Preparation of Polymeric aluminium ferric chloride from bauxite tailings

Bauxite tailings are the main solid wastes in the ore dressing process. The Al2O3 and Fe2O3 contents in bauxite tailings can reach 50% and 13% respectively. The present study proposed a feasible method to use bauxite tailings to prepare polymeric aluminium ferric chloride (PAFC), a new composite inorganic polymer for water purification. Bauxite tailings roasted reacting with hydrochloric acid under air, pickle liquor which mainly contains Fe, Al was generated, then calcium aluminate was used to adjust pH value and the basicity of the pickle liquor, the PAFC was subsequently prepared after the polymerization process. The optimal synthesizing parameters for the preparation of PAFC obtained were as follows: the concentration of hydrochloric acid of 24 wt%, ratio of hydrochloric acid to bauxite of 6:1, temperature of 90°C, leaching time of 2.5 hours, ration of pickle liquor to calcium aluminate of 12:1, polymerization temperature of 90°C and polymerization time of about 3 hours. The basicity of PAFC was higher than 68%, the sum concentration of Al2O3 and Fe2O3 was beyond 12.5%. The results of flocculation tests indicate that the PAFC has a better performance of removing the turbidity of wastewater compared to PAC, and PAFC prepared by bauxite tailings is a kind of high quality flocculants.


Introduction
Bauxite tailings are one of the main solid wastes generated in the flotation-Bayer process which uses low grade bauxite ore to produce alumina in China [1] .About 30wt% bauxite tailings are produced in every one ton Al 2 O 3 producing.At present, most of them are deposited on land surface without any utilization, which pollutes environments seriously.In recent years, much attention has been paid to utilize bauxite tailings to produce refractory, construction material, absorbent material and alloy material [2][3][4][5] .The main chemical components of bauxite tailings include Al 2 O 3 , SiO 2 , Fe 2 O 3 , TiO 2 and K 2 O [6] .The content of alumina and ferric oxide in bauxite tailings are as high as 50% and 13%, respectively, while Fe 3+ salts and Al 3+ salts are most commonly used as coagulation in water treatment.Hence, according to the chemical composition of bauxite tailings, it is suitable for the production of polymeric aluminium ferric chloride.
Polymeric aluminum ferric chloride (PAFC in abbreviation) is a new inorganic compound coagulant which is widely used in water purification.The main chemical components in PAFC are Fe 3+ , Al 3+ , Cl -, and OH -.It has been developed from pre-hydrolyzing AlCl 3 and FeCl 3 by alkali [7] .It has a higher capacity on flocculating and condensation, and could produce large floc aggregates which can be removed from the waste water in subsequent filtration processes [8] .Raw Material for synthesis of PAFC are various, Genli Wang prepared PAFC by bauxite, but waste acid containing Fe were needed to increase the iron content of PAFC [9] .Shihua Pang prepared PAFC by red mud, but its low content of Al [10] .Other material like mineral (such as gibbsite, bauxite, hematite), pure reagent (such as aluminium chloride, pure ferric chloride, aluminium hydroxide) and industrial wastes (such as blast furnace dust, gangue, hydrochloric pickle liquor ) [11][12][13] for preparing PAFC are either high-cost or low content of Al and Fe.Bauxite tailings are a kind of solid wastes which contain large amounts of Al and Fe (up to 49% and 13% respectively), so that it is reasonable suggest that if the high cost of raw materials is replaced by bauxite tailings, it may not only reduce the cost for preparing PAFC, but also be an effective way to utilize bauxite tailings.However, relative works has seldom been reported.
In this paper, a feasibility of the preparation route from bauxite tailings to polymeric aluminum ferric chloride (PAFC) was investigated.The composition of PAFC was characterized, and the flocculation performance of PAFC was investigated as well.

Material and methods
The bauxite tailings (Zhengzhou, Henan Province, China), hydrochloric acid (analytical grade, 37wt %) and calcium aluminate (Gongyi, Henan Province, China), were chosen as raw materials for the preparation of PAFC.Kaolin (Lingshou, Hebei Province, China), Polymeric aluminium chloride (PAC in abbreviation, from Gongyi, Henan Province, China), NaOH and HCl (analytical grade) were chosen as materials for the flocculation performance test of PAFC.All aqueous solutions in this study were prepared with de-ionized water.
The chemical composition of bauxite tailings and calcium aluminate were shown in Table 1.The X-ray diffraction pattern of bauxite tailings were shown in Fig. 1.It was observed that Al 2 O 3 , SiO 2 , Fe 2 O 3 , TiO 2 , K 2 O had constituted the major main chemical compositions of bauxite tailings, while its main mineral compositions are diaspore, kaolinite, with a small amount of hematite, anatase, etc.

Fig.1 XRD pattern of bauxite tailings
The PAC was provided by Huaming Chemical material Co., in Gongyi, Henan Province, China.It is a kind of buff liquid produce, the content of Al 2 O 3 in PAC is higher than 8%, and basicity of PAC is higher than 70%.

Preparation and characterization of PAFC
The preparation of PAFC consists of 2 main steps, hydrochloric acid leaching and polymerization process.
The objective of hydrochloric acid leaching process is to dissolve Al 2 O 3 and Fe 2 O 3 in bauxite tailings as much as possible.In this process, bauxite tailings were firstly shattered, grinded and roasted.The preheated bauxite tailings were directly reacted with hydrochloric acid in the air.Aluminum oxide and ferric oxide were dissolved in hydrochloric acid gradually.At the end of this process, the liquor mainly contained AlCl 3 , FeCl 3 and residual acid, while the solid mainly contained SiO 2 .After the solid-liquid separation, pickle liquor which mainly contained Al 3+ , Fe 3+ and residual acid.
The concentration of hydrochloric acid, the ratio of hydrochloric acid to bauxite tailings, the leaching temperature and the leaching time affected the dissolution ratio of Al 2 O 3 and Fe 2 O 3 in bauxite tailings.
Therefore, these 4 parameters were detailed investigated and their synthesis composition were shown in Table 2.The next process was followed by polymerization process.Al 3+ and Fe 3+ in pickle liquor were expected to be hydrolyzed in this process and then the macromolecule hydroxyl polymer of Al and Fe would generate.
At the beginning of polymerization, Al 3+  Basicity (B) is the most characterizations to represent the degree of polymerization of PAFC.It represents the averages quantity of hydroxyl ligands combined with metal atoms.The higher the basicity is, the higher the degree of polymerization is, the better the flocculation performance of PAFC would be.
The basicity of PAFC was deeply depended on the ratio of calcium aluminate to pickle liquor, the polymerization reaction time and the polymerization reaction temperature, respectively.Hence, the relationship between the basicity of PAFC and the polymerization parameters was detailed investigated and their synthesis composition were shown in Table 3.

Flocculation test
Turbidity removal is the most characterizations of the performance of PAFC in flocculation test.Turbidity removal tests were performed in kaolin turbid liquid.The kaolin turbid liquid was prepared by adding 100mg kaolin to 1.0 L deionized water.The performance of PAFC were investigated compared with PAC which is a traditional flocculant widely applied.In the turbidity removal test, varied volumes of PAFC (PAC) were loaded into 1.0L turbid liquid with varied pH value, respectively.Then the mixture was stirred under a rotate speed of 300r/min for 3min, and then 30r/min for 7min, after that deposited and kept static for 10min.Finally, the water below surface's 10cm was fetched to perform turbidity removal test by turbid meter.
The Turbidity was measured under varied dose of PAFC and PAC (D P ), pH value of kaolin turbid water (pH K ), respectively.Their synthesis composition was shown in Table 4.

Chemical and phase analysis
The Basicity (B) of PAFC was measured as equation shown in Equal.(3) [14] .
In the equation ( 3), [OH] -were the mole amounts of hydroxyl in pickle liquor after the caluminate aluminate was hydrolyzed.Fe T and Al T were the mole amounts Fe 3+ and Al 3+ in pickle liquor, it was measured by ICP.
Turbidity of kaolin turbid water was measured by turbid meter WGZ-800 supplied by from Shanghai, China.

Acid leaching process
Preparation of PAFC began with the acid leaching process.The preheated bauxite tailings were directly reacted with hydrochloric acid.Al 2 O 3 and Fe 2 O 3 were dissolved in hydrochloric acid, and pickle liquor which mainly contained Al 3+ and Fe 3+ was generated.The main reactions in this process were shown in equation ( 4)- (5), Al 3+ and Fe 3+ in pickle liquor were the most important components of PAFC.High contents of Al 3+ and Fe 3+ will enhance the polymerization in the next process.Hence, higher dissolution ratios of Al2O3 and Fe2O3 in bauxite tailings were expected.In order to increase the dissolution ratio of Al 2 O 3 and Fe 2 O 3 , the parameters of acid leaching process were detailed investigated, namely, the concentration of hydrochloric acid (C L ), ratio of hydrochloric acid to bauxite tailings (R L ), leaching temperature (T L ) and leaching time (H L ).
The dissolution ratios of Al 2 O 3 and Fe 2 O 3 in bauxite tailings depend on each parameter were shown in Fig. 2 to Fig. 5. residual acid in pickle liquor which would consume large amounts of calcium aluminate to neutralize in the next process.Therefore, R L of 6:1 was chosen as the optimal parameter.With the increase of H L , leaching reaction continued, but the reaction reached equilibrium at H L of 2.5 hours, so it was selected as the best parameter.Finally, according to the analysis above, C L of 24wt%, R L of 6:1, T L of 90℃ and H L of 2.5 hours were the optimal parameters of the acid leaching process, the dissolution ratio of Al 2 O 3 and Fe 2 O 3 reached 62.4% and 93.5% under this circumstances, respectively.
At the end of the polymerization process, most Al and Fe will exist as polymeric aluminium ferric chloride, PAFC was successfully synthesized.
Basicity (B) is the most characterizations representing the degree of polymerization of PAFC.Basicity signifies the average quantity of hydroxyl ligands combined with Al and Fe atoms.Higher basicity will enhance the flocculation performance, but the PAFC will precipitate easily when the basicity is higher than about 80%.
Therefore within the approved basicity, higher basicity will achieve a better flocculation performance of PAFC.
In the condition that PAFC was stable without precipitation, the highest basicity was reported to be around 70% [15] .
The ratio of pickle liquor to calcium aluminate (R P ), polymerization temperature (T P ) and polymerization time (H P ) affected the basicity of PAFC, therefore, these 3 parameters were investigated as shown in Fig. 8 to Fig. 10.From Fig. 8 it was show that, with the decrease of R P , the dosage of calcium aluminate added into pickle liquor increased.Therefore the hydroxyl generated by hydrolyzing of calcium aluminate increased, as a result the basicity of pickle liquor increased.While the optimum basicity of PAFC is about 70% according references [15] , So R P of 12:1 was chosen to be the optimal parameter.From Fig. 9 it was shown that the basicity of pickle liquor was increased with the increase of polymerization temperature (T P ).That's because, with the increase of T P , kinetic condition of polymerization process improved, therefore the dissolution rate of calcium aluminate and polymerization rate of PAFC increased, so the basicity increased.However as the T P is 90℃, basicity reached a maximum, exorbitant T P would lead to the evaporation of liquid which was useless to the polymerization process.So T p of 90℃ was selected to be the best parameter.Fig. 10 showed that with the increase of H P , the basicity of pickle liquor was increased.Because, as H P increased, the dissolution ratio of calcium aluminate and polymerization ratio of PAFC increased.However, the reaction reached equilibrium as polymerization time was 3 hours.So H P of 3 hours was chosen to be the optimal parameter.Finally, according to the analysis above, R P of 12:1, T p of 90℃ and H P of 3 hours were the optimal parameters of polymerization process.
PAFC was successfully synthesized after the polymerization process under the optimal synthesis parameters.The basicity of PAFC was higher than 68%.According to the ICP measurement, the sum  As shown in Fig. 11, in the range of dosage, the turbidity removal ratios of PAFC were evidently higher than that of PAC.Turbidity removal ratios reached about 80% as the dosage of PAFC was around 6ml•L -1 , compared with PAC of 8 ml•L -1 .It proved PAFC was more effective than PAC on turbidity removal test.
From fig. 12, it was shown that the turbidity removal ratio of PAFC and PAC both increased as pH value of turbid liquid increased from 2 to 8, and both slightly decreased as pH value increased from 8 to 12. Therefore, too high pH value of turbid liquid was not conducive to the coagulant efficiency, the optimal pH value for the PAFC flocculation performance is about 8.In the range of pH value of turbid liquid researched, basically, turbidity removal ratio of PAFC was higher than that of PAC.
According to the flocculation test results, it indicates that PAFC prepared by bauxite tailings had an excellent flocculation performance, and it was more effective than that of PAC.

Conclusions
PAFC was successfully synthesized form bauxite tailing in the present study, and the synthesizing parameters was investigated.The optimal synthesizing parameters attained include: the concentration of hydrochloric acid of 24 wt%, ratio of hydrochloric acid to bauxite of 6:1, temperature of 90℃, leaching tim e of 2.5 hours, ration of pickle liquor to calcium aluminate of 12:1, polymerization temperature of 90℃ and polymerization time of about 3 hours respectively.The basicity of PAFC is higher than 68%, the sum concentration of Al 2 O 3 and Fe 2 O 3 was beyond 12.5%.According to flocculation tests, it indicated that the PAFC has a better performance of removing the turbidity of wastewater compared to the PAC.PAFC prepared by bauxite tailings is a kind of high quality flocculants.
mineral composition of bauxite tailings was measured by X-Ray Diffract meter (XRD) with Cu K radiation in the range 10-100° (2θ) at a scan rate of 5°/min supplied from Japan.The pH value of pickle liquor was measured by pH meter supplied by PHS-29A from Shanghai, China.The dissolution ratio of Al 2 O 3 (Dis A ) and Fe 2 O 3 (Dis F ) of bauxite tailings were calculated as shown in equation (1) and (2). the equation (1), Con AB (Con FB ) was content of Al 2 O 3 (Fe 2 O 3 ) in bauxite tailings.Con AL (Con FL ) was content of Al 2 O 3 (Fe 2 O 3 ) in pickle liquor, which was measured by ICP (Inductive Coupled Plasma Emission Spectrometer).

Fig. 8 Fig. 9 Fig. 10
Fig.8 Basicity of PAFC under varied R P (T P =90℃, H P = 4h) concentration of Al 2 O 3 and Fe 2 O 3 was beyond 12.5% (the concentration of Al 2 O 3 and Fe 2 O 3 were calculated from the concentration of Al 3+ and Fe 3+ in the pickle liquor )。 3.3.Flocculation test Turbidity removal test was carried out in kaolin turbid liquid.The pH value and turbidity of the kaolin turbid liquid were 7.2 and 130NTU, respectively.Dose of PAFC and PAC (D P ) affected the performance of flocculation test.Also, pH value of kaolin turbid liquid (pH K ) was an important factor for the turbidity removal test as well.The turbidity removal performance of PAFC and PAC under varied D P and pH k are investigated as shown in fig.11 and fig.12.

Table 1
Chemical composition of bauxite tailings and calcium aluminate (wt.%)

Table 2
the synthesis parameters in hydrochloric acid leaching process pickle liquor to neutralize residual acid and raise the pH value of the pickle liquor.With increased the pH value, Al(H 2 O) 6 + and Fe(H 2 O) 6 + were gradually hydrolyzed, polymerized and copolymerized.After polymerization and copolymerization reaction reached equilibrium, most Al and Fe will exist as polymeric aluminium ferric chloride (PAFC), PAFC was then successfully synthesized.
and Fe 3+ in pickle liquor were exist as Al (H 2 O) 63 + and Fe (H 2 O) 6 3+ , they could not hydrolyze and polymerize in acidic conditions cause of residual acid.Calcium aluminate was added into the

Table 3
The Parameters in polymerization process

Table 4
The Parameters in Turbidity removal With the progressing of reaction, residual hydrochloric acid was gradually neutralized, the content of Al 3+ in pickle liquor slowly increased, and pH value of pickle liquor increased too, Al (H 2 O) 6+ and Fe (H 2 O)