Reaction Runaway Prevention For The Preparation Of Red Iron Oxide Pigment By Nitric Acid Method And Optimization For The Exhaust Treatment Process

The production process of iron oxide pigment by nitric acid method involved the preparation of iron nitrate,the synthesis of iron oxide crystal and the re-oxidation of iron nitrate.All these three reaction steps will produce a large number of nitrogen oxide(NOx)exhaust gas.Moreover,the re-oxidation of iron nitrate has the potential risk of runaway.Once the runaway happened,a lot of waste gas contained with high concentration NOx will be released from reactors within very short time.The purification ability of processing unit was designed based on normal operation condition.It was difficult to achieve full purification for NOx under the runaway conditions,which would result in the phenomenon of yellow gas and the accident of excessive emissions,causing air pollution and complaint of surrounding residents and even the public panic and public security incident.This paper is a case study of process of a synthetic iron oxide pigment plant located in Zhejiang Province.Based on several iron nitrate decomposition runaway incidents happened during the plant was put into operation three years,the paper investigated the mechanism of reaction runaway.Iron nitrate will be catalytically decomposed in several minutes when the pH value inside reactor is lower 2.0 and no oxygen exists simultaneously,leading to reaction runaway.At this time,by imminently dosing caustic soda to the reactor then mixing homogeneously to increase the pH from 2.0 to 2.3 within 3 minutes can suppress the autocatalysis of iron nitrate and prevent reaction runaway.In addition,according to the analysis of NOx exhaust gas source strength and purification capacity of waste gas treatment facilities under different production conditions,it is found that the purification capacity of waste gas treatment facilities cannot ensure the stable emission after the emission standard is improved.Therefore,the existing waste gas treatment process was optimized that the waste gas scrubber absorption solution was replaced by 10%urea solution from industrial water,and then the absorption efficiency was increased by 10%.Meanwhile,an additional layer of catalyst was added in the SCR reactor,then the treatment efficiency of SCR increased by about 3.5%.The optimized waste gas treatment process can effectively avoid the occurrence of yellow smoke,so as to ensure the stable emission of waste gas standards.Finally,the process after the optimization transformation was observed for a long time.According to the actual operation results,the runaway reaction of iron nitrate decomposition was successfully restrained,and no subsequent runaway events occurred.Meanwhile,the on-line monitor and on-site sampling analysis data proved the NOx removal efficiency of the tail gas treatment facility after process optimization reached 98%while the N2O removal efficiency was no less than 97%,and the emission satisfied the standard.This investigation effectively prevented the environmental problems caused by the reaction runaway in the production of iron oxide red pigment,reduced the pollution of NOx,and had good industrial significance.