined by two factors: the condition of vanadium in solution (anion or cation form) and the affinity of exchanging anion to anionit, i.e. the form of anionit at the moment of sorption. Therefore the appropriate рН level of solution and anion form is required for optimal sorption. The chemical research showed that АМ-п has the greatest sorptive capacity and the affinity of exchanging anion of vanadium with anionit. The dynamic conditions of sorption are preferable for more efficient separation of vanadium and potassium. The speed of solution circulation in the sorption column is about 0,5-2,0 volume/volume in hour. The vanadium extraction reaches 100% if more that 110-115 volumes of solutions passes through sorbent. The maximum vanadium capacity of ionite reaches 250-600 mg/g АМ-п.
After sorption the mother solution contains 110 g/dm3 K2SO4 and is used to prepare fluidic complex fertilizers (FCF) after strengthening and clearing.
To achieve fullest extraction of vanadium from ionite, desorption is carried out at рН=4,5 by 160 g/dm3 of solution of nitrate of ammonium (NH4NO3). After the process is finished, ionite is recharged by solution of sulphate of ammonium and eluant conditions with solution of NH4ОН up to рН 7,5-8,0 at agitation with simultaneous heating up to 800C, which results in sedimentation of metavanadate of ammonium (MVA). Thermal decomposition MVA is carried out in the electro oven at 5500С to receive V2О5 as a commercial product.
The new technological process (Figure 2) allows:
• complex and wasteless recycling of the catalysts’ base components;
• the production of commercial products, i.e.: a) oxide of a vanadium with contents 99,8 % of V2O5; b) the hydrochemically activated carrier which is free of contact toxins and can be repeatedly used for manufacture of vanadium catalysts; c) complex liquid fertilizers.
Figure 2. Complex recycling of DVC
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