Study on Sodium Modification of Inferior Ca-Based Bentonite by Suspension Method

Abstract

Hunyuan Ca-based bentonite is one of large-type bentonite deposits in China, the reserve of which is more than 0.15 billion tons. However, they are not completely utilized in the pellet production. Process mineralogy investigation shows that the bentonite is a kind of typical Ca-bentonite (Ca-Bent). The sodium modification of the sample is studied by suspension method in this study. Results present the alkali coefficient K of modified Na-bentonite that is increased from 0.34 to 1.33, and the 2HWA, dilation, and colloid index are, respectively, increased to 601%, 32.4 mL/g and 87.6 mL/(3 g) under optimal conditions of Na₂CO₃ dosage 3.0%, pulp density 20%, sodium temperature 55°C, and sodium time 0.5 h. The XRD patterns show that d(001) of the sample bentonite is reduced from 1.5539 nm down to 1.2467 nm, and 2θ(001) of the sample bentonite is increased from 5.6875° to 7.0907°, indicating that the sample Ca-Bent is effectively modified into Na-Bent.

1. Introduction

Being high dispersity, hydrophilic property, and viscosity, bentonite is widely used in the pellet production to improve the strength of green and dry pellets as well as the bursting temperature of green pellets [1–5]. In commercial production, bentonite quality has obvious effects on the production cost and the quality of products. The practice has shown that about 90% of bentonite is residual in the finished pellets, reducing total iron (TFe) grade of roasted pellets. For blast furnaces, the coke rate is lowered by 2%, and output is increased by 3% if TFe grade of charging is enhanced by 1%. Thus, improving the bentonite quality has great importance to the iron and steel industry.

Presently, average dosage of bentonite, most of them are Ca-Bent, is between 2.5% and 4% in domestic pellet plants. Generally speaking, the balling effect using Na-Bent is better than that using Ca-Bent if the dosage is equal [6]. Therefore, in order to reduce the bentonite dosage, it is necessary to modify the inferior Ca-Bent into superior Na-Bent.

Hunyuan bentonite is one of large-type bentonite deposits in China, the reserve of which is more than 0.15 billion tons. It is a typical kind of Ca-Bent, and the dosage of it is high to 3.0%~3.5% when used as pellet binder. So, they should be firstly subjected to sodium modification so as to be widely used in pellet production. Three methods are usually used to sodium modification [7, 8]: suspension method, semidry process, and screw extrusion method. Comparatively, suspension method makes the sodium reaction happen in solution, and the time for this method is the shortest.

In this investigation, based on the physicochemical properties and process mineralogy of the Ca-Bent, effects of different factors on the sodium modification by means of suspension method are studied, which provides an effective route for the utilization of Hunyuan Ca-Bent.

2. Materials

The main chemical compositions of the sample bentonite are shown in Table 1. According to the formula 𝐾=(𝐸_Na⁺+𝐸_K⁺)/(𝐸_Ca²⁺+𝐸_Mg²⁺), the alkali coefficient 𝐾 is 0.34 (<1), indicating that this bentonite is a typical Ca-Bent [1].

Table 1 

Chemical compositions of the sample Ca-Bent (wt.%).

The chemical phases of silicon in the Ca-Bent show that 94.08% SiO₂ exists in silicates, and 5.92% SiO₂ exists as the dissociated form. As far as calcium is concerned, 78.06% of CaO exists in Ca-montmorillonite, and 12.66% of CaO is in the insoluble silicates.

It can be seen from Figure 1 that Ca-montmorillonite is the leading minerals in the bentonite apart from a small quantity of dissociated quartz. Figure 2 shows the flaky and plate-like particles of Ca-montmorillonite and Figure 3 presents the dissociated quartz enwrapped by the montmorillonite.

Figure 1 

XRD pattern of Hunyuan Ca-Bent.

Figure 2 

SEM image of Ca-montmorillonite.

Figure 3 

SEM image of quartz enwrapped by Ca-montmorillonite (a, b, d-quartz, and c-montmorillonite).

The basic physical indexes of the sample as pellet binders are presented in Table 2.

Table 2 

Basic physical properties of the sample Ca-Bent.

All the sodium agents, including NaF, Na₂CO₃, NaOH, Na₃PO₄, and NaCl, are chemical pure.

3. Experimental Methods

The experimental procedures are described as follows: to begin with, the Ca-Bent is ground, of which the granularity below 0.1 mm accounts for 90%. 100 mL of distilled water is put into a 200 mL beaker. After that, a certain quality of dried Ca-Bent and sodium salt is added into the same beaker and stirred for 3 min in order to disperse the materials evenly, and, then, the beaker is laid in a homothermal water-bath and heated at a given temperature for a certain time. In the bentonite-water system, there exist plenty of Ca²⁺ and Na⁺, the following reaction will occur:Ca-Bent+2Na⁺=2Na-Bent+Ca²⁺.(1)

After the sodium modification is finished, the slurry is filtered, then the filter residues are dried at 95±5°C for more than 6 hours. Finally, the dried bentonite is ground to the granularity of 99% below 0.075 mm.

Literatures indicate [7, 8] that the 2HWA and the dilation of Na-Bent are better than those of Ca-Bent, because Na-montmorillonite can be dispersed into single crystal while Ca-montmorillonite can be only dispersed into the polymer composed of tens of single crystals. So, 2HWA and dilation are often used to evaluate the sodium-modification effect in these investigations, and so is in this research.

XRD is applied to distinguish Ca-Bent from Na-Bent. Literatures [7, 9, 10] reveal that the montmorillonite has the strongest intensities and bright peaks on (001) crystal face. Moreover, d(001) of Ca-Bent is about 1.55 nm, and 2θ(001) is 5.66°; relatively, d(001) of Na-Bent is 1.25 nm, and 2θ(001) is 7.1°.

4. Results and Discussion

4.1. Factors Affecting Sodium Modification

Many factors have obvious effects on the sodium modification of Ca-Bent. In this paper, the following primary parameters, including category and dosage of sodium agent, sodium temperature and time and pulp density, are investigated.

4.1.1. Dosage of Na₂CO₃

To start with, effects of Na₂CO₃ dosage on the 2HWA and dilation of modified bentonite are studied, fixing the conditions of pulp density 20%, sodium temperature 25°C, and sodium time 0.5 h, and the final results are plotted in Figure 4.

Figure 4 

Effects of Na2CO3 dosage on sodium modification.

Seen from Figure 4, 2HWA and dilation of the unmodified Ca-Bent are far less than those of modified Na-Bent. With ascending of Na₂CO₃ dosage, 2HWA and dilation have the similar changing rule of first increase and then decrease, which reaches the maximum at the dosage of 3.0% and 4.0%, respectively. Studies mentioned above have shown that 1.85% CaO is contained in the Ca-montmorillonite. Supposing that Ca²⁺ can be completely replaced with Na⁺, theoretic dosage of Na₂CO₃ is calculated as 3.5% according to reaction (1) mentioned before, which is almost equivalent to the actual dosage used. When Na₂CO₃ dosage is below the suitable quantity, Na⁺ content in the liquid is evidently increased if the Na₂CO₃ dosage is heightened, which improves the reaction probability between Na⁺ and Ca²⁺. After the Na₂CO₃ dosage exceeds the right value, the excessive free Na⁺ exists in the solution, and partial Na⁺ ions are absorbed to the surface of the montmorillonite crystal grains to form a hydrated shell, preventing the outer moisture from entering between the crystal layers. Moreover, the balance of ion-exchange reaction between Na⁺ and Ca²⁺ is destroyed owing to free Na⁺ having high ionization rate and activity. The interlamellar spacing between crystal grains is compressed, so part of the interlayer water is extruded out, which is adverse to the sodium reaction [11].

From the results obtained, the suitable Na₂CO₃ dosage is 3.0%~3.5%.

4.1.2. Pulp Density

Effects of pulp density on the sodium-modification are performed when Na₂CO₃ dosage is 3.0%, sodium temperature is 25°C, and sodium time is 0.5 h, and the variation curves of 2HWA and dilation of modified bentonite are shown in Figure 5.

Figure 5 

Effects of pulp density on sodium modification.

Figure 5 indicates that the 2HWA and dilation are relatively the best when pulp density is 20%, being 593% and 33.0 mL/g, respectively. With the rising of pulp density, the content of Na⁺ and Ca²⁺ in solution is improved, increasing the probability of ion exchange between Na⁺ and Ca²⁺. While the pulp density is in excess of 20%, the montmorillonite crystal grains cannot be dispersed adequately due to the apparent increase of solution viscosity, resulting in the diffusion velocity of Na⁺ decreased to a certain extent. In addition, electrical double layers of montmorillonite mutually repulse and are suppressed [12, 13], which hinder the sodium reaction. Thus, the effect of sodium modification becomes worse.

4.1.3. Sodium Temperature

Figure 6 is the changing curves of sodium temperature affecting the 2HWA and dilation of the modified bentonite. The other parameters are fixed as the following: 3.0% Na₂CO₃ dosage, 20% pulp density, and 0.5 h sodium time.

Figure 6 

Effects of sodium temperature on sodium modification.

As shown in Figure 6, sodium temperature also has perceivable impact on the 2HWA and dilation. Comparatively speaking, the effect of sodium modification is the best when the sodium temperature is 55°C. In the double electric layer of bentonite-water system, Na⁺ in the diffusion layer presents a trend of moving to montmorillonite surfaces [10]. When the temperature is below 55°C, the migration rate of Na⁺ is increased with the temperature improved, which increases the content of Na⁺ on the montmorillonite surface. Therefore, the reaction velocity between Na⁺ and Ca²⁺ is growing. When the temperature is over 55°C, part of non ionization water combined with Ca²⁺ between montmorillonite crystal layers is escaped out because of the high temperature [14], resulting in the increase of migration resistance pressure of interlamination Ca²⁺, which hinders the ion exchange between Na⁺ and Ca²⁺ to a certain extent. Above the right temperature, the higher sodium temperature is, the more interlamination water is out, and the harder Ca²⁺ is replaced of with Na⁺.

4.1.4. Sodium Time

Effects of sodium time on sodium-modification are also studied, and the results are shown in Figure 7. The following parameters are fixed: Na₂CO₃ dosage of 3.0%, pulp density of 20%, and sodium temperature of 55°C.

Figure 7 

Effects of sodium time on the sodium modification.

The essence of sodium modification is the ion exchange between Na⁺ and Ca²⁺. Sodium time will have obvious effects on the sodium reaction balance. It can be seen from Figure 7 that 2HWA and dilation has a sharp increase when the sodium time is less than 0.5 h. With the sodium time increased to 0.5 h, the ion exchange reaction is almost balanced, and the effects of sodium modification nearly reach the optimum. When the time is changed between 0.5 h and 2.5 h, 2HWA and dilation of modified bentonite are kept constantly. So in the commercial production, the sodium time is recommended as about 0.5 h.

4.1.5. Comparisons of Different Sodium Agents

Effects of different sodium agents on sodium modification are performed and the results are displayed in Figure 8. The experimental parameters are fixed as follows: sodium agent dosage of 3.0%, pulp density of 20%, sodium temperature of 55°C, and sodium time of 0.5 h.

Figure 8 

Comparison of different sodium agents affecting sodium modification.

At the same mass percent, Na⁺ mole number of different sodium agents has obvious difference, which is listed in Table 3.

Table 3 

Na⁺ mole number of different sodium agents at the same mass percent.*

From Figure 8, it can be apparently observed that the sodium modification effects of different sodium agents is NaF > Na₂CO₃ > NaOH > Na₃PO₄ > NaCl.

As given in Table 3, Na⁺ mole number of NaF and NaOH is relatively the most, that of Na₂CO₃ and NaCl is less, and that of Na₃PO₄ is the least. When we use NaF, NaOH, and Na₂CO₃ as sodium agents, there are sediments, colloids, or slightly soluble substances formed in the bentonite-water system, which has promoting effects on the sodium reaction. Although NaCl and Na₂CO₃ have the equivalent Na⁺ mole number, Cl⁻ and Ca²⁺ cannot form colloid or sediments, which is the real reason for the sodium effect of NaCl worse than that of Na₂CO₃. At the same mass percent, Na₃PO₄ only provides 0.018 mol of Na⁺, far less than the theoretic quantity of 0.066 mol.

As well known, fluorine is a harmful element. So, Na₂CO₃ is the best sodium agent for the sodium modification of Ca-Bent.

4.2. Integrated Experiments

From the results mentioned above, the suitable conditions of sodium modification for Hunyuan Ca-Bent are including Na₂CO₃ dosage 3.0%, pulp density 20%, sodium temperature 55°C and sodium time 0.5 h.

Repeating the experiment under the optimal conditions for three times, the main physical and chemical properties are measured and given in Tables 4 and 5.

Table 4 

Main physical properties of the bentonite.

Table 5 

Chemical compositions of the modified bentonite.

It can be seen from Table 5 that the content of Na₂O is obviously increased to 2.33%, by contrast, the content of CaO is decreased to 0.77%. According to 𝐾=(𝐸_Na⁺+𝐸_K⁺)/(𝐸_Ca²⁺+𝐸_Mg²⁺), alkali coefficient 𝐾 of modified bentonite is calculated as 1.33 (more than 1.0).

Figure 9 presents the XRD patterns of Hunyuan Ca-bent and the modified Na-Bent obtained under the integrated experimental conditions. As shown, d(001) is reduced from 1.5539 nm down to 1.2467 nm, 2θ(001) is increased from 5.6875° to 7.0907°. And the peak (001) of curve 1 is cragged and dissymmetry; however, the peak (001) of curve 2 is relatively flat and symmetry. These characteristics apparently indicate that curve 1 is the Ca-Bent XRD pattern, and curve 2 is the Na-Bent XRD pattern.

Figure 9 

XRD patterns of Ca-Bent (curve 1) and modified Na-Bent (curve 2).

4.3. Process Flow Recommended

Based on the above results, the sodium-modification process flow is recommended in Figure 10. Suspension method is an effective sodium modification for Ca-Bent. However, the drying of the slurry is the limited section of this method, which is not applied in the Na-Bent commercial production because of the low drying efficiency and long drying time.

Figure 10 

Process flow for sodium modification.

In recent years, the spray high-speed centrifugal dryer is developed and successfully used to dry the liquids as emulsion, suspending liquid, colloid, and so on. Compared with traditional drying equipment, the spray high-speed centrifugal dryer possesses high drying speed and thermal efficiency. Practice shows that the drying time is only 5~15 seconds in general as the drying temperature is about 120~150°C. So, it is feasible that this dryer is recommended to dry bentonite.

5. Conclusions

(1)Process mineralogy investigations show that Hunyuan bentonite, the reserve of which is more than 0.15 billion tons, is one of large-type typical Ca-Bent deposits in China. The 2HWA, dilation, and colloid index of the Ca-Bent are 195%, 8.3 mL/g, and 19.0 mL/(3 g), respectively. Presently, they are not adequately utilized in iron ore pellet production. (2)By means of suspension method, the alkali coefficient 𝐾 of modified bentonite is increased from 0.34 to 1.33, and the 2HWA, dilation, and colloid index are respectively increased to 601%, 32.4 mL/g, and 87.6 mL/(3 g), under the optimal conditions of Na₂CO₃ dosage 3.0%, pulp density 20%, sodium temperature 55°C, and sodium time 0.5 h. (3)The XRD patterns of bentonite before and after sodium modification show that the d(001) from 1.5539 nm reduces to 1.2467 nm, and 2θ(001) from 5.6875° increases to 7.0907°. It indicated that the Ca-based Hunyuan bentonite changed to Na-Bent successfully.

Acknowledgments

This research is supported by National Natural Science Foundation of China (No. 50804059), National Science Fund for Distinguished Young Scholars (50725416), and Specialized Research Fund for the Doctoral Program of Higher Education (No. 200805331080).