Inhibition of Cr(VI) in the Al2O3-CaO-Cr2O3 castables by using (Al1-x,Crx)2O3 solid solution


 Al2O3-CaO-Cr2O3 castables are required for various furnaces due to excellent corrosion resistance and sufficient early strength. However, generation of toxic Cr(VI) caused subsequent problems with disposal. The present work aimed at achieving Cr(VI) reduction by replacing Cr2O3 with (Al1 − x,Crx)2O3 solid solution. The phase evolutions and Cr(VI) formation in castables were systemically investigated. Compared with Cr2O3, stability of (Al1 − x,Crx)2O3 in CAC was much higher and improved gradually with Al3+ proportion. The substitution of Cr2O3 with (Al1 − x,Crx)2O3 completely inhibited CaCrO4 formation at 300–1100 oC, and drastically suppressed Ca4Al6CrO16 generation at 900–1300 °C. Thus, a remarkable Cr(VI) reduction of 98.1% could be achieved. Moreover, In comparison with CA and CA2, CA6 was much more stable, which would not take chemical reaction with (Al1 − x,Crx)2O3. Thus, incorporating some Al2O3 powders in Al2O3-CaO-Cr2O3 castables to form CA6 at temperature above 1300 oC was also essential for inhibiting Cr(VI) formation when using (Al1 − x,Crx)2O3 as substitute for Cr2O3.


Introduction
Al 2 O 3 -Cr 2 O 3 refractories have high corrosion resistance due to the extremely low solubility and high chemical stability of Cr 2 O 3 in molten slag, and are therefore widely used as lining materials in incinerators, gasi ers, glass furnaces and non-ferrous smelting, etc. [1][2][3][4][5][6]. Recently, the Al 2 O 3 -Cr 2 O 3 refractories as castables are applied successfully since they are much more energy saving as well as convenient for installation and repairing compared with the bricks or shaped refractory products [7,8].
During fabrication and application processes, binders play a key role in performance of the castables, of which calcium alumina cement (CAC, generally CA and CA 2 phases) is most widely used as it exhibits fast strength development and stable thermo-mechanical behavior, and even resistant to slag attack [9,10]. However, the Cr 2 O 3 can be oxidized into toxic Cr(VI) products at high temperature in the presence of alkali metal or alkaline earths metal oxides such as Na 2 O, K 2 O and CaO in oxidizing atmosphere [11][12][13][14].
Mithun et al investigated the phase evolution of the Al 2 O 3 -CaO-Cr 2 O 3 castables after treatment at various temperature, and they found that CAC can facilitate the conversion of Cr(III) into Cr(VI) due to the presence of CaO [15]. Song and Garbers-Craig also con rmed that both CA and CA 2 , as main phases of the CAC, can react with Cr 2 O 3 in air to produce Ca 4 Al 6 CrO 16 (hauyne) at 1300 °C [16]. The Cr(VI) compounds poses a serious threat to the human and environment since they are toxic, carcinogenic and highly soluble in water [17,18]. Therefore, it is of great environmental and practical signi cance to inhibit the Cr(VI) generation when applying Al 2 O 3 -CaO-Cr 2 O 3 castables as lining materials.
Up till now, several research work focusing on the Cr(VI) formation and minimization have been carried out [19][20][21]. Generally, it was believed that the formation of Cr(VI) was closely related to atmosphere and basicity of other components in the Cr 2 O 3 containing materials [22]. For the Cr 2 O 3 containing refractory linings, since the operation conditions and service atmosphere in a given furnace can hardly be changed in practical production, most of the related work have focused on Cr(VI) minimization using some additives at high temperatures. And the results suggested that acidic components such as SiO 2 , TiO 2 , Fe 2 O 3 and P 2 O 5 can effectively suppress the Cr(III) oxidation during thermal treatment of Cr 2 O 3 containing refractories [4,[23][24][25][26][27][28][29]. However, these oxide additives usually result in formation of low melting point phases in the matrix, which would obviously deteriorate the thermo-mechanical properties or slag corrosion resistance of the refractories [30][31][32].
Previous research work indicated that incorporating chromium into spinel phases can also reduce the risk of Cr(VI) formation in the Cr 2 O 3 containing materials [25,33,34]. For example, the investigation on the Al 2 O 3 -Cr 2 O 3 -CaO-MgO system con rmed that composite spinel Mg(Al,Cr) 2 O 4 could co-exist with CA 2 , where chromium existed in + 3 state [25,34]. And for the magnesia-chrome refractories, adding a certain amounts of Al 2 O 3 and TiO 2 can effectively lower the concentrations of Cr(VI) as chromium mainly exists in the form of Mg(Al,Cr,Ti) 2 O 4 composite spinel [4]. Recent [23]. Again, our recent research work showed that the formation of glasses phases in the Al 2 O 3 -CaO-Cr 2 O 3 castables could promote the (Al 1 − x ,Cr x ) 2 O 3 formation at high temperature, which drastically lowered the formation of Cr(VI) phases simultaneously [35]. These observations indicated that besides spinel phases, chromium existing in the form of (Al 1 − x ,Cr x ) 2  CaO-Cr 2 O 3 castables with temperature and the corresponding mechanism were studied by means of XRD and related software, SEM, and leaching tests. Furthermore, since the (Al 1 − x ,Cr x ) 2 O 3 could be formed in the Al 2 O 3 -CaO-Cr 2 O 3 castables at high temperature [23], castables with Cr 2 O 3 was pre-heated at 1500 o C to produce the in situ formed (Al 1 − x ,Cr x ) 2 O 3 , whose effect on the Cr(VI) formation for the castables at various temperature was also eveluated. solution pre-synthesized at 1300 °C, 1600 °C and 1650 °C were labeled as C-S13, C-S16 and C-S165, respectively. Specimen C-R was pre-heated at 1500 o C for 3h (labeled as C-F15) to produce the in situ formed (Al 1-x ,Cr x ) 2 O 3 , whose effect on the Cr(VI) formation in the castables at various temperature was also eveluated then. The castables were formulated based on the Andreasen distribution co-e cient (q) value of 0.31. And the speci c formulation is shown Tab. 1. Each batch was dry-mixed for 3 minutes in a Hobart mixer followed by wet-mixing (4.0wt% water, 25 °C) for further 3 minutes, and then castables were moulded in vibrating table (1 min) into bars of size 160 mm× 40 mm× 40 mm at room temperature. All specimens were cured at 25 °C and 75±5% relative humidity for 24 h in standrad cement maintainer, and dried at 110 °C for 24 h in an electric air oven after demoulding. Dried specimens C-R, C-S13, C-S16 and C-S165 togther with specimen C-F15 were nally heated in the temperature range of 300-1500 °C for 3h at peak temperature in air.

Characterization methods
The crystalline phase compositions were identi ed by X-ray diffraction (XRD) patterns using a PANalytical X′Pert Pro MPD diffractometer (copper Kα radiation (λ=1.5418Å) at 40 kV/40mA, step size 0.02 over a 2θ range of 5-90°) and analyzed by the software of Philips X′pert Pro High Score. Lattice parameters were calculated using Philips X′pert Pro High Score and Celref 2.0 software. Microstructuremorphology was analyzed by scanning electron microscopy (SEM, Nova 400 Nano-SEM, FEI Company, USA) equipped with energy dispersive spectroscopy (EDS, Oxford UK). Cr(VI) leachability was evaluated using leaching test according to TRGS 613 standard procedure, which is suitable for the determination of water-soluble Cr(VI) compounds in cement and products containing cement [37]. Leaching specimens were prepared by crushing and grounding properly before passing through 200 mesh sieve (≤ 74μm). Fine specimens were stirred with deionized water as leaching solution using a magnetic stirrer at speed of 300 rpm for 15 minutes (at room temperature) with solid-liquid ratio of 1:20, and then leachates were obtained through a 0.45 μm membrane lter with a glass ber by vacuum. The Cr(VI) concentration in the leachates was determined using a colorimetric method. The Cr(VI) can react in acid condition with the 1,5-diohenylcarbazide to form 1,5-diohenylcarbazone which is a red complex (0.02-0.2 mg/l chrome). Then the absorbance of the leachates after nal treatment with 1,5diphenylcarbazide (DPC) method was recorded at 540 nm, using a 722 Vis spectrophotometer (Jinghua Instruments, China). with the temperature (Fig. 1b), which demonstrated that more Al 2 O 3 was dissolved into the (Al 1-x ,Cr x ) 2 O 3 solid solution at higher temperature. To further investigate the crystal structure of the (Al 1-x ,Cr x ) 2 O 3 , the lattice parameters of the (Al 1-x ,Cr x ) 2 O 3 solid solution at various temperature were calculated. As shown in Tab. 2, the lattice parameters decreased constantly with increasing the temperature. This was attributed to the fact that Al 2 O 3 (reference code: JCPDS 01-081-2266, a=b=4.7569 Å and c=12.9830 Å) has lower lattice parameters than that of Cr 2 O 3 (reference code: JCPDS 00-038-1479, a=b=4.9540 Å and c=13.5842 Å). Therefore, theoretically the decreasing trend of lattice parameters of (Al 1-x ,Cr x ) 2 O 3 solid solution with an increase of Al 2 O 3 dissolution at higher temperature was expected.

Cr(VI) leachability
The Cr(VI) concentration in Al 2 O 3 -CaO-Cr 2 O 3 castables treated at various temperature was evaluated by leaching test according to the TRGS 613 standard procedure and the results are shown in Fig. 2. The details of Cr(VI) reduction compared to the reference specimen C-R is presented in Table. 3. It is revealed that with the addition of pre-synthesized (Al 1-x ,Cr x ) 2 O 3 solid solution, a noticeable decrease in Cr(VI) concentration in the specimens was observed and specimens with (Al 1-x ,Cr x ) 2 O 3 pre-synthesized at higher temperature exhibited relative lower Cr(VI) concentration at the same heat treatment temperature (exception for specimen C-S165 at 1300 o C and 1500 o C). For example, at 700 o C, the total amount of Cr(VI) reduced drastically from 1233.2 mg/kg in specimen C-R (without (Al 1-x ,Cr x ) 2 O 3 ) to 223.7 mg/kg in specimen C-S13 (a reduction of 81.9%), and reduced further to 24.0 mg/kg in specimen C-S165 (a reduction of 98.1%). However, at 1300 o C, specimen C-S165 exhibited even higher Cr(VI) concentration than that of reference specimen. Moreover, the temperature corresponding to the maximum Cr(VI) concentration shifted from 900 o C for specimen C-R to 1100 o C for the specimens with pre-synthesized (Al 1-x ,Cr x ) 2 O 3 . The specimen C-F15, which pre-heated at 1500 o C, exhibited extrememly low Cr(VI) concentration at all heat treatment temperatures studied. Although the mid-temperature (700-1100 o C) was favorable for the formation of Cr(VI), the total amount of Cr(VI) in specimen C-F15 at 700 o C-1300 o C was still only 13.0-17.3 mg/kg (a reduction of ~98.9%-99.1% compared to that of specimen C-R), which was below the allowable Cr(VI) limit values of Environmental Protection Agency (EPA), United States (5 mg/L equivalent to 100 mg/kg) [38].

Phase evolution of the castables
In order to study the effect of the pre-synthesized (Al 1-x ,Cr x ) 2 O 3 solid solution on the phase evolution of the castables, phase compositions of the specimens treated at 110-1500 o C were analyzed and the results are shown in Fig. 3. It could be found that in all specimens, main phase corundum together with NaAl 11 O 17 impurity could be detected at all temperatures, and hydrate phase C 3 AH 6 was generated at 110 o C but then disappeared at 300 o C due to dehydration. For specimen C-R, CaCrO 4 phase could be detected at 300 o C, whose peak intensity increased with the increase of temperature from 300 to 900 °C but then decreased with further increasing temperature until disappearance at 1300 °C. The hauyne (Ca 4 Al 6 CrO 16 ) was generated at 900 o C, whose peak intensity reached a maximum at 1100 °C but dropped down with further increasing temperature until disappearance at 1500 °C. Moreover, eskolaite existing in the range of 110 °C to 1100 °C reduced in peak intensity with temperature and disappeared at 1300 °C, while (Al 1x ,Cr x ) 2 O 3 solid solution and CaAl 12 O 19 (CA 6 , calcium hexa-aluminate) increased in peak intensity after generating at 1100 °C and 1300 °C, respectively. However, for specimens C-S13, C-S16 and C-S165, no CaCrO 4 phase was detected at 300-1100 °C, indicating that chromium in the (Al 1-x ,Cr x ) 2 O 3 solid solution would not be oxidized by the CAC in this temperature range. At 900-1300 °C, although the hauyne phase was still formed in these specimens with pre-synthesized (Al 1-x ,Cr x ) 2 O 3 , the peak intensity of this Cr(VI) compound was much lower compared with that in specimen C-R at the same temperature. The peak intensity of Ca 4 Al 6 CrO 16 phase reached the maximum at 1100 °C in Al 2 O 3 -CaO-Cr 2 O 3 castables and therefore, the highest Cr(VI) concentration for the specimens with pre-synthesized (Al 1-x ,Cr x ) 2 O 3 was detected at 1100 o C (Fig. 2). In general, the substitution of Cr 2 O 3 with (Al 1-x ,Cr x ) 2 O 3 in the Al 2 O 3 -CaO-Cr 2 O 3 castables can almost completely restrict the formation of CaCrO 4 compound at 300-1100 °C and effectively lower the Cr(VI) compound Ca 4 Al 6 CrO 16 formation at 900-1300 °C, which was in accordance with the results of Cr(VI) leachability shown in Fig. 2. After treated at 1500 °C, only corundum (with NaAl 11 O 17 impurity), (Al 1-x ,Cr x ) 2 O 3 solid solution and CA 6 phases was found in specimens C-R, C-S13, C-S16 and C-S165. Whereas, the enlarged XRD patterns of the castables in Fig. 3c indicated that specimens with (Al 1-x ,Cr x ) 2 O 3 pre-synthesized at higher temperature exhibited relative lower peak intensity of CA 6 phase after treated at 1300 °C. Besides, the specimen C-F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

Discussion
The above results demnstrated that in the then the mixed powders (formulation shown in Tab. 4 ) were pressed into Ф20 mm×20 mm cylindrical specimens under a pressure of 50 MPa. After treated at 900 o C and 1300 o C for 3 hours in air, the phase compositions and microstructures the of the specimens were analyzed by XRD (Fig. 4) and SEM (Fig. 5), respectively. And the plausible chemical reaction equations discussed below in various specimens heated at 900 o C and 1300 o C were list in Tab. 5.
After treated at 900 o C, the CA phase from CAC disappeared in specimen C-C with the formation of many granular CaCrO 4 grains (Fig. 5a) via reaction 1, whereas specimen C-S was still composed of the initial main phases (CA, CA 2 and (Al 1-x ,Cr x ) 2 O 3 ) (Fig. 5b) in addition to forming minute amounts of hauyne (reaction 2). As the heat treatment temperature increased to 1300 o C, plenty of hauyne and (Al 1-x ,Cr x ) 2 O 3 solid solution (Fig. 5e) were generated in specimen C-C (via reactions 3-5) accompanying with the disappearance of CA and signi cant reduction of CA 2 phase, while specimen C-S possessed relative lower peak intensity of hauyne although it had the same kind of phases as specimen C-C. Combining the observations of Cr(VI) in Fig 2 with the phase evolution results (Fig. 3 and Fig. 4), it can be deduced that compared with Cr 2 O 3 , the (Al 1-x ,Cr x ) 2 O 3 solid solution was more stable that would not form CaCrO 4 and could effectively hinder the hauyne formation when contacted with CAC, and therefore the substitution of Cr 2 O 3 with (Al 1-x ,Cr x ) 2 O 3 can effectively lower the Cr(VI) concentration of the castables after treated at various temperature (Fig. 2). The castables with (Al 1-x ,Cr x ) 2 O 3 pre-synthesized at higher temperature For specimens CH-C, no new phases were occurred after heat treatment at 900 o C, and only miniscule amount of hauyne was generated at 1300 o C (Fig. 5g) via Eqs. 6, which also produced Al 2 O 3 that subsequently interacted with Cr 2 O 3 to generate the (Al 1-x ,Cr x ) 2 O 3 solid solution via Eqs. 5. It is worth mentioning that no changes in the phase compositions were detected in specimen CH-S after heat treatment at both 900 o C and 1300 o C. These observations demonstrated that calcium in CA 6 was much more stable comparing with that in both CA and CA 2 , which only caused slight oxidation of Cr 2 O 3 and would not take chemical reaction with (Al 1-x ,Cr x ) 2 O 3 solid solution. Therefore, specimen C-F15, in which chromium and calcium existed in (Al 1-x ,Cr x ) 2 O 3 and CA 6 respectively, showed no changes in phase composition and extremely low Cr(VI) concentration at various heat treatment temperature. In the Al 2 O 3 -CaO-Cr 2 O 3 castables, CA 6 could be generated from the reaction between CAC and Al 2 O 3 powders in the matrix at 1300 o C (Fig. 3). However, for specimen C-S165, since no Al 2 O 3 existed in the (Al 1-x ,Cr x ) 2 O 3 powder pre-synthesized at 1650 o C (Fig. 1), the calcium would be still exist as CA and CA 2 rather than CA 6 at 1300 o C. As a result, specimen C-S165 possessed even higher Cr(VI) concentration than that of the reference specimen C-R at 1300 o C, attributing to the fact that CA and CA 2 can more easily react with (Al 1x ,Cr x ) 2 O 3 to produce hauyne compared with CA 6 .

Conclusions
In the present work, (Al 1 − x ,Cr x ) 2 O 3 solid solution was pre-synthesized at different temperature, whose inhibition on the Cr(VI) formation in the Al 2 O 3 -CaO-Cr 2 O 3 castables was systematically investigated. And the following conclusions can be drawn: (1) Compared with Cr 2 O 3 , the stability of (Al 1 − x ,Cr x ) 2  can effectively lower the Cr(VI) concentration of the castables after treated at various temperature and a reduction of Cr(VI) amounts up to 98.1% with (Al 1 − x ,Cr x ) 2 O 3 addition could be achieved.
(2) In comparison with CA 2 phase, CA was more likely to react with Cr 2 O 3 /(Al 1 − x ,Cr x ) 2 O 3 resulting in Cr(VI) compounds formation, while calcium in CA 6 was much more stable comparing with that in both CA and CA 2 , which only caused slight oxidation of Cr 2 O 3 and would not take chemical reaction with (Al 1 − x ,Cr x ) 2 O 3 solid solution. Thus, incorporating some Al 2 O 3 powders in the matrix of the Al 2 O 3 -CaO-Cr 2 O 3 castables to form CA 6 at temperature above 1300 o C was also essential for inhibiting Cr(VI) formation when using (Al 1 − x ,Cr x ) 2