Crystal Phase Regulation And Green Manufacturing Of Preparation Of ? High-strength Gypsum From Phosphogypsum

Phosphogypsum(PG)is the main solid waste in phosphorus chemical industry.In the“wet process”production of phosphoric acid(mainstream process of phosphorus chemical industry),4.5-5.5 tons of PG is emitted for each ton of P₂O₅ production.China's emission of phosphogypsum ranks first in the world.In China,PG has accumulated for more than 400 million tons of storage,and its annual output is over 80million tons.Whereas,the comprehensive utilization rate is merely about 30%.A large amount of PG,difficult to be handled,induces resource waste and environmental pollution,which seriously restricts the sustainable development of the phosphorus chemical industry.Therefore,the comprehensive utilization of PG has significant necessity and urgency.In this paper,based on the“13th Five-Year”Plan for Major State Research Development Program of China(2016YFC0700904)and Hubei Province Major Science and Technology Innovation Project(2015ACA060),a technical solution of PG problem for preparing?high-strength gypsum from PG by atmospheric hydrothermal reaction was proposed.The regulation mechanism of crystal phase transformation,crystal morphology and macroscopic properties of?hemihydrate gypsum(?-HH)were systematically investigated.The recycling utilizations of waste reaction medium and waste gypsum have been carried out to improve the green level of the overall process.The main work and the innovative results are as follows:(1)The rules of crystal phase transformation of“PG-?-HH”under multi-factor conditions were systematically determined,and the high-efficiency regulation methods of the reaction were proposed.The correction coefficient based on the characteristics of PG was introduced into the relation for quantitatively characterizing reaction conversion rate in TG mass loss method.Hence a conversion ratio calculation formula(R={36.03·[1-?(t)]-172.17·L_TG}/{27.02·[1-?(t)]-27.02·L_TG}),suitable for“PG-?-HH”crystal phase transformation,was derived.Based on the above methods,the effects and mechanisms of multiple factors(temperature,Ca Cl₂ concentration,p H value,impurity,liquid-solid ratio,and modifier dosage)on the phase transformation and crystal morphology were systematically revealed.It was found that the high acidity and high impurity contents of PG could shorten the induction time and crystal growth time of the crystal phase transformation,and cause the crystal size of the product to become smaller,and increase the micro-scale body defects on the crystal surface.The chemisorption of anionic groups of modifier(R-(COO)_n^n-)on the top planes of?-HH was detected with the help of FTIR,XPS,EDS and other methods.Furthermore,the influence mechanisms of the aspect ratio regulation were explained from the perspective of“selective adsorption”.Based on these results,high-efficiency regulation methods of the atmospheric hydrothermal reaction were obtained,namely,the reaction rate and?-HH crystal size being controlled by reaction temperature and Ca Cl₂ concentration,and?-HH aspect ratios being controlled by the modifier dosage.(2)The comprehensive evaluation of the performance and synthesis efficiency of?-HH was carried out,and the optimization method of atmospheric hydrothermal reaction was proposed.Based on the characteristics of PG atmospheric hydrothermal process and industrial practical requirements,the preparation of phosphogypsum-based?high-strength gypsum were comprehensively evaluated in terms of strength performance,filtration performance and synthesis efficiency.It was found that the main influencing factor of?-HH strength performance is the average aspect ratio,and the aspect ratio is optimal at 1-2.The main factor of filtration performance is crystal size.The smaller of the crystals and the less of small particles(?10?m),the better the filtration performance.However,fine strength and filtration performances generally correspond to lower synthesis efficiency.Based on this contradiction,an optimization method of the reaction was further proposed(increasing temperature progressively).Compared with the control group,the mechanical strength and filtration performance of the product obtained by this method were not significantly deteriorated,and the synthesis efficiency was obviously improved(reaction time decreased by over 20%).(3)Multiple recycling of reaction medium in PG atmospheric hydrothermal system was realized.It was revealed that in direct recycling the acidity and impurity increased obviously with the increase of number of cycles,leading to significant increase in the average aspect ratio of the?-HH(from 0.9 to 7.6)and significant deterioration of its strength performance.In the 6-cycle experiment,the 2 h bending strength decreased by 66%,and the 3 d compressive strength decreased by 85%and merely the strength of the first cycle product reached the JC/T 2038-2010 standard.It was proposed to improve the product performance by changing modifier dosage in the multiple recycling of reaction medium.After 6 cycles of experiments,the degree of deterioration of strength was significantly reduced compared with the control group,and all the strengths of 6 cycles exceeded JC/T 2038-2010 requirements.Furthermore,an empirical relation between the modifier dosage and number of cycles(M=0.086wt%+0.057 wt%·N)was summarized.The industrial pilottest of the overall process further confirmed the feasibility of recycling the reaction medium industrially.Results showed that when reaction medium was recycled,the consumption of industrial Ca Cl₂was reduced by about 80% and the waste liquid discharge was reduced by about 84%compared with that without reaction medium recycling.Consequently,the overall process was significantly improved in terms of low cost and greening.(4)Recycled?high-strength gypsum was successfully prepared from waste gypsum with atmospheric hydrothermal method.From the perspective of green design of the whole life cycle,the recycling utilization of waste phosphogypsum-based high-strength gypsum products was explored,and the recycling route by atmospheric hydrothermal reaction was proposed.It was proved that the raw material characteristics of waste gypsum powder(including fine grains and residual R-(COO)_n^n-groups)could cause deterioration of morphology and macroscopic performance of recycled?-HH,resulting in the product could not reach the JC/T 2038-2010 standard.Based on this problem it was proposed that increasing the acidity of the reaction medium to control the?-HH morphology,thereby obtaining recycled?-HH with better performances.It was found that when the p H value was adjusted to 4-5,the comprehensive performance were fine.The 2 h bending strength and 3 d compressive strength were 4.1±0.4 MPa and 29.2±0.9 MPa,respectively,higher than the relevant requirements of JC/T 2038-2010.Therefore,it can be concluded that recycled?high-strength gypsum was successfully prepared from waste phosphogypsum-based high-strength gypsum products by atmospheric hydrothermal method.