Abstract: Oil sludge treatment is an important topic in the field of organic solvent contaminated soil remediation. Dewatering is the key technology in sludge treatment, which has a great influence on the operation volume and cost of removal. In this paper, the results of dehydration research are summarized from three aspects of agent, process and equipment, in order to provide further research direction and suggestions.
Oily sludge runs through the whole production process of petroleum products. Every year, there are 800000 tons of new oily sludge in China. A large number of aromatic organic compounds in oily sludge may cause pollution to soil, groundwater, etc., which is difficult to degrade, thus affecting the health of residents around the operation area.
The sludge treatment of petrochemical industry related enterprises needs more thorough and environmental protection.
The treatment of oily sludge has always been a challenge for environmental protection technicians.
Dehydration technology is the key link in sludge treatment. On the one hand, it can reduce the operation amount, on the other hand, it can reduce the energy consumption in the process of heat treatment, so as to reduce the treatment cost.
The water forms in oily sludge can be divided into four categories: surface adsorbed water, interstitial water, capillary combined water and internal combined water.
- When dehydration is carried out, the surface adsorbed water needs to be added with flocculant to destroy the surface tension.
- Because interstitial water is free, it can be separated by gravity and centrifugation.
- The binding force of capillary binding water, which accounts for about 15% ~ 25% of the total sludge, and sludge particles is strong, which needs high mechanical force and energy to be removed.
- A large amount of internal binding water exists in the biological cell, which needs to be removed by heating or biodegradation.
Dehydrating agent
Oil sludge is a complex mixture, and the analysis of its components is the preliminary work of dehydration.
Pentane, heptane and toluene were used to extract and analyze the oil sludge.
The results of elemental analysis show that the extraction of pentane and heptane are mainly light alkanes and waxy solids, both of which are important components of crude oil and can be recovered by 90 × petroleum ether heat washing.
The results of FTIR showed that the toluene extract was mainly carboxyl rich asphaltene with acid value close to 360mg · g-1.
It is suggested that the addition of Fe3 +, Al3 + and other metal cations can reduce the bound water, and the addition of amphoteric polymer flocculant can form a polymer crosslinked network structure, which can free the water molecules in the inner structure and enhance the dehydration effect.
In the experiment, adding calcium oxide to adjust the pH value of sludge stored for a long time to 6.0 ~ 6.5 can increase the number of hydroxide, weaken the flocculation effect and promote dehydration. Finally, the water content of sludge can be reduced to 33.6%, and the volume can be reduced by 20 times.
Polyacrylamide (PAM) is a surfactant widely used in the dehydration of oily sludge.
Dehydration process
Fenton and Fenton like oxidation processes are the most reported dehydration processes.
The experimental results show that when pH is 4, the concentration of hydrogen peroxide is 2G · L-1, the mass ratio of hydrogen peroxide to iron is 4:1, the temperature is 35 ℃, the reaction time is 1 h, the concentration of calcium oxide is 7g · L-1, the speed is 3000r · min-1, and the time is 5 min, the moisture content of mud cake is less than 75%, and the content of petroleum is less than 2%.
The increase of the amount of hydrogen peroxide is beneficial to reduce the content of petroleum and turbidity of the supernatant.
The main principle of this method is that the hydroxyl radical oxidizes and decomposes the extracellular polymer, which destroys the flocculent formed by the extracellular polymer as the chain bridge, and makes the micelle of the extracellular polymer become filiform.
Strong oxidation is not necessarily conducive to dehydration, but at the same time, it increases the metal cations with flocculation, so as to reduce the specific resistance of the sludge.
The metal cations can form a chain bridge with a large number of negative charges and extracellular polymers on the surface of the sludge, accelerating the sedimentation.
In addition to the oxidation of hydroxyl radicals, iron ions can accelerate the disintegration of sludge flocculation group.
In the later stage of oxidation, iron ions are embedded in the activated flocculation group, which is conducive to the removal of internal bound water in the subsequent treatment.
Microwave can accelerate the movement of electron rich particles in sludge, increase the frequency of mutual collision, and then change the stability of flocs, and improve the concentration of extracellular polymers.
Extracellular polymer is also considered to be an important cause of sludge dehydration.
Dewatering equipment
Filtration is a traditional method of dehydration. The analysis of particle size and oil content of oily sludge is conducive to the implementation of filtration section.
According to the single factor experiment and the orthogonal experiment optimization method, the centrifugal process of oily sludge with high viscosity and wide distribution of water droplet size was studied. The results showed that temperature and rotation speed were the key factors to determine the dehydration rate.
For the sludge whose particle size is less than 5 μ m, the effect of rotation speed is great, when the particle size is larger than 10 μ m, the total time of 40 minutes can reach 85% dehydration rate.
Oil content and solid particle size are the key factors affecting the specific resistance of filter cake.
- The sludge with an oil content of 20.97% and an average particle size of 289 μ m does not need to be added with a filter aid. When the pressure difference is 0.2MPa, the pressure filtration can be carried out smoothly.
The sludge with an oil content of 45.41% and an average particle size of 45.36 μ M can’t be pressed and filtered under the same pressure difference. It needs to add a filter aid of 15% dry sludge to reduce the water content to 60%.
The flocculant PAM and PAC can reduce the specific resistance and facilitate the pressure filtration, but the use of PAC will cause the problem of yellow and turbid filtrate.
The combination of 1% Cationic PAM and 0.3% filter aid can meet the needs of molding and transportation in 3min.
Conclusions and recommendations
At present, most of the surfactants used in sludge dewatering are general-purpose reagents, which are suitable for many fields. However, there is no report on the surfactants based on sludge components and properties. Therefore, the research and development of special surfactants for oily sludge is the direction that can be considered in the future.
The process treatment is mainly focused on Fenton and Fenton like oxidation modification. There are few reports about other oxidants, so it is suggested to carry out systematic research.
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