Study On Mechanism Of Alkaline Hydrothermal Reduction Of Paint Sludge And Treatment Of Hydrothermal Solution

Incineration is the main method for a variety kinds of hazardous waste disposal.However,the continuous yield of hazardous waste,requires higher disposal capacity,resulting in the contradiction between the supply and demand.Meanwhile,the the outsourcing price increased anaually.Thus,the reduction of the hazadous waste become an inevitable trend for further hazardous waste disposal.Paint sludge,a kind of typical organic hazardous waste,is characterized by huge volume,high water content,organic content,viscosity and low flash point.The reduction effect of conventional dewatering is very limited.Thermal drying process is normally energy intensity,which could result in high potential of explosion.In our research group,the alkaline hydrothermal reduction of paint sludge was proposed.The total mass reduction reached 79.1%and the dry mass reduction reached52.1%.However,the reduction mechanism unveiled,and the treatment of wastewater and the adapation for the hydrothermal reduction of organic hazardous waste are still large unknown.To solve these problems,the following work has been carried out in this paper:Firstly,the analysis of hydrothermal product of paint sludge shows that the main components of gas products are olefins?51.31%?and alcohols?32.22%?.Compared with the raw simple,the biological toxicity of alkaline hydrothermal residues and its comprehensive impact on the ecological environment are significantly enhanced.The inhibition rate of luminescence increases from 36.9%to 41.6%.The flash point increases from 67.1°to 85.7°and the calorific value is from 26.2 MJ/kg increased to 27.0 MJ/kg,which indicates that the safety of subsequent heat disposal of residues improved and it has the potential to develop into fuel.Separated wastewater is brown with pH=10.2,COD,TN and ammonia nitrogen are 1421 53mg/L,14156 mg/L and 13327 mg/L,respectively.Wastewater contains heavy metal elements such as Ba,Cu,Ag,Zr,Se and Pd,which are harmful to the environment.It is further found that after alkaline hydrothermal treatment?AHT?,1)Dewatering performance of residues improved significantly:the contact angle of residues increases from 13.1°to 106.8°,paint sludge turns into hydrophobic substance.FTIR spectra shows that the polar bonds such as O-H,N-H,C-O-C in hydrothermal solid products decreased,while the non-polar bonds such as C-H,C=C increased,indicating the hydrophobic functional groups enriched.After AHT,the absorption peaks of hydrophobic functional groups are further enhanced.The hydrophobicity index HI/HB in ¹³C-NMR spectra decreased from 0.35?raw simple?to 0.34?hydrothermal product?and0.28?alkaline hydrothermal product?respectively,which also indicated that the hydrophobicity of the residues further improved after AHT.In addition,the transfer of hydrophilic functional groups with negative charge increases the surface charge of the residues from-7.17±0.07 mV to-1.17±0.11mV,lead to the electrostatic repulsion between particles decrease,which is conducive to reunion of particles and easy to separate free water.Finally,the volume of free water in residues increases from1.10g/gDS?raw sludge?to 2.98g/gDS?hydrothermal residues?and 4.57g/gDS?alkaline hydrothermal residues?.2)Enhanced dry mass conversion:Themogravimetric curve shows that the weight loss peak of hydrothermal solid products decreases significantly in the range of 310-320?,even disappears under alkaline conditions.This part of organic matter enters into aqueous phase and increases the concentration of COD and ammonia nitrogen in hydrothermal solution.The molecular weight distribution indicates that the macromolecules convert into small molecules after hydrothermal treatment,and the proportion of Mw<1K increased from 0.04%?raw solution?to 8.24%?hydrothermal products?and 17.54%?alkaline hydrothermal products?.In the paradigm diagram,the dehydration and dehydroxylation of paint sludge are obvious with the raising of temperature and time,and the trend of demethylation is notable with the increase of NaOH dosage.Experiments have proved that the breakage of ether bond is the specific cause of enhanced demethylation of raw sludge under alkaline conditions.GC-MS analysis shows that there are a large number of alcoholic organic compounds in hydrothermal solution.Besides,the discovery of PMMA monomer methyl methacrylate in hydrothermal solution also proves the occurrence of depolymerization in hydrothermal process.Under alkaline conditions,Esters also form alcohols by saponification.Finally,the organic matter in the raw sludge enters into liquid phase fully through the above chemical reaction and realizes dry mass reduction.In general,the water separated from paint sludge forms a liquid environment,which helps to dissolve dry mass and accelerate chemical mass transfer.The reduction of organic matter in solid improves the electronegativity solid surface,leads to the transformation of bound water to free water.That is to say,moisture and dry mass reduction in hydrothermal process promote synergistically,this phenomenon is more obvious in alkaline environment.The hydrothermal wastewater separated from the paint sludge polluted seriously and must be treated harmlessly.Blow-off method can effectively remove 84.3%of ammonia nitrogen at pH=12and temperature of 50?for 8h.The dosage of H₂O₂ is 0.5ml/L and stirs for 4h,the removal efficiencies of COD in wastewater were 45.3%and 24.5%respectively for the first and second stage of advanced oxidation.The first stage of advanced oxidation mainly relied on acidification during acid regulation process to reduce COD concentration.The hydroxyl radicals play a key role in the oxidation of organic compounds in the second stage of advanced oxidation.Then,effluent enters the aerobic biological system?sludge concentration 10g/L?at the reflux ratio of 15:1 and stays for4 days,the COD is effectively degraded by 61.7%.High throughput analysis showed that Proteus and Bacteroides are the main microorganisms in activated sludge,which had strong adaptability to the environment.A small number of Actinomycetes and Miracoccus could resist various extreme environments.After further advanced oxidation treatment,COD was removed by 47.4%.Finally,COD and ammonia nitrogen in effluent were 1175 mg/L and 112 mg/L,respectively.Due to the difficulty of degradation of residual organic matter in wastewater,it is suggested that the wastewater should be discharged into the comprehensive wastewater treatment station of industrial zone.In order to explore the universality of hydrothermal reduction of organic hazardous waste,the effects of hydrothermal temperature,reaction time,material moisture content and catalyst dosage on the hydrothermal reduction of six organic hazardous wastes?distilled drug residue,pharmaceutical sludge,white paint sludge,biochemical sludge,chromium-containing sludge and phosphating sludge?were investigated by single factor control method.Among them,pharmaceutical sludge and white paint sludge have the potential of hydrothermal reduction engineering application was due to their high moisture content,organic matter concentration and large reduction space.The total mass reduction?TMR?and dry mass reduction?DMR?were 74.9%and 50.6%respectively when 0.018 g/gDS H₂SO₄ was added to pharmaceutical sludge?water content 78.0%?and the sludge was heated at 240?for 3h.White paint sludge?moisture content70.3%,NaOH dosage was 0.13g/gDS?at 240?for 3h,the reduction effect is 66.9%?TMR?and56.2%?DMR?.Generally speaking,the basic requirement of hydrothermal technology for organic hazardous wastes is that the water moisture should not be too low,the organic concentration was high,and it is convenient for input and discharge in the equipment.