Flotation Decarbonization and Desulfurization of a High-Sulfur Bauxite in China

Abstract

A high sulfur content is one of the challenges in the processing of refractory bauxites in China. The high carbon content of bauxite makes it more difficult to deal with. Desulfurization and decarbonization are the critical issues in the efficient exploitation of high-sulfur bauxite resources. An experimental study on the removal of sulfur and carbon in high-sulfur bauxite is proposed. The sulfur and carbon phases in high-sulfur bauxite were studied via X-ray diffraction and chemistry quantitative analyses. The results show that the sulfur phases in the high-sulfur bauxite mainly exist in the form of sulfide sulfur (pyrite), and the carbon phases in the sample mainly exist in the form of elementary substance carbon. The morphological features of pyrite in the high-sulfur bauxite were analyzed using a microscopic analysis and a scanning electron microscope (SEM). The pyrite exists in minerals in the forms of euhedral, semi-euhedral and other crystalline structures, with a particle size varying from several hundred microns to only a few microns. The raw ore, with a sulfur content of 4.78%, a sulfide sulfur content of 4.20%, a carbon content of 3.10% and an elementary substance carbon content of 2.46% goes through the “one roughing, one cleaning, two scavenging” process after a pre-decarburization, obtaining the total desulfurization recovery of 96.20% with a desulfurization tailing sulfur content of 0.38%, a carbon content of 0.27% and an alumina content of 71.85%, respectively. This work provides technical support for the efficient utilization of high-sulfur and high-carbon bauxite.

1. Introduction

Aluminum is the world’s second-largest source of metal after steel, whereas bauxite provides the main industrial raw material for alumina production [1,2,3]. Rapid developments in the alumina industry have made bauxite resources scarce. About 0.8 billion tons of high-sulfur bauxite (in which the sulfur content is higher than 0.7%) are not effectively utilized in China [4]. The above-mentioned high-sulfur bauxite ore is mostly found in the provinces of Henan, Guizhou, Guangxi and Shandong, respectively [5,6,7,8]. The scarcity of bauxite resources drives the exploitation of abundant high-sulfur bauxite resources, which has become a hot spot of current research [9].

Generally, the sulfur content is one of the important indicators in the quality assessment of bauxite. During the Bayer process of alumina production, the sulfur in high-sulfur bauxite firstly enters the solution in the form of S²⁻ and then the S²⁻ is gradually oxidized into various forms of S₂O₃²⁻, SO₃²⁻ and SO₄²⁻. Thus, the removal of S²⁻ is very important. The direct use of high-sulfur bauxite in the Bayer process would cause some harmful effects, such as the corrosion of the equipment, the increase in the consumption of alkali, the decline in the dissolution of alumina and the reduction in the yield and quality of alumina production [10,11,12]. Therefore, high-sulfur bauxite must be desulfurized before being used in the manufacture of alumina products.

Recently, many approaches have been used for the desulfurization of high-sulfur bauxite, including pre-roasting [13,14,15], flotation [16,17,18], bioleaching desulfurization [19,20], electrolysis desulfurization [21,22,23], wet oxidation desulfurization [24] and desulfurization by precipitators [25,26,27]. These methods have disadvantages, such as a low desulfurization degree, a high cost and high SO₂ emissions, so they have not been widely applied in the industry. On the other hand, when the organic matter content of circulating mother liquor accumulates to a certain extent in the process of alumina production, the particles of aluminum hydroxide will be refined, causing a reduction in the aluminum hydroxide production. In addition, the impurity content of aluminum oxide products will be increased, and the aluminum oxide products will be colored, resulting in severe scarring of the equipment. Therefore, the direct use of high-sulfur bauxite in the case of high carbon content will bring some adverse effects. For instance, high-sulfur bauxite successfully produces high levels of alstonite in the subsequent alumina production, which would affect the dissolution production energy, energy consumption, safety risks, the evaporation steam consumption and the cauterization consumption. Additionally, bauxite can decrease the effect of red mud washing, the seed decomposition rate, etc. [28,29,30]. Among them, flotation remains one of the most economical and effective methods to utilize this material.

In this study, butyl xanthate and terpenic oil were applied in the reverse flotation for the simultaneous decarbonization and desulfurization of high-sulfur bauxite. After the batch flotation and open-circuit flotation tests, it was found that the products meet the requirements of the Bayer method for industrial application. This work will provide technical support for the flotation process for the effective application of high-sulfur bauxite ores.

2. Materials and Methods

2.1. Materials

The high-sulfur bauxite was collected from Guangxi province, China, then treated by milling, screening and drying. The X-ray diffractometer (XRD, model D8 ADVANCE, Bruker Co., Ltd., Romanshorn, Switzerland) and X-ray fluorescence spectrometer (XRF, model Axios mAX, Malvern Panalytical Co., Ltd., Almelo, The Netherlands) analyses are given in

Table 1. XRF test results of high-sulfur bauxite sample.

Element	Content (%)	Element	Content (%)
Al	30.47	Mg	0.01
Fe	4.66	Cr	0.12
Si	3.81	O	47.40
Ti	2.95	STotal l	4.78
Na	0.031	CTotal 1	3.10
K	0.083	L.O.I. 2	19.13

¹ Based on CS analysis. ² Loss of ignition.

and Figure 1, respectively.

Figure 1 indicates that the main mineral phases of high-sulfur bauxite are chlorite, diaspore, pyrite and rutile. Table 1 demonstrates that the total sulfur (S_Total) content of the bauxite is 4.78%, the aluminum content is 30.47%, the A/S (mass ratio of Al₂O₃ to SiO₂) of the bauxite is 7.05 and the total carbon (C_Total) content of the bauxite is 3.10%, illustrating that the sample is a high-sulfur and high-carbon bauxite with a high grade of alumina.

The chemical phase analytic results of sulfur and carbon are listed in

Table 2. Chemical phase analytic results of sulfur in high-sulfur bauxite.

Phase	Content (%)	Distribution (%)
Sulfur in sulfide	4.20	87.90
Sulfur in sulfate	0.57	11.85
Elementary substance sulfur	0.01	0.25
STotal	4.78	100.00

and

Table 3. Chemical phase analytic results of carbon in high-sulfur bauxite.

Phase	Content (%)	Distribution (%)
Carbon in organic	0.05	1.46
Carbon in carbonate	0.60	19.27
Elementary substance carbon	2.46	79.27
CTotal	3.10	100.00

.

As shown in Table 2 and Table 3, the main existing form of sulfur is sulfide sulfur, which accounts for 87.90% of the total sulfur content. And the main existing form of carbon is elementary substance carbon, which accounts for nearly 80% of the total carbon content. The elementary substance carbon exists mainly in the forms of coal and graphite. Elementary substance carbon can absorb the flotation agents during the flotation process, decreasing the adsorption of xanthate on the mineral surface while reducing the aggregation of pyrite. At the same time, elementary substance carbon tends to adsorb the frother, preferentially forming a “carbon foam layer”, resulting in unsatisfactory desulfurization results. Therefore, decarbonization is required before desulfurization to achieve satisfactory desulfurization results.

2.2. Reagents

AR-grade ethyl xanthogenate, butyl xanthate, cupric sulfate, sodium carbonate, terpenic oil, methanol, HCl (hydrochloric acid), sulfuric acid and CP-grade NaOH (sodium hydroxide) were obtained from Kermel Chemical Reagent Co., Ltd., Tianjin, China. All chemicals were directly used without further purification and distilled water was used in all tests.

2.3. Flotation Experiments

Flotation experiments were performed on an XFGII flotation machine assembled with a 1500 mL plexiglass cell at 1920 rpm speed. The flow chart of the flotation experimental procedure is shown in Figure 2.

3. Results and Discussions

3.1. Microscopic Identification

The morphological features of the high-sulfur bauxite were analyzed using microscopic analysis, as displayed in Figure 3.

As shown in Figure 3, the main minerals in the ore are diaspore, pyrite, rutile, chlorite and a small amount of biotite, kaolinite, etc. Diaspore is the main aluminum-bearing mineral with a fine crystalline size and mainly exists in aggregate form, and pyrite is the primary sulfur-bearing mineral.

Figure 3a–d show that pyrite and rutile have punctate distribution in the diaspore, and the granular diaspore is closely embedded among the particles; the rutile is distributed in a star-dotted pattern. Pyrite exists in minerals in euhedral, semi-euhedral and other crystalline forms (Figure 3d–g). The particle size varies from several hundred microns to only a few microns, and the distribution relationship between pyrite and other minerals is remarkably complex.

The coarse-grained pyrite can reach more than 1 mm, while the granular pyrite is usually in the form of eukaryotic and semi-eukaryotic grains or angular structures distributed in diaspore (Figure 3e,f). The coarse-grained pyrite has a large particle size, which can be easily separated from the original minerals by crushing and grinding, and then separated via flotation.

The medium-grained pyrite with the largest particle size of 100 μm is mostly distributed in other crystalline forms (Figure 3d–g), which is wrapped by diaspore and is embedded by some chlorite, diaspore and other minerals. Some medium-grained pyrite is occasionally seen in strip distribution (Figure 3d), and the pyrite edges are filled with rutile, diaspore, chlorite, etc. Similar to the coarse-grained pyrite, the medium-grained pyrite can also be separated by crushing and grinding, and finally separated via flotation.

The fine and micro-fine pyrite with a few microns in size are scattered in the diaspora in the shape of stars. The distribution of the partial pyrite, the diaspore in the oolitic structure and partial chlorite in a network structure along the dispersed micro-pyrite is presented in Figure 3h,i. However, the small particle size of this pyrite makes it difficult to dissociate it by crushing and grinding. Consequently, it is hard to separate it via flotation.

The morphological features of the high-sulfur bauxite were analyzed using a scanning electron microscope (SEM), as displayed in Figure 4. The main minerals in the ore were analyzed for the diaspore, pyrite, rutile and chlorite.

By comparing the results of Figure 3 and Figure 4, it can be found that the diaspore is an oolitic aggregate with a smooth surface. There is a pure diaspore existing in the high-sulfur bauxite, as illustrated in Figure 4a,b. At the same time, the diaspore is covered with fine granular rutile and chlorite. The mineral in Figure 4c is chlorite. Combined with an XRD pattern analysis (Figure 1), it can be seen that chlorite contains magnesium, iron and other elements. It is clear from Figure 3b that the chlorite is mainly plagioclase or oolitic chlorite.

Figure 4d is relatively pure kaolinite, and a part of the kaolinite is embedded around the pyrite and other minerals in granular form. Combined with the energy-dispersive spectrometer (EDS) analysis in Figure 4d and the mineral facies analysis in Figure 3b, it can be seen that part of the iron oxide in the high-sulfur bauxite is associated with kaolinite and embedded around chlorite and pyrite.

The high-sulfur bauxite sample in Figure 3c,d and Figure 4e,f contain relatively pure rutile with trace iron and a smooth surface, embedding in chlorite and other minerals. Figure 4f illustrates that the rutile is a euhedral crystal, which encloses some chlorite and other calcium-containing minerals.

3.2. Trial Tests

3.2.1. Batch Flotation Tests

In the batch flotation test, the slurry which was ground for 5 min was transferred to the flotation cell. The pH of the slurry was adjusted by adding sodium hydroxide or hydrochloric acid, and the required 40 g/t terpene oil and 80 g/t butyl xanthate were added with stirring for 2 min. The flotation results are listed in

Table 4. The results of direct flotation tests.

Product	Productivity/%	Content/%	Recovery/%
Sulfur concentrate	16.15	14.63	49.42
Tailing	83.85	2.88	50.58
Raw ore	100.00	4.78	100.00

.

As can be observed from Table 4, while the productivity of the sulfur concentrate is 16.15%, the sulfur removal rate is only 49.42%.

3.2.2. Pre-Decarbonization Test

Elementary substance carbon had the potential to adsorb agents and interfere with the collection of sulfide ore, resulting in a failure to achieve the desired level of desulfurization. In consequence, decarbonization experiments are required to decrease the adsorption of agents and achieve the desired level of desulfurization. In the decarbonization tests, terpenic oil was used as the carbon collector, and the decarbonization effect was investigated based on the desulfurization effect after decarbonization. The results are displayed in Figure 5.

Figure 5 displays that the desulfurization degree increased substantially, and the decarburization rate increased slightly with the increase in the dosage of terpenic oil. When the dosage of terpineol oil reached 40 g/t, the desulfurization degree roughly stabilized. Therefore, a terpenic oil dosage of 40 g/t was employed.

3.3. Grinding

The samples of 500 g with the particle size below 3 mm were first wetly ground and passed through a 0.074 mm sieve with a grinding density of 60 wt% in an RK/ZQM ball mill. The yield of the −0.074 mm fine ground sample under different grinding times is shown in Figure 6.

As shown in Figure 6, the yield of −0.074 mm sample increased almost linearly with the increase in the grinding time, showing that the high-sulfur bauxite ore processes a high quality of grind ability. A series of flotations were tested to obtain better grinding times under the conditions of a butyl xanthate dosage of 150 g/t, a terpenic oil dosage of 40 g/t, and with all the tests being conducted at room temperature. The slurry derived from the grinding times of 4, 4.5, 5, 6 and 8 min with each −0.074 mm grain size content of 60.3%, 61.88%, 63.68%, 71.8% and 88.89% were tested, respectively. Figure 7 shows the effects of the grinding fineness on the desulfurization degree.

The results pictured in Figure 7 confirmed that the degree of desulfurization increased and then decreased as the grinding fineness increased, while the sulfur content in the tailings showed an opposite trend. If the grinding fineness was too coarse, the pyrite monomer dissociation rate was low, which was not conducive to flotation recovery, whereas excessive grinding fineness will produce a large amount of slime, which will seriously deteriorate the flotation and lead to the decrease in the desulfurization degree. When the content of the grinding fineness of −0.074 mm is 63.68%, the content of sulfur in the tailings is 0.76%, and the desulfurization degree is 90.72%. Thus, −0.074 mm is the suitable grinding fineness, accounting for 63.68%. In other words, the corresponding appropriate grinding time is 5 min.

3.4. Dosage of Collector

The degree of desulfurization calculated from the raw ore sulfur content, the tailing sulfur content and the tailing yield is described as follows.

$$R=\frac{\alpha -{\gamma }_x\beta }{\alpha }\times 100\%$$

(1)

where R is the desulfurization degree, %; α and β represent the sulfur content of the raw ore and tailings, respectively, wt%; γ_x is the yield of the tailings, %.

The effects of the xanthate dosage on desulfurization are illustrated in Figure 8.

The results in Figure 8 demonstrate that the content of sulfur in the tailings decreased gradually with the increase in the butyl xanthate dosage, while the increase in the sulfur content in the tailings was limited as the butyl xanthate dosage exceeded 150 g/t. The desulfurization degree increased as the dosage of the collector increased, while it decreased when the dosage of the collector was over 150 g/t. Consequently, the optimum dosage of the butyl xanthate collector was 150 g/t in the concentration range. Under these conditions, the sulfur content of the tailing was 0.56% and the desulfurization degree reached up to 94.9%, which indicated that the butyl xanthate was an efficient collector for pyrite in the flotation of high-sulfur bauxite.

3.5. Open-Circuit Flotation Test

The open-circuit tests adopt the “one roughing, one cleaning and two scavenging” process, the flow chart is shown in Figure 9, and the results are illustrated in

Table 5. Results of open-circuit tests.

Product	Productivity/%	S		C		Al2O3
		Content/%	Recovery/%	Content/%	Recovery/%	Content/%	Recovery/%
C concentrate	26.97	4.68	26.45	9.69	84.37	40.38	19.42
S concentrate	11.34	27.21	64.60	1.58	5.78	19.58	3.96
S middling 1	4.84	1.40	1.42	1.95	3.05	59.82	5.17
S middling 2	5.24	2.55	2.79	1.13	1.91	64.02	5.99
S middling 3	3.35	1.35	0.94	0.63	0.68	60.67	3.63
S tailing	48.26	0.38	3.80	0.27	4.21	71.85	61.83
Raw ore	100.00	4.78	100.00	3.10	100.00	56.07	100.00

.

Table 5 shows that the desulfurization tailing (bauxite concentrate) had a sulfur content of 0.38%, a carbon content of 0.27% and an alumina content of 71.85%.

In contrast, a total desulfurization rate of 96.20% and a total decarbonization rate of 95.79% can be obtained after the “one roughing, one selection and two scavenging” process. The tailings meet the requirements of the Bayer process for industrial application, but the sulfur concentrate obtained by the process does not meet the industrial requirements and should be further selected.

4. Conclusions

The raw ore contains 4.78% sulfur, 4.20% sulfide sulfur, 3.10% carbon and 2.46% elementary substance carbon. Sulfur in the high-sulfur bauxite mainly exists in the form of sulfide ore (pyrite), whereas the particle size of pyrite varies from several hundred microns to only a few microns. The coarse-grained pyrite and the medium-grained pyrite can be separated by crushing and grinding, and finally separated via flotation, while the fine and micro-fine pyrite is scattered in the diaspora in the shape of stars, which is hardly dissociated by crushing and grinding, resulting in difficult separation via flotation.

The high-sulfur and high-carbon bauxite ore was obtained using the “one roughing, one cleaning, two scavengings” process after pre-decarbonization with the desulfurization recovery of 96.20%, the total decarbonization rate of 95.79% and the desulfurization tailing with a sulfur content of 0.38%, the carbon content of 0.27% and the alumina content of 71.85%. This work will provide technical support for the efficient utilization of high-sulfur and high-carbon bauxite.

Based on what high-sulfur bauxite desulfurization technology has achieved, the recovery has been substantially improved and the desulfurization and decarbonization have been made better, providing ideas for the flotation of actual high-sulfur bauxite ores.