Translated Abstract
Organophosphorus pesticide is the most important system of pesticide production in China, and its output accounts for over 70% of the total national pesticide production, but at the same time the organophosphorus pesticide industry is an industry with the largest investment and most difficult for pollution control in the fine chemical industry in our country. The wastewater derive from production processes of organophosphorus pesticides, containing high pollutant concentration, salinity, complex composition, and high toxicity etc. Its conventional processing methods have some disadvantages such us high operation cost, secondary pollution, and covering much land. As a new type of treatment technology for high concentration of refractory organic wastewater, supercritical water oxidation (SCWO) has many advantages such thorough, quick reaction, no secondary pollution, and requiring less land. Thus, this article introduced the supercritical water oxidation technology of organic phosphorus pesticide wastewater treatment, and studied its key problems for ultilization, and the main research contents and innovative results are as follows:
In this work, the optimization research of operation parameters for organophosphorus pesticide wastewater SCWO were conducted, involving reaction temperature, reaction time, oxidation coefficient and alkaline catalyst. We analyzed the nitrogen balance, carbon balance in the organic matter degradation process. Compared with oxidation coefficient and reaction time, reaction temperature has the most remarkable influence on the XCOD improvement, and speeding up the preheating of feed can also significantly improve XCOD. At 600°C, 25MPa, OC=3.0 and t=2.0min conditions, XCOD and XTN can reach up to 99.42% and 86.70%，respectively, and more than 92.0 wt % TOC and 86.7 wt % TN were correspondinglly converted to CO2 and N2. The concentration increment of ammonia nitrogen in reactor effluent, indicates that the amount of organic nitrogen converting into ammonia nitrogen is higher than the amount of ammonia converting into nitrogen.
This paper studied the corrosion behaviors of typical nickel-based alloy in SCWO of pesticide wastewater under the conditions of subcritical water(350°C, 25MPa) and supercritical water (450°C, 25MPa). Under supercritical conditions, the synergy of corrosion caused by chlorine and oxygen in comparison to a single chlorine or oxygen condition, can lead to more serious corrosion. Supercritical chlorine solution can induce Incoloy 800 severe pitting corrosion, however, the other tested alloys show good corrosion resistance. Ni shows serious dissolution and erosion, and the Cr under the condition of oxidation can generate stable chromium oxide. In nickel based alloys, Mo improving the corrosion resistant ability depends on the existence of the Cr. Fe in nickel based alloys can also form stable Fe3O4. Incomplete carbon decomposition of organic matter, can form carbon deposition on the alloy surface. In supercritical water, corrosion characteristics of three kinds of nickel based alloy as Incoloy 825, Inconel 625 and Hastelloy C276 were studied. The results show that Incoloy 825 has weak corrosion resistance, under the conditions of high chlorine and oxygen, it presents the highest 14.6mmpy corrosion rate among all tested alloys. Due to the high content of Cr, Inconel 625 oxidation corrosion resistance is higher than that of Hastelloy C276. Hastelloy C276 exhibits the lowest 0.9mmpy corrosion rate in the case of supercritical salt solution due to more Mo concentration.
The corrosion behavior characteristics of austenitic stainless steel, nickel based alloy and titanium alloy were studied in supercritical water (450°C, 25MPa) containing chlorine, oxygen and phosphate. Corrosion inhibition mechanisms of phosphate to three kinds of alloy under the condition of chlorine and oxygen are also illuminated. 316 SS and TA 10 separately presents the weakest and the strongest corrosion resistance. In supercritical water containing chlorine and oxygen, 316 SS shows a serious loss of scale and Inconel 600 displays the most severe pitting corrosion morphology. In TA 10 corrosion layer, the content of oxygen increase from outside to inside, and titanium oxides as TiO, Ti2O3 and TiO2. TiO2, Fe3O4, Cr2O3 and MoO2 are generated in the sample surface. Under the condition of coexistence of chlorine and oxygen, 316 SS has the highest 16.1mmpy corrosion rate. TA 10 has the lowest corrosion rate lower than 1.8mmpy. Due to phosphate adsorption on the alloy surface and the stability of the phosphate film, it shows effective mechanism of corrosion inhibition effect. The loss of metal ions is one of the important fact leading to corrosion, and K3PO4 can obviously decrease the loss of metal ions in corrosion test, because of the high solubility of CrPO4. The K3PO4 shows good inhibitory effect on Ni and Fe than does on Cr. A salt counter-current filtration separation method can well separate the inorganic salt. Owing to the same ion effect, the solubility of K2SO4 can reduce the solubility of Na2SO4. Through the electric double layer structure and K3PO4 is able to keep Na2SO4 reunion after precipitation, so improving transport in supercritical fluid. When the mass concentration ratio of Na2SO4: K3PO4 is 2:1, the solubility of Na2SO4 reduces only from 45g/kg in subcritical water to 40g/kg in supercritical water.
The protection of coating to material corrosion, flow accelerated corrosion properties and coating inhibits flow accelerated corrosion were studied in SCWO. Two kinds of surface treatment technologies, i.e. arc ion plating(TiN and Cr plating) and pickling passivation, were used to improve nickel based alloy corrosion resistance in SCWO. Cr plating of Incoloy 825 in supercritical water shows the best corrosion resistance. The corrosion resistance of TiN plating is weaken than Cr plating, and pickling passivation presents the weakest corrosion resistance, however they are better than that of untreated samples. In a high temperature and high pressure flow experimental apparatus, we explored influences mechanisms of angle, flow velocity and corrosion environment on the flow corrosion. The influence of Cr plating has obvious inhibitory effect on flow accelerated corrosion. Under the conditions of the attack angle of 45°, the velocity of 1.0m/s, the corrosion rate reachs up to 22.7mmpy, which is about 2 times under that in stationary state. In the corrosion test of influence of attack angle, the corrosion rate at the attack angle of 45° is significantly higher than that in the attack angle of 0°. Under the conditions of supercritical water containing oxygen and chlorine, flow accelerated corrosion is mainly due to the oxidation film stripping.
Translated Keyword
[Organophosphorous pesticide wastewater, Supercritical water, Oxidation, corrosion, Phosphate]
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