Wang Weixiao, Ha Jing, Li Zhaoyang, Liu Kui, Gao Leihong Abstract Chromium(III) salts are
the most popularly chemical agent used in the tanning process. Chrome tanned leathers are
characterized by top handling quality, high hydro-thermal stability and excellent user
properties. But in the tanning process with chromium(III) salts only 60%~70% of the total chromium(III) are up-taken by the hides. So the
recovery of chromium from tanning wastewater is essential for environmental protection
& economic reason. To this purpose, three precipitation and anion polyacrylamide (PAM) as flocculent are used in this research. The effects
of pH and dosage on settling rate, Cr concentration in the handled wastewater,sludge size and volume were studied in this paper. The results show
that the optimum method for recovery chromium is adjust pH of tanning wastewater about 7
by sodium hydroxide, then a small amount of PAM is put into the wastewater at stirring. At
the optimum condition, not only high settling velocity,
low volume of sludge and above 99.9% recovery rate of chrome can be obtained, but also the
handled wastewater can be discharged to environment without alkalization to the soil
because the pH of disposed wastewater is at neutral. Furthermore, the cost discharge is
low with flocculation-sedimentation. One of the most significant problems of the leather industry is waste generation. One tone of wet salted hide yields only 200kg of leather, but over 50m3 of waste water[1,2]. Chrome tanning is the most common type of tanning in the world. In the tanning process using chromium salt, only about 60%-70% of the total chromium are up-taken by the hides, in the other word, about 30%~40% of the chromium are discharged with the wastewater. So wastewater from tanning processing poses a significant chrome pollutant to environment and waste lots of chromium. In most countries, regulations governing chrome discharge from tanneries are stringent. All tanneries must thoroughly check their waste streams. Studies show that the concentration of chrome in the tanning wastewater varies from 2500 to 8000 mg.L-1 as the count of Cr2O3[3].The environmental impact of chrome waste from tanneries has been a subject of extensive scientific and technical dispute. Several methods to remove chromium from tanning wastewater have been used. There are chemical precipitation, ion exchange, membrane filtration, extraction, reverse osmosis and absorption[4,5]. Chemical precipitation is the most popular method for recovery chrome and sodium hydroxide is the common precipitating regents. However, Cr(OH)3 precipitation forming from NaOH is very gelatinous. The settling rate for NaOH is very slow and the dewatering sludge is very difficult[6]. In order to get a grainy, dense, easily settable precipitation, MgO and Ca(OH) 2 is used as precipitating regents[7], but MgO is more expensive than NaOH and precipitation forming from Ca(OH) 2 is not desirable. In order to find a method to remove the chrome from wastewater with low cost and the precipitation easily settable, PAM has been used in this research. By comparing the size of precipitation' grain, the velocity of precipitation, the settling rate and sludge volume, we confirmed that the optimum method to recover chrome from wastewater is to adjust pH of tanning wastewater 7-8 by sodium hydroxide and then to use a small amount of PAM as flocculent. 2. MATERIAL AND METHODS 2.1 chemical reagents The chemicals used in this research are all of analytical reagent grade. In this research, saturated lime, 1.0 mol.L-1 sodium hydroxide and saturated MgO were used to adjust pH of sample. PAM solution was prepared to yield concentration of 0.5g.mL-1 with deioned water. 2.2 Sample collecting and determination of the most important parameters Wastewater sample from tanning process were collected in polythethylene bottles after tannery was finished (within 12h), then transported to laboratory and stored in a refrigerator at approximately 4°C. The most important parameters including total chromium concentration, pH, total dissolved solids (TDS), color and COD were determined as soon as possible. 2.3 Treatment of tannery wastewater Jar tests had been used to determine the effect of chromium removal and property of precipitation or flocculent object. 400mL of wastewater was added to each baker. Precipitant or flocculent were added to each sample separately to adjust pH from 6 to 11. Sample were mixed by magnetic stirrer at 100 rpm for one minute and then with the speed of 30 rpm for 15 min. In order to determine settling velocity and sludge volumes, discharged samples were poured into 500mL scaled cylinders and supernatant or sludge volume was read during the settling period. Then precipitation supernatant solution was filtered, chromium concentration was determined by atomic absorption spectroscopy method. 3. RESULTS AND DISCUSSION 3.1 Characteristic of the tannery wastewater The properties of the tannery wastewater are shown in Table 1. Table 1 Chemical analysis of tannery wastewater
The average of chromium concentration was
2660 mg.L-1 in the raw tanning wastewater. The results showed that the
wastewater from tannery process is one of the most important sources which pollutes
environment as the concentration of chromium and other harmful material in the wastewater
is extremely high. This is also confirmed by many other studies[8]. That is why
tanning wastewater should be disposed. Fig.1 shows the minimum
solution of chromium is presented at pH 8-9.5 when using NaOH as the precipitant. In case
of using Ca(OH)2 and MgO as the precipitant, the minimum chromium concentration
of settled wastewater are present at pH8-10. Ether using NaOH as precipitant or using
Ca(OH)2 and MgO, increasing pH above 10 will result in increasing the chromium
concentration in filter solution. That is because the precipitation of Cr(OH)3 is
amphoteric colloid, it will redissolve in acidic or basic solution. When pH is above 10,
parts of Cr(OH)3 will be transferred to CrO2-, and CrO2-
can be dissolved in water. So the pH should be control carefully when using precipitants
to remove chrome from wastewater. Figure 1 also shows the removal rate of chromium from
wastewater has little relationship with the kinds of precipitants, it depends mainly on pH
of solution. The results show that
precipitation forming from NaOH is very gelatinous and settling rate of precipitation is
very slow. Using Ca (OH) 2 as precipitant can improve velocity of settling, but
the improvement is a little and the precipitation is also gelatinous and difficult to
filter. However, grainy precipitation with higher settling rate and lower volume of sludge
was obtained by MgO, but the price of MgO is more expensive than NaOH and lime. This is
the agreement with results obtained by Panswad. Table 2 Comparison of different methods
Sludge volume of the
precipitation in Table 2 is determined after 3 h. Flocculation-sedimentation is an appropriate technology for recovery chromium from tanning wastewater. The results show that the optimum method for recovery chromium is adjust pH about 7 by sodium hydroxide, then a small amount of PAM solution is put into the wastewater at stirring. At the optimum condition, Not only a grainy, dense, easily settable and filtrate precipitation can be formed, but also the cost discharge is low and the handled wastewater can be discharged to environment without alkalization to the soil. Acknowledgement This work was supported by the Natural Science Foundation of Hebei Province(No. 2006000305) REFERENCES
沉淀及沉淀絮凝法回收铬鞣废液中铬的研究
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