Cr2O3 containing refractories castables are widely used as linings of various furnaces because of its remarkable corrosion resistance. However, Cr2O3 can be oxidized to toxic Cr(VI) by calcium ...aluminate cement phases (CA and CA2) which are used as hydraulic binder in castables. In the present research, one of the most stable Cr(III) phase viz., (Al,Cr)2O3 solid solution was synthesized and then reacted with cement phases in order to check the formation of Cr(VI). The (Al, Cr)2O3 solid solution and calcium aluminate cement (70% Al2O3) were mixed in the ratio 1:1 (mass%) and heated in the temperature range of 500–1500°C in air. The phase compositions and formation/leachability of Cr(VI) were investigated using XRD, XPS and the leaching tests. The (Al,Cr)2O3 solid solution didn’t react with calcium aluminate cement at 500–900°C, while it partially converted to a Cr(VI)-containing phase at 1100–1300°C. However, (Al,Cr)2O3 and cement phases react to form a ternary Cr(III) phase (Ca(Al,Cr)12O19) at 1500°C.
Saprolitic nickel laterite is characterized by relatively low iron and nickel contents. Iron and nickel oxides are reduced to form fine ferronickel particles that disperse and embed in silicates in ...the reduction process, limiting the application of magnetic separation to extract ferronickel. Additives are applied to promote the aggregation and growth of ferronickel particles, then the large ferronickel particles will be separated by fine grinding and recovered via magnetic separation. Calcium sulphate is considered to be capable of increasing the size of ferronickel particles considerably. Due to the decomposition of calcium sulphate in the reduction process, the mechanism of calcium sulphate on the aggregation and growth of ferronickel particles should be conducted studied in-depth. The current work explores the effects of calcium sulphate, elemental sulphur, and calcium oxide on the formation of ferronickel particles in a saprolitic nickel laterite ore. The results showed that the formation of an Fe-FeS eutectic and the mineral structure transformation contributed by calcium oxide were all conducive to the mass transfer of ferronickel particles in gangue, ferronickel particles aggregated and grew up at the boundary between the hole and the gangue. The self-reduction, fine grinding, and magnetic separation of nickel laterite ore in the presence of three types of additive were examined. Nickel laterite ore with 7.88 wt% coal, 12 wt% calcium sulphate reduced at 1200 °C for 30 min, a ferronickel concentrate of Ni 8.08 wt%, and Fe 79.98 wt% was obtained at a nickel and iron recovery of 92.6% and 79.9%, respectively.
An Impulse Model for Computer Viruses Zhang, Chunming; Zhao, Yun; Wu, Yingjiang
Discrete Dynamics in Nature and Society,
01/2012, Letnik:
2012
Journal Article
Recenzirano
Odprti dostop
Computer virus spread model concerning impulsive control strategy is proposed and analyzed. We prove that there exists a globally attractive infection-free periodic solution when the vaccination rate ...is larger than θ0. Moreover, we show that the system is uniformly persistent if the vaccination rate is less than θ1. Some numerical simulations are finally given to illustrate the main results.
A stochastic computer virus spread model is proposed and its dynamic behavior is fully investigated. Specifically, we prove the existence and uniqueness of positive solutions, and the stability of ...the virus-free equilibrium and viral equilibrium by constructing Lyapunov functions and applying Ito's formula. Some numerical simulations are finally given to illustrate our main results.
•A novel method of high-grade ferronickel production was developed using self-reduction and selective oxidation.•Oxidation by CO2 at a concentration of 70 vol% allowed for a nickel recovery >95% and ...grade of ~32 wt%.•Metallic nickel and iron were separated by selective oxidation with CO2, and CO2 was efficiently utilised.
The known resources of nickel sulfide ores are quickly diminishing; to satisfy future nickel demands, nickel laterite deposits are now being investigated as an alternative. The use of classic mineral processing techniques to economically extract nickel and prepare high-grade ferronickel from low-grade nickel laterite entails numerous challenges. In this study, we developed a new reduction and selective oxidation process for nickel laterite to separate metallic iron from ferronickel in reduced laterite and produce a higher-grade ferronickel, as compared with current techniques. The thermodynamics of the oxidation of metallic nickel and iron in CO2-CO gas mixtures were investigated, and the self-reduction and selective oxidation of nickel laterite with CO2-CO gas mixtures were examined. We found that ferronickel with a nickel grade of 37.2 wt% could be obtained at a nickel recovery of 95.89% when nickel laterite was reduced at 1200 °C for 25 min, followed by oxidation with CO2 at a concentration of 70 vol% at 1100 °C for 50 min. Therefore, the process proposed here is an effective means of deriving high-grade ferronickel from low-grade nickel laterite whilst using CO2 efficiently.
Cr2O3 is the best-known corrosion resistant refractory materials under high temperature extremely harsh environment. However, the formation of toxic, carcinogenic & water-soluble Cr(VI) compounds ...during its application remained a major predicament. Here, we have introduced a facile route to fabricate the Al2O3-CaO-Cr2O3 refractory castables to mitigate Cr(VI) using basic silica sol (pH∼9) via in-situ secondary phase modification. Basic silica sols 0.25, 0.49 and 0.98 wt% (CaO/SiO2 ratio of 5.8, 2.9 and 1.45 respectively) were doped by keeping the castables properties reasonable. Addition of silica drastically reduces the mid-temperature (500–1100 °C) formation of Cr(VI) phase (CaCrO4), while relatively higher temperature Cr(VI) phase (hauyne, Ca4Al6CrO16) totally disappeared from 900 to 1300 °C. However, at 1300–1500 °C, silica addition promote the solid solution phase formation like Ca(Al,Cr)12O19 and (Al,Cr)2O3 where chromium exists as +3 oxidation state. Also, silica addition reduces the dimensional tolerance of the castables at a higher temperature associated with the secondary phases (dehydration of cement phases and expansive formation of CaAl12O19). A comparable mechanical strength (up to 156 MPa) and a remarkable Cr(VI) reduction (up to 94%) could be achieved upon silica addition.
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•Al2O3-CaO-Cr2O3 castables were fabricated via doping SiO2 sol to mitigate Cr(VI).•CaCr6+O4 reduces drastically on SiO2 addition, while Ca4Al6Cr6+O16 doesn't form.•SiO2 promotes the formation of Ca(Al,Cr3+)12O19 & (Al,Cr3+)2O3 at 1300–1500 °C.•SiO2 reduces dimensional tolerance of the castables at the higher temperature.•A remarkable mechanical strength (∼156 MPa) & Cr(VI) reduction (∼94%) was achieved.
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The Al2O3-CaO-MgO-Cr2O3 system has immense potential for refractory castables applications. However, Cr(III) can potentially be oxidized to carcinogenic Cr(VI) during the usage, which ...can cause subsequent problems with disposal. Equilibrium experiments on the Al2O3-CaO-MgO-4wt%Cr2O3 system were performed at 1500 °C in air. The effect of MgO addition (0 and 20 wt%) on the formation and leachability of hexavalent chromium was investigated using XPS, XRD, SEM-EDS, the TRGS 613 standard Cr(VI) leaching test and multiple leaching tests. A Cr(VI)-containing phase Ca4Al6CrO16 predominantly formed in the samples with 0 and 5 wt% MgO, while the Cr(III)-containing Mg(Al, Cr)2O4 spinel phase formed in the samples with 10 and 20 wt% MgO. Addition of MgO suppressed the formation of Cr(VI) while favored the formation of spinel phase. Concentrations of Cr(VI) in the leachates from TRGS 613 tests exceeded the European limit of 0.0002 wt% in all samples, although decreased significantly with 20 wt% MgO addition.
A worm spread model concerning impulsive control strategy is proposed and analyzed. We prove that there exists a globally attractive virus-free periodic solution when the vaccination rate is larger ...than θ1. Moreover, we show that the system is uniformly persistent if the vaccination rate is less than θ1. Some numerical simulations are also given to illustrate our main results.
This paper exactly formulates the kth-order fixation probabilities on complete star digraphs (CSDs), which extend the results from Broom and Rychtář (2008). By applying these probability formulae, ...some asymptotic properties on CBDs are analyzed, and certain CSDs are determined to be amplifiers of selection for arbitrary relative fitness larger than 1, while all the CSDs are proved to be amplifiers of selection for fixed relative fitness slightly larger than 1. A numerical method for fixed population structure (by solving a linear system) is developed to calculate the fixation probabilities on complete bipartite digraphs (CBDs), and some conjectures are finally given through simulations.
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