J. Cent. South Univ. Technol. (2008) 15: 824-829

DOI: 10.1007/s11771-008-0152-2

Selective leaching of chromium-containing slag by HCl

YANG Zhi-hui(杨志辉), CHAI Li-yuan(柴立元), WANG Yun-yan(王云燕),

ZHAO Kun(赵 堃), SHU Yu-de(舒余德)

 (School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China)

Abstract: A batch of column experiments was carried out to investigate the change of Cr(Ⅵ) concentration leached out from chromium-containing slag with HCl as leaching agent, and to study influences of pH, ratio of solid mass to solution volume, flow velocity and temperature on Cr(Ⅵ) leaching. The optimal parameters were obtained for Cr(Ⅵ) leaching and a fitting model was established to describe the procedure of Cr(Ⅵ) leaching. The results show that Cr(Ⅵ) concentration in leachate increases with decreasing pH and increasing flow velocity and temperature. Moreover, Cr(Ⅵ) leaching percentage increases with increasing ratio of solid mass to solution volume. The optimal parameters for Cr(Ⅵ) selective leaching are as follows: pH=3.0, 1?5 of ratio of solid mass to solution volume, 180 mL/min of flow velocity and 40 ℃ of temperature. The procedure of Cr(Ⅵ) leaching fits well with the model: v=1.87t^-0.54, indicating that the leaching rate of Cr(Ⅵ) declines in an exponential order of -0.54.

Key words: chromium-containing slag; Cr(Ⅵ) leaching; leaching rate; dynamic model

1 Introduction

Chromium-containing slag is one kind of hazardous solid wastes discharged from the chromate manufacture, electroplating and leather tanning^[1]. Hexavalent chromium (Cr(Ⅵ)) in the chromium-containing slag is considered one of the most harmful chemical matters to human beings, one of the carcinogenic metals and one of crucial contaminations recognized by EPA of USA^[2]. In recent years, the lack of appropriate disposal facilities has led to serious water and soil pollution^[3-4]. As reported in previous literatures, chemical stabilization and physical removal were documented for detoxicity of chromium-containing slag^[5-7]. However, these methods were not widely explored due to the high expenditure and incompleteness of Cr(Ⅵ) removal. Therefore, more and more attention was paid on microbial method for Cr(Ⅵ) removal^[8]. In our previous study, one novel strain Leucobacter sp, nominated CRB1, was isolated from chromate slag for reduction detoxification of chromium-containing slag. This strain can reduce Cr(Ⅵ) of high concentration in alkaline media^[9].

The hexavalent chromium in the chromium- containing slag can be fractionated into water-soluble and acid-soluble hexavalent chromium according to their solubility. The water-solubility hexavalent chromium consists of Na₂CrO₄ from the leaching and baking process. And acid-solubility hexavalent chromium mainly contains dissociated CaCrO₄, Ca₂SiO₄ and 4CaO·Al₂O₃·Fe₂O₃. Although CaCrO₄ is dissociated from chromium-containing slag, most chromium components are distributed throughout the particles rather than just as surface contaminant^[10-11]. For instance, Ca₂SiO₄ and 4CaO·Al₂O₃·Fe₂O₃ exist in the crystal lattice. The above characteristics of Cr(Ⅵ) components result in persistent leaching of Cr(Ⅵ) from chromium-containing slag. JAMES^[12] found that Cr(Ⅵ) leaching behavior was highly dependent on the soluble behavior of acid-soluble CaCrO₄ in the outer circumstance. Among various Cr(Ⅵ) forms, Na₂CrO₄?4H₂O, CaCrO₄, 4CaO?Al₂O₃?CrO₃? 12H₂O, Fe(OH)CrO₄, and adsorbed Cr(Ⅵ) have fast diffusion rate in aqueous or acid media, while CaCrO₄·Ca₂SiO₄·CaCrO₄ and 4CaO?Al₂O₃?Fe₂O₃? CaCrO₄ have slow diffusion rate^[13]. Previous research demonstrated that decreasing pH value and increasing pressure and temperature could promote Cr(Ⅵ) leaching^[11]. Moreover, CaCrO₄·Ca₂SiO₄·CaCrO₄ and 4CaO·Al₂O₃·Fe₂O₃·CaCrO₄ could entirely be dissolved with prolonging time, leading to CaCrO₄ release.

In order to completely detoxicify Cr(Ⅵ) from the chromium-containing slag, it is important to investigate its leaching procedure and influencing factors. Therefore, the objectives of this research were: 1) to study the influences of pH, ratio of solid mass to solution volume, flow velocity, temperature on Cr(Ⅵ) leaching from chromium-containing slag; 2) to obtain the optimal parameters of Cr(Ⅵ) leaching by HCl; 3) to establish the dynamic model to describe the procedure of Cr(Ⅵ) leaching.

2 Materials and methods

2.1 Materials

The samples of chromium-containing slag were collected from the previous Changsha Chromium Slat Factory, and their chemical compositions are listed in Table 1. The slag samples were collected in accordance with the waste residue heap sampling method^[14-15]. The coarse particles were removed manually. Samples were kept on a drying oven at 100 ℃ for 4 h, and then passed through a sieve with sieve pore of 0.1 mm by ground with a vibration grinder. The ground samples were kept in a drying oven at 100 ℃ again. Cement was added into samples in proportion of 4% before granulation, and the average diameter of particles was 5 mm. Samples were placed in airtight container.

Table 1 Chemical compositions of chromium-containing slag (mass fraction, %)

2.2 Methods

2.2.1 Experimental apparatus

The PVC columns used in the present study were 10.6 cm in inner diameter and 60 cm in length. The column was hand-packed with 500 g chromium- containing slag and it was placed on nylon net (with sieve pore of 25 μm) rested on a Whatman 41 filter to retain solid material in the column (Fig.1). A support base with several drainage holes (1.5 mm of inner diameter) was placed at the bottom of the column. A funnel, filled with fiberglass beads of 5 mm in diameter, was attached to the column bottom to conduct the leaching experiment. Leachates were collected in a 2 500 mL glass bottle. A collecting tank was connected with above glass bottle. The top of the chromium- containing slag column was covered with fine sand to facilitate the uniform flow. Ambient temperature throughout the experimental period was 22 ℃. The leaching solution was supplied by a peristaltic pump.

Fig.1 Sketch map of set-up for leaching experiment

2.2.2 Experimental procedure

The leaching experiments were carried out using single factor experiment to investigate the effects of pH, ratio of solid mass to solution volume, flow velocity and temperature on Cr(Ⅵ) leaching. Five pH values (2, 3, 4, 5 and 6), four ratios of solid mass to solution volume (1?1, 1?2, 1?5 and 1?10), four flow velocities (80, 120, 160 and 180 mL/min) and four temperatures (25, 30, 35 and 40 ℃) were chosen in the present study.

1) In order to investigate the effect of pH on   Cr(Ⅵ) leaching, the concentrated hydrochloric acid was diluted to pH=2, 3, 4, 5 and 6, respectively. The temperature of leaching solution in the collecting tank was 30℃ during the experiment. The flow velocity was maintained at 180 mL/min. At 0, 5, 15, 30, 60, 90, 120, 180, 240, 300, 360, 420, 480, 540, 600 and 660 min, 0.1 mL of leachate was taken out for determining of Cr(Ⅵ) concentration, respectively. The corresponding volume of distilled water was added into collecting tank to maintain the constant volume of solution.

2) In order to investigate the effect of the ratio of solid mass to solution volume on Cr(Ⅵ) leaching, the diluted hydrochloric acid with pH=3.0 was used as leaching agent. The temperature of leaching solution in the collecting tank was maintained at 30 ℃. The flow velocity was maintained at 180 mL/min. The volume of inlet solution in the column was controlled at 1?1, 1?2, 1?5 and 1?10 in ratios of solid mass to solution volume. The rest procedure was the same as that described in the experiment of Cr(Ⅵ) leaching as affected by pH.

3) In the flow velocity experiment, hydrochloric acid with pH=3.0 was chosen as leaching agent. The flow velocities were maintained at 80, 120, 160 and 180 mL/min, respectively. The temperature of leaching solution in the collecting tank was maintained at 30 ℃. The rest procedure was the same as that described in the experiment of Cr(Ⅵ) leaching as affected by pH.

4) In order to compare the Cr(Ⅵ) leaching at different temperatures, four temperatures including 25, 30, 35 and 40 ℃ for leaching solution were chosen. The diluted hydrochloric acid with pH=3.0 was used as leaching agent in column experiment. The flow velocity was maintained at 180 mL/ min. The ratio of solid mass (g) to solution volume (mL) was controlled at 1?5. The rest procedure was the same as that described in the experiment of Cr(Ⅵ) leaching as affected by pH.

2.2.3 Determination of Cr(Ⅵ) concentration

The concentration of Cr(Ⅵ) was determined by using diphenylcarbazide (DPC) method described by PATTANAPIPITPAISAL et al^[16]. 0.1 mL of leachate was diluted with water to 50 mL, and then mixed with 2.0 mL of DPC and 0.5 mL of H₂SO₄ solution (1?1 in volume ratio). After 10 min, the absorbance at 540 nm was determined with a spectrophotometer (Hitachi U2010).

3 Results

3.1 Effect of pH on Cr(Ⅵ) concentration in leachate

The effect of pH on Cr(Ⅵ) concentration was investigated by a continuous flow column leaching system under conditions: 30 ℃ of temperature, pH range of 2-6, 1?5 of solid mass (g) to solution volume (g) ratio and 180 mL/min of flow velocity.

As shown in Fig.2, Cr(Ⅵ) concentrations in leachates increase rapidly in 180 min at pH values of 2.0 and 3.0 and in 90 min at pH values of 4.0, 5.0 and 6.0,  thereafter, the Cr(Ⅵ) concentrations increase gradually.  At the end of experiments, Cr(Ⅵ) concentrations are 72, 65, 49, 44 and 41 mg/L at pH values of 2.0, 3.0, 4.0, 5.0 and 6.0, respectively. In Fig.3, the highest leaching percentage of Cr(Ⅵ), 14.7%, is obtained at pH 2.0. Meanwhile, leaching percentage of Cr(Ⅵ) increases linearly with decreasing pH in the range of 2-4 with a high correlation coefficiency (R²=0.964 5, P＜0.01).

Fig.2 Cr(Ⅵ) concentrations in leachate at different pH values

Fig.3 Cr(VI) leaching rates at different pH values

3.2 Effect of pH on HCl consumption

Fig.4 shows HCl consumption amount at different pH values when 1% of Cr(Ⅵ) is leached out. Based on pH value and the volume of leaching agent, the amount of HCl is used to compare HCl consumption at different pH values. At pH values of 2.0, 3.0, 4.0, 5.0 and 6.0, the amounts of HCl consumption are 0.13, 0.06, 0.05, 0.02 and 0.02 mol, respectively. Although the leaching solution with pH=2.0 exhibits the highest Cr(Ⅵ) percentage (Fig.3), it also has the largest amount of HCl consumption. Technically, in order to get the economical HCl utilization, the optimal parameters with a relatively large Cr(Ⅵ) leaching percentage and small HCl consumption are preferred. Therefore, pH=3.0 is chosen as the optimal pH value in the present study.

Fig.4 HCl consumption amount at different pH values when 1% of Cr(Ⅵ) was leached out

3.3 Effect of ratio of solid mass to solution volume on Cr(Ⅵ) leaching percentage

Fig.5 shows the changes of Cr(Ⅵ) leaching percentage affected by ratio of solid mass to solution volume. At 1?1 of solid mass (g) to solution volume (mL), only 6% of Cr(Ⅵ) is released from chromium-containing slag at the end of experiment. This leaching percentage can be achieved within a short time (30 min), and then there is minor variation of Cr(Ⅵ) leaching percentage during the latter period. Moreover, Cr(Ⅵ) leaching percentage is distinctly dependent on the ratio of solid mass to solution volume. The ratio of 1?1 reveals the lowest Cr(Ⅵ) leaching percentage during the whole experiment, while the ratios of 1?5 and 1?10 show the high Cr(Ⅵ) leaching percentage. After 660 min, 13%-14% of Cr(Ⅵ) was released into aquatic phases for the ratios of 1?5 and 1?10. However, the difference of Cr(Ⅴ) leaching percentage between the ratios of 1?5 and 1?10 is minor at all sampling periods. In this study, 1?5 of ratio of solid mass (g) to solution volume (mL) is chosen for optimizing parameters because the solution consumption with this ratio is lower than that with 1?10.

Fig.5 Effects of ratios of solid mass (g) to solution volume (mL) on Cr(Ⅵ) leaching percentage

3.4 Effect of flow velocity on Cr(Ⅵ) concentration

The changes of Cr(Ⅵ) concentration in leachate at different flow velocities are shown in Fig.6. It is obvious that Cr(Ⅵ) concentration elevates with increasing time at all velocities. The rapid increment of Cr(Ⅵ) concentra- tion is observed before 180 min. The velocity distinctly influences the Cr(Ⅵ) concentration in leachate (Fig.6).

Fig.6 Effects of flow velocity on Cr(Ⅵ) concentration

The highest Cr(Ⅵ) concentration is obtained at velocity of 180 mL/min, followed by 120 and 80 mL /min at all sampling periods.

3.5 Effect of temperature on Cr(Ⅵ) concentration

The effect of temperature on Cr(Ⅵ) concentration in leachate was investigated using a continuous flow column leaching system at pH=3, in the temperature range of 25-40 ℃, 1?5 of ratio of solid mass (g) to solution volume (mL) and 180 mL/min of flow velocity. As shown in Fig.7, Cr(Ⅵ) concentration in leachate increases very fast in 180 min, thereafter, the value gradually increases until the end of experiment. It is noted that Cr(Ⅵ) concentration in leachate increases with increasing temperature over the whole experimental period.

Fig.7 Effects of temperature on Cr(Ⅵ) concentration

3.6 Dynamic model describing Cr(Ⅵ) leaching process

Based on the above results, the optimal conditions for Cr(Ⅵ) leaching are as follows: pH=3.0, 1?5 of ratio of solid mass (g) to solution volume (mL), 180 mL/min of flow velocity and 40 ℃ of temperature. Under the optimal conditions, Cr(Ⅵ) leaching dynamics was investigated. The dynamic curve of Cr(Ⅵ) leaching is shown in Fig.8.

Fig.8 Cr(Ⅵ) concentration in leachate against time under optimal conditions

Chromium(Ⅵ) concentration in leachate against time fits well to the following equation:

c=at^b                                       (1)

where  c is Cr(Ⅵ) concentration, mg/L; t is leaching time, min; a and b are constants.

Taking logarithm of Eqn.(1), the following equation is obtained:

ln c=ln a+bln t                                (2)

A straight line was obtained by plotting ln c against ln t with a high correlation coefficient (R²=0.995 2, P＜0.01) (Fig.9). Thus, the calculated slope (b) of the line is 0.46 and the intercept (ln a) is 1.4011. The corresponding value for constant a is 4.06. Therefore, the dynamic model of the Cr(Ⅵ) leaching by HCl can be described by the following equation:

c=4.06t^0.46                                   (3)

The differential coefficient of Eqn.(3) is expressed as follows:

v=dc/dt=4.06×0.46t^0.46-1=1.87t^-0.54             (4)

where  v represents the apparent leaching rate, mg/(L?min). As shown in Eqn.(4), the leaching rate of Cr(Ⅵ) declines in an exponential order of -0.54.

Fig.9 Relationship between ln c and ln t

In the present study, Cr(Ⅵ) concentration in leachate increases rapidly within a short tine at the beginning (in 180 min). Thereafter, Cr(Ⅵ) concentration increases slowly. The results imply that leaching of Cr(Ⅵ) from chromium-containing slag consists of two-step procedure, a fast leaching and a slow leaching. In the fast leaching procedure, Cr(Ⅵ) is rapidly released into leachate because of presence of water soluble substances in chromium-containing slag such as Na₂CrO₄. In the slow leaching procedure, Cr(Ⅵ) leaching can be contributed to acid dissolution of other chromium-containing metals such as CaCrO₄. However, CaCrO₄ is encompassed by the insoluble substances or matrix components, which leads to the decrease of diffusion and the proliferation with later leaching processing. The results in this study further demonstrate that fast leaching velocity results in an increase of Cr(Ⅵ) concentration in leachate (Fig.6). In fact, fast leaching velocity can enhance the recycle of leachate and Cr(Ⅵ) diffusion from chromium-containing metals. In addition, Cr(Ⅵ) concentration in leachate can increase with increasing temperature (Fig.7). It contributes to the enhanced activity of reaction particle at high temperature. In this study, low pH value stimulates Cr(Ⅵ) leaching (Fig.2). Probably, low pH enhances the dissolution of CaO·Al₂O₃·CrO₃·12H₂O, Fe(OH)CrO₄, CaCrO₄?Ca₂SiO₄? CaCrO₄, CaO·Al₂O₃·Fe₂O₃·CaCrO₄ and CaCrO₄, leading to large amount of Cr(Ⅵ) leached out from chromium- containing slag.

4 Conclusions

1) Cr(Ⅵ) is effectively leached out by diluted HCl from chromium-containing slag. Cr(Ⅵ) leaching can be improved by adjusting pH of leaching agent, flow velocity, ratio of solid mass to solution volume and temperature.

2) The optimal experimental conditions for Cr(Ⅵ) leaching by HCl are as follows: pH=3, 1?5 of ratio of solid mass (g) to solution volume (mL), 180 mL/min of flow velocity and 40 ℃ of temperature.

3) The dynamic model of the chromium-containing slag leached by HCl can be described by the expression: v=1.87t^-0.54. The leaching rate of Cr(Ⅵ) declines in an exponential order of -0.54.

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Foundation item: Projects(2006AA06Z374; 2007AA021304) supported by the National High-Tech Research and Development Program of China

Received date: 2008-05-24; Accepted date: 2008-07-19

Corresponding author: CHAI Li-yuan, Professor, PhD; Tel: +86-731-8836921; E-mail: lychai@mail.csu.edu.cn

(Edited by YANG Hua)