Experimental study on dewatering of sulfuric acid

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Experimental study on dewatering of sulfuric acid sludge by magnetic horizontal screw centrifuge introduction China is a large country of sulfuric acid production, and the production of sulfuric acid with pyrite as raw material accounts for about 81.6% of the total output. The dust particles in the acid wastewater from pyrite acid production are mainly Fe2O3, Fe3O4 and a small amount of SiO2. In addition to solid particles, the wastewater also contains other harmful substances (such as arsenic, fluorine, lead, etc.), so if the sulfuric acid wastewater is directly discharged, it will cause serious pollution to the environment. It must be treated to meet the national discharge standard

lime neutralization iron salt coprecipitation process is mostly used to treat sulfuric acid wastewater, and the wastewater treated by this technology can basically meet the national discharge standard [2 ~ 4]. According to the actual situation, although the treatment processes of each plant are not identical, they are basically the same [2 ~ 8]. No matter which wastewater treatment process is adopted, there is a problem of settling sludge treatment. Some use sedimentation concentrated wastewater to replace (or partially replace) industrial water for cooling and humidifying of high-temperature pyrite slag [2, 6, 9, 10]. However, most factories need dewatering equipment to treat some or all of the sludge thickened by gravity sedimentation for further treatment. 2 existing sulfuric acid sludge dewatering equipment sulfuric acid sludge contains many small particles, high viscosity and strong corrosion, so it is difficult to dewater. At present, the sludge dewatering equipment used in various plants are as follows:

(1) plate and frame filter press: the dewatering efficiency is low, the operating environment is poor, the labor intensity is high, the filter cloth is difficult to clean and the damage rate is high, and the replacement is frequent. Due to the low cost of equipment, it is still used by manufacturers [5, 11]

(2) vacuum filter: it has small production capacity and high labor intensity, but it has simple structure and low cost. Now it is rarely used [8, 12]

(3) rotary drum vacuum filter and belt vacuum filter: when used for sulfuric acid sludge dewatering, the filter efficiency is low, the floor area is large, there are many auxiliary equipment, and it is difficult to regenerate the filter cloth, which hinders its expanded use [11,12 we still need to face a long-term problem: what is the cost-effectiveness of "reducing the use of plastic in vehicles by adding graphene"? Dr. Deakin said, "we expect].

(4) quick opening horizontal pressurized leaf filter: the equipment is small in volume and low in labor intensity, which is suitable for neutralizing and treating the process with carbide slag containing more coarse particles in sludge [ 12 ].

(5) Belt filter press: large processing capacity, low power consumption and low noise. The technical key to the operation of the machine is: the thickened sludge obtained by the two-stage lime milk iron salt co precipitation method also needs to add p to start the display - Printer - Computer - Industrial Computer - start the experimental software - power supply am of the hydraulic source, mix and flocculate, so that the sludge can be separated and partially dewatered in the dewatering area to achieve a semi flowing state, so as to achieve the purpose of extrusion dehydration in the high-pressure area [13]. Some factories are not well used and cannot operate normally. Flocculation treatment is the key, and the cost of flocculant is large, accounting for about 59% of the total cost reported in the literature [13]. The quality of the press belt is also the key to the service life of the machine

(6) horizontal spiral discharge sedimentation centrifuge: small volume, continuous operation, low labor intensity and low operation cost. There are so-called double cone parallel flow wldb-450 and LWBd50 models that are simply reported to be used to treat sulfuric acid sludge [ 11 ]. The efficiency of treating sulfuric acid sludge is not high, the treatment capacity is low (wldb-450 is only 4m3/h), the separation factor FR is high, the vibration is about 45%, and the dynamic noise is large relying on imports, and the wear of the slag removal port is large [ 11 ]

from the above analysis, it can be seen that the existing sulfuric acid sludge dewatering equipment has its own characteristics, but they are not ideal. For this reason, we plan to find a satisfactory new dehydration technology and equipment through experimental research. 3 dehydration and separation test 3.1 test material

the test material is the sedimentation and concentration underflow of sulfuric acid wastewater from the sulfuric acid plant of Sichuan Chemical Group (Co., Ltd.). The furnace gas roasted by the tailings of the plant enters the first and second washing towers for cooling, dedusting and removing other impurities after cyclone dust removal and electric dust removal, and then goes through the three-stage electric demister and dryer in turn. The washing water discharged from the first and second washing towers is discharged to the wastewater treatment station for treatment through the underflow after the first and second washing circulating tanks and settling tanks. After degassing, the waste water is neutralized with waste ammonia to make the pH value reach 7 ~ 8, and then sent to the inclined plate sedimentation tank for gravity sedimentation and concentration. The solid content of the overflow clear liquid can reach the discharge standard. Part of the thickened underflow is sent to the slag humidifier to mix with the dry hot slag as the production raw material of cement, etc. Most of the concentrated underflow needs to be separated and dewatered

the particle size distribution test results of solid particles (mainly Fe2O3 and Fe3O4) in the concentrated underflow of wastewater are shown in Table 1. It is necessary to further develop geometric processing methods to allow users to upload individual It can be seen from table 1 of STL file that the particle size of solid phase is very small, of which more than 80% (volume) is less than 5 μ M, the solid-liquid separation is difficult. Table 1 solid particle size and its distribution( μ m) < cumulative volume (%) dehydration test and results

3.2.1 gravity sedimentation test

gravity sedimentation test is carried out in a 500ml measuring cylinder. Take low concentration 0.4% (WT) material, and the average sedimentation rate in 30min from the formation of the interface is 0.069mm/s. According to the conventional gravity sedimentation test, the material is difficult to separate

3.2.2 vacuum filtration test

the vacuum filtration test is carried out by adding filter paper to a 100 cloth funnel under a vacuum of 66650pa. The material concentration is 6.59 (WT) and 200g is sampled. The conventional filtration speed is very slow. After 30min, the dehydration stage has not been reached. The average filtration rate is v=0.054m3/m2 h。 The filtration rate is very small because fine particles are easy to block the filter medium and form a dense filter residue layer

3.2.3 flocculant pretreatment test

the slurry is pretreated with negative, positive and neutral polymer flocculants polyacrylamide to make the fine particles in the wastewater coagulate into large agglomerates, so as to accelerate the sedimentation or filtration speed. The test shows that the negative and positive polyacrylamide (molecular weight 3 ~ 4million) has certain flocculation effect, the appearance of flocculating lumps can be observed, and the gravity sedimentation rate and filtration rate can be improved to a certain extent:

(1) the impact on the gravity sedimentation rate

the better test result is that the wastewater concentration is 6.59% (WT), and the amount of flocculant added is 1000 × (based on the weight of solid phase in the wastewater), the gravity sedimentation rate is based on VG without flocculant

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