Preparation of Potassium Dichromate Crystals from the Chromite Concentrate by Microwave Assisted Leaching

Home , Chromite (compound), Potassium dichromate

crystals

Article Preparation of Potassium Dichromate Crystals from the Chromite Concentrate by Microwave Assisted Leaching

Hua Liu 1,2,3,4, Shenghui Guo 1,2,3,4,*, Jinhui Peng 1,2,3, Duan Yu 1,2,3,4, Libo Zhang 1,2,3,4 and Linqing Dai 1,2,3,* 1 State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China; [email protected] (H.L.); [email protected] (J.P.); [email protected] (D.Y.); [email protected] (L.Z.) 2 Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China 3 Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming 650093, China 4 National Local Joint Laboratory of Engineering Application of Microwave Energy and Equipment Technology, Kunming 650093, China * Correspondence: [email protected] (S.G.); [email protected] (L.D.)

Academic Editor: Shujun Zhang Received: 28 August 2017; Accepted: 11 October 2017; Published: 18 October 2017

Abstract: In the present investigation, the oxidizing roasting process of chromite with sodium carbonate to prepare potassium dichromate crystals was studied in the microwave ﬁeld with air, by heating the chromite and sodium carbonate. The chromite and sodium carbonate heated separately at 1000 ◦C in the microwave oven (frequency: 2.45 GHz; power 1.5 kW) in order to study the microwave absorption properties. The dielectric constant and dielectric loss factor of the chromite and sodium carbonate examined. Then, chromite with sodium carbonate taken in (1:2) ratio and heated at 750 ◦C. Thus obtained samples were characterized using various techniques includes Powder-XRD (XRD), Scanning Electron Microscopy (SEM), and X-ray ﬂuorescence (XRF). The XRD pattern reveals the existence of Fe3O4, Fe2O3, NaAlO2, and Na2CrO4. The iron and aluminum were leached out as Fe2O3 and Al(OH)3 respectively. The resulting sample treated with the KCl to prepare potassium dichromate crystals. Finally, potassium dichromate crystals formed.

Keywords: chromite; microwave; dielectric; ultrasonic extraction; potassium dichromate

1. Introduction The application of microwave heating in metallurgy has been developed in recent years. Some of the world’s developed countries such as United States, United Kingdom, Japan, and Canada have given the great importance to this new technology. China has started the research work in this ﬁeld from 1980s. Tiny although the application of microwave heating in metallurgy is still in the development stage [1,2], many attractive research results have been obtained, and microwave heating has been used in the pretreatment of the ore crushing, refractory gold ore, and the recovery of gold from tailings, extraction of rare metals from ores and heavy metals [3–6]. The microwave is an electromagnetic wave, with the frequency of 0.3–300 GHz (wavelength in 1–1000 mm), and located in the electromagnetic spectrum of red between the external radiation and radio wave. The heating effects of microwave are characterized as follows: (1) the powder material can be heated quickly, and the heating effect can shorten reaction time; (2) can selectively heat the powdery material to make the ore mine and the gangue minerals produce thermal dissociation; (3) can be used without contact with thermal powder material to avoid the external pollution of the material; (4) with

Crystals 2017, 7, 312; doi:10.3390/cryst7100312 www.mdpi.com/journal/crystals Crystals 2017, 7, 312 2 of 11 molecular ruler degree of stirring, can create a strong solid-solid state reduction kinetics, microwave energy is a new type of heating energy, which has obvious advantages. It is possible to realize that the rapid and selective heating of powder material [7–9]. In order to avoid the traditional heating method, it is required to bring the powder material heat and mass transfer to the phenomenon of uneven, using microwave heating method, which can effectively heat material [10]. Potassium dichromate crystals are orange-red monoclinic crystal plate-shaped crystals [11]. The main raw material mainly used for the production of chromate products such as Cr2O3 etc. [12]. In the chemical industry, it is used as an oxidizing agent to make a match head. Manufacture of enamel powder used for enamel, enamel to green. The glass industry as a colorant. Printing and dyeing industry as a mordant. In the spice industry it is used as an oxidizing agent. In addition, it is one of the important reagents to test the Chemical Oxygen Demand (COD) of the water body. The acid water solution can detect the ethanol, and the ethanol can quickly generate the blue and green trivalent Cr ion [13,14], and the solution becomes green. Acidiﬁcation of potassium dichromate can be ethanol oxidation and discoloration, to test whether the driver drunk driving [15]. In nature, it is easy to form a symbiosis with iron Chromium Picotite (Cr2O3·FeO), this type of symbiotic ore is commensurate with the chromium iron ore (chromite) [16,17]. Chromite is China’s shortage of mineral resources, reserves accounted for only 15% of the world, and the size of deposit is small, scattered and ore grade is low [18–20], the dependence on the import of chrome ore is high up to about 95% in our country. Chromite easily forms a symbiosis with picotite [(Mg, Fe)Cr2O4] in nature, this type of symbiotic ore is commensurate with the chromite [21,22]. The Chromite is the main raw material for the production of chromium compounds, which is the main component of FeO·Cr2O3, in general, chromite content is about 40%. It is commonly used in the chromite preparation of potassium dichromate in Laboratory [23]. But the conventional heating and extraction are slow in this process. Importantly, the conversion rate of Cr is not very high. Ultrasonic extraction can accelerate the conversion rate and shorten the extraction time and strengthen the effect of extraction and also can improve the conversion rate of some elements [24–28]. In this paper, mainly research using chromite to prepare potassium dichromate crystals in the ﬁeld of the microwave is reasonable and by microwave heating and ultrasonic extraction can get a high purity of potassium dichromate crystals.

2. Experimental Work

2.1. Materials and Characterization The chromite concentrate sample originated from South Africa and provided by Jinzhou Company in Liaoning Province, China. The brief chemical components of chromite analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) and the results listed in Table1. Phase analysis of the chromite concentrate was performed with the XRD (Bruker, Billerica, MA, USA; CuKα, wavelength: 1.5405980) and the patterns were shown in Figure1. The SEM images of the chromite concentrate were recorded using JSM-35CF SEM equipment (JAPAN Electron Optics Laboratory Co, Ltd., Tokyo, Japan) and shown in Figure2. The particle size distribution was measured using particle size analyzer (LS230, Beckman Coulter, Brea, CA, USA).

Table 1. Brief chemical components of chromite concentrate (wt. %).

Cr2O3 Fe Al2O3 MgO H2O SiO2 CaO P S 51.44 13.56 11.11 10.44 10.44 3.95 1.45 0.05 0.05 Crystals 2017, 7, 312 3 of 11 CrystalsCrystals 2017 2017, ,7 7, ,312 312 33 of of 11 11

Figure 1. XRD pattern of chromite concentrate. FigureFigure 1. 1. XRD XRD pattern pattern of of chromite chromite concentrate. concentrate.

FigureFigure 2. 2. SEM SEM images images of of chromite chromite concentrate. concentrate.

2.2. Dielectric Property Measurement System 2.2. Dielectric Property Measurement System The microwave absorbing property of the material is an important physical indicator in the field The microwave absorbing property of the material is an important physical indicator in the field ﬁeld of microwave chemistry. In this work, a hybrid experimental/computational permittivity measuring of microwave chemistry. InIn thisthis work,work, aa hybridhybrid experimental/computationalexperimental/computational permittivity measuring system developed by the Institute of Applied Electromagnetics at Sichuan University used to system developeddeveloped by by the the Institute Institut ofe Applied of Applied Electromagnetics Electromagnetics at Sichuan at Sichuan University University used to determine used to determine the complex permittivity of the chromite and sodium carbonate at different sample determinethe complex the permittivity complex permittivity of the chromite of the and chromi sodiumte and carbonate sodium at carbonate different sampleat different temperature. sample temperature. First, the quartz tube without measuring material calibrated to get the resonant temperature.First, the quartz First, tube the without quartz measuring tube without material measuring calibrated material to get thecalibrated resonant to frequencyget the resonant and the frequency and the quality factor of empty cylindrical cavity, then the sample was placed in the frequencyquality factor and of the empty quality cylindrical factor of cavity, empty then cylindric the sampleal cavity, was then placed the in sample the calibration, was placed which in hasthe calibration, which has already completed with the diameter of 4 mm quartz tube. At the beginning calibration,already completed which has with already the diameter completed of 4 with mm quartzthe diameter tube. Atof the4 mm beginning quartz tube. of the At experiment, the beginning the of the experiment, the quartz tube kept into the test chamber for calibration, the calibration is ofquartz the tubeexperiment, kept into the the quartz test chamber tube kept for calibration,into the test the chamber calibration for is calibration, completed. the Sample calibration materials is completed. Sample materials kept into quartz tube measured at the start 2.45 GHz electromagnetic completed.kept into quartz Sample tube materials measured kept at into the startquartz 2.45 tube GHz measured electromagnetic at the start wave 2.45 propagation GHz electromagnetic along the wave propagation along the waveguide section. When it encountered the material of the quartz tube, waveguidewave propagation section. along When the itwaveguide encountered section. the material When it ofencountered the quartz the tube, material the energy of the ofquartz the wavetube, the energy of the wave absorption, transmission, and reflection, change the microwave output theabsorption, energy transmission,of the wave andabsorption, reﬂection, transmission change the, microwave and reflection, output change frequency the andmicrowave qualityfactor output of frequency and quality factor of the material observed before and after the entry of the quartz tube, frequencythe material and observed quality beforefactor andof the after material the entry observ of theed quartzbefore tube,and after which the can entry calculate of the the quartz absorbing tube, which can calculate the absorbing ability of the size of the quartz tube material. The structure of whichability ofcan the calculate size of thethe quartz absorbing tube material.ability of Thethe structuresize of the of quartz dielectric tube constant material. measurement The structure device of dielectric constant measurement device shown in Figure 3. dielectricshown in constant Figure3. measurement device shown in Figure 3.

Crystals 2017, 7, 312 4 of 11 Crystals 2017, 7, 312 4 of 11

Crystals 2017, 7, 312 4 of 11

FigureFigure 3. The 3. structureThe structure diagram diagram of of dielectric dielectric constant constant measurementmeasurement device device (1-water (1-water cooling cooling system, system, 2-reaction2-reaction system, system, 3-control 3-control system, system, 4-microwave 4-microwave network network analyzer,analyzer, 5-PC 5-PC collection collection system). system). Figure 3. The structure diagram of dielectric constant measurement device (1-water cooling system, 2-reaction system, 3-control system, 4-microwave network analyzer, 5-PC collection system). 2.3. Experimental2.3. Experimental Setup Setup and and Procedure Procedure All experiments were carried out in a microwave oven, which was developed by Kunming All2.3. Experimental experiments Setup were and carried Procedure out in a microwave oven, which was developed by Kunming University and it has a good performance. Its adjustable power range was 0–6 kW, microwave UniversityAll and experiments it has a good were performance. carried out Itsin adjustablea microwave power oven, range which was was 0–6 developed kW, microwave by Kunming frequency frequency was 2.45 GHz; with a tungsten-rhenium thermocouple (measuring temperature range: 0– University and it has a good performance. Its adjustable power range was 0–6 kW, microwave ◦ was 2.451800 GHz; °C). The with particle a tungsten-rhenium size of chromite was thermocouple 74 μm and it (measuringwas measured temperature using particle range: size analyzer. 0–1800 C). frequency was 2.45 GHz; with a tungsten-rhenium thermocouple (measuring temperature range: 0– The particleThe chromite size of powder chromite (50 was g) and 74 µ sodiumm and itcarbonate was measured (50 g) separately using particle heated size in analyzer.microwave The heating chromite 1800 °C). The particle size of chromite was 74 μm and it was measured using particle size analyzer. powderfurnace (50 g) (frequency and sodium 2.45 carbonate GHz, power (50 1.5 g) separatelykW) using silicon heated carbide in microwave crucible. heatingSimilarly, furnace chromite (frequency and The chromite powder (50 g) and sodium carbonate (50 g) separately heated in microwave heating 2.45 GHz,sodium power carbonate 1.5 kW) mixed using (weight silicon ratio carbide of 1:2) and crucible. roasted Similarly,using a microwave chromite oven and for sodium 30 min carbonateat 750 furnace (frequency 2.45 GHz, power 1.5 kW) using silicon carbide crucible. Similarly,◦ chromite and mixed°C. (weight Thus, the ratio obtained of 1:2) andsample roasted contains using Fe3 aO microwave4, Fe2O3, NaAlO oven2, and for Na 302 minCrO4 at. The 750 sampleC. Thus, was theground, obtained sodium carbonate mixed (weight ratio of 1:2) and roasted using a microwave oven for 30 min at 750 samplewater contains added Fe andO ultrasonic, Fe O , NaAlOextraction, andcarried Na toCrO remove. The Iron sample particles. was Then ground, the iron-free water addedsample and °C. Thus, the obtained3 4 sample2 3 contains2 Fe3O4, Fe22O3, NaAlO4 2, and Na2CrO4. The sample was ground, was neutralized (pH: 7 to 8) using glacial acetic acid to leach out the aluminum particles. Further, ultrasonicwater extractionadded and carriedultrasonic to removeextraction Iron carried particles. to remove Then Iron the particles. iron-free Then sample the wasiron-free neutralized sample (pH: glacial acetic acid was added (pH: 5) in order to convert CrO42− into Cr2O72−. Then the KCl solution 7 to 8)was using neutralized glacial acetic(pH: 7 acidto 8) tousing leach glacial out acetic the aluminum acid to leach particles. out the aluminum Further, glacial particles. acetic Further, acid was added and heated. When crystals precipitation2− appeared,2− the heating stopped and crystals washed addedglacial (pH: acetic 5) in acid order was to added convert (pH: CrO 5) in orderinto to Cr convertO .CrO Then42− into the Cr KCl2O7 solution2−. Then the added KCl solution and heated. to remove KCl in a hot environment. Afterwards,4 the2 7 filtrate cooled and transferred into the ice bath. added and heated. When crystals precipitation appeared, the heating stopped and crystals washed WhenThen crystals a large precipitation amount of appeared,K2Cr2O7 formed the heating [29]. It washed stopped with and ice crystals water, washedat this time to removebecause KClK2Cr in2O7 a hot to remove KCl in a hot environment. Afterwards, the filtrate cooled and transferred into the ice bath. environment.has a relatively Afterwards, large solubility the filtrate and cooledretained and in the transferred filtrate. Finally, into thethe iceorange-red bath. Then crystals a large obtained amount Then a large amount of K2Cr2O7 formed [29]. It washed with ice water, at this time because K2Cr2O7 of K2Cr(Figure2O7 formed 4). [29]. It washed with ice water, at this time because K2Cr2O7 has a relatively large has a relatively large solubility and retained in the filtrate. Finally, the orange-red crystals obtained solubility and retained in the filtrate. Finally, the orange-red crystals obtained (Figure4). (Figure 4).

Figure 4. Flowchart for the preparation of K2Cr2O7 from chromite concentrate with Na2CO3.

FigureFigure 4. Flowchart 4. Flowchart for for the the preparation preparation of of K KCr2Cr2O7 fromfrom chromite chromite concentrate concentrate with with Na2CO Na3. CO . 2 2 7 2 3

Crystals 2017, 7, 312 5 of 11

The waste liquid must be recycled because Cr is poisonous. The conversion rate of chromite calculated according to the following formula [30]:

%η(Cr) = [Cr]r·Vr/m0 × 100 (1) where [Cr]r,Vr and m0 denotes the Cr concentration (g/L) in the leach liquor, the volume (L) of the leach liquor, and the Cr mass (g) in the chromite concentrate, respectively.

2.4. Determination of Purity of the Resultant Product

2.5 g (msample) of K2Cr2O7 weighed, dissolved in 250 mL volumetric flask (VK2Cr2O7 ), and transferred

25 mL solution to an iodine flask (V1K2Cr2O7 ). Then, put 10 mL (2 mol/L) H2SO4 solution and 2 g KI solid mixture for 5 min in the dark and added to 100 mL of distilled water, the solution changed to yellow-green with the prepared standard 0.1 mol/L Na2S2O3(CNa2S2O3 ) solution. Then, starch 3 mL (density 1.5 g/cm3) added and continued titration solution until the blue faded presenting a bright green, according to Na2S2O3 standard the concentration and use of the volume (VNa2S2O3 ) to calculate the conversion rate of the product. The main reactions involved in this process as follows:

K2Cr2O7 + 6KI + 7H2SO4 = Cr2(SO4)3 + 4K2SO4 + 3I2 + 7H2O (2)

2Na2S2O3 + I2 = Na2S4O6 + NaI (3)

CNa2S2O3 VNa2S2O3 MK2Cr2O7 /6 K2Cr2O7% = × 100% (4) msample V1K2Cr2O7 /VK2Cr2O7

3. Results and Discussion According to the previous reports, in the microwave field, the process of material heating mainly decided by the dielectric loss in the electromagnetic field. The dielectric properties characterized by two parameters (1) dielectric constant and (2) dielectric loss factor. The numerical value reflects the interaction between the reduced material and the electromagnetic field. When the reducing material placed in the microwave field, the dielectric polarization will occur due to the presence of a series of different particle reduction materials, namely the dielectric polarization when the particles were not the same, polarization and elimination time was not the same. The dielectric constant and dielectric loss factor of the material calculated theoretically to explain the strength of the material wave absorbing property [31–37]. During the microwave heating process, the FeO·Cr2O3 was oxidized to Na2CrO4 in the presence of O2 and Na2CO3. At the same time, the components of Al2O3, Fe2O3, and SiO2 all transformed into the corresponding soluble salts NaAlO2, NaFeO2 and Na2SiO3, all of them dissolve easily in water. Action equations of the main reactions are as follows:

4FeO · Cr2O3 + 8Na2CO3 + 7O2 = 8Na2CrO4 + 2Fe2O3 + 8CO2 ↑ (5)

Na2CO3 + Al2O3 = 2NaAlO2 + CO2 ↑ (6)

Na2CO3 + Fe2O3 = 2NaFeO2 + CO2 ↑ (7)

Na2CO3 + SiO2 = Na2SiO3 + CO2 ↑ (8)

Thus formed NaFeO2 is leached in the form of Fe(OH)3 the main reaction equation is as follows:

NaFeO2 + 2H2O = NaOH + Fe(OH)3 ↓ (9) Crystals 2017, 7, 312 6 of 11 Crystals 2017, 7, 312 6 of 11 Crystals 2017, 7, 312 6 of 11

Controlling the pH in the process helpshelps inin thethe leachingleaching ofof NaAlONaAlO22 andand NaNa22SiO3 byby precipitation Controlling the pH in the process helps in the leaching of NaAlO2 and Na2SiO3 by precipitation of Al(OH)3 and H2SiO3,, while while Na 2CrOCrO44 convertedconverted to to Na 22CrCr22OO7.7 .The The reaction reaction equations equations of of the the main of Al(OH)3 and H2SiO3, while Na2CrO4 converted to Na2Cr2O7. The reaction equations of the main reactions are as follows: reactions are as follows: NaAlO2 + 2H2O = 2NaOH + Al(OH)3 ↓ (10) NaAlO + 2H O = Al(OH ) ↓ +2NaOH 2 + 2 = 3 ↓ + (10) NaAlO 2 2H 2O Al(OH )3 2NaOH (10) Na SiO + 2H O = 2NaOH + H SiO ↓ (11) Na 2SiO 3+ 2H 2O = H SiO ↓ +2 NaOH3 (11) Na2 SiO3 + 2H 2O = H 2 SiO3 ↓ +2NaOH (11) 2 3 2 2 3 2− + 2− 42− + + = 2 27− + 2CrOCrO 2− +2H + = Cr OO 2−+ HHO2O (12) 22CrO4 + 2H = Cr22O77 + H2 O (12)(12) 2 The Na2Cr2O7 react with KCl and forms the K2Cr2O7 as fellows [38]: The Na2Cr2O7 react with KCl and forms the K2Cr2O7 as fellows [38]: The Na2Cr2O7 react with KCl and forms+ the= K2Cr2O7 as+ fellows [38]: Na2Cr2O7 + KCl =K 2Cr2O7 +2NaCl (13) Na2Cr2O7 KCl K 2Cr2O7 2NaCl (13) Na2Cr2O7 + KCl = K2Cr2O7 + 2NaCl (13) 3.1. The Absorption Properties of Raw Materials 3.1. The Absorption Properties of Raw Materials Figure 5 shows the temperature rise characteristic curves of the chromite and the solid sodium Figure 55 showsshows thethe temperaturetemperature riserise characteristiccharacteristic curvescurves ofof thethe chromitechromite andand thethe solidsolid sodiumsodium carbonate respectively in the microwave field. Chromite concentrate heated to 1000◦ °C in 6 min and carbonate respectivelyrespectively inin the the microwave microwave ﬁeld. field. Chromite Chromite concentrate concentrate heated heated to 1000to 1000C °C in 6in min 6 min and and the the heating rate was 167◦ °C/min, then the solid sodium carbonate was raised 1000◦ °C in 13 min and theheating heating rate rate was was 167 167C/min, °C/min, then then the the solid solid sodium sodium carbonate carbonate was was raised raised 1000 1000C in°C 13 in min13 min and and the the heating rate was◦ 77 °C/min. From the temperature rise curves, chromite and solid sodium theheating heating rate wasrate 77wasC/min. 77 °C/min. From From the temperature the temperature rise curves, rise chromitecurves, chromite and solid and sodium solid carbonate sodium carbonate have good wave absorbing properties. carbonatehave good have wave good absorbing wave absorbing properties. properties.

(a) (b) (a) (b)

Figure 5. Temperature rise characteristic curves in the microwave field of chromite (a) and Figure 5. Temperature riserise characteristiccharacteristic curves curves in in the the microwave microwave ﬁeld field of of chromite chromite (a )(a and) and Na 2 CO3 (b). Na2CO3 (b). Na2CO3 (b). 3.2. The Study Dielectric Constant and Dielectric Loss Factor of Raw Material 3.2. The Study Dielectric Constant and Dielectric Loss Factor of Raw Material 3.2. The Study Dielectric Constant and Dielectric Loss Factor of Raw Material Figures6 and7 show the dielectric constant and dielectric loss factor of the chromite and Figures 6 and 7 show the dielectric constant and dielectric loss factor of the chromite and sodiumFigures carbonate 6 and respectively. 7 show the dielectric The increasing constant trend and of dielectric dielectric loss loss factor and dielectricof the chromite loss factor and sodium carbonate respectively. The increasing trend of dielectric loss and dielectric loss factor with withsodium the carbonate temperature, respectively. indicates The the increasing chromite concentratetrend of dielectric and sodium loss and carbonate dielectric have loss goodfactorwave with the temperature, indicates the chromite concentrate and sodium carbonate have good wave theabsorbing temperature, properties. indicates the chromite concentrate and sodium carbonate have good wave absorbing properties. absorbing properties.

Figure 6. DielectricDielectric constant constant ( (aa)) and and loss loss factor factor ( (b)) of of chromite chromite with temperature. Figure 6. Dielectric constant (a) and loss factor (b) of chromite with temperature.

Crystals 2017, 7, 312 7 of 11 Crystals 2017, 7, 312 7 of 11 Crystals 2017, 7, 312 7 of 11 Crystals 2017, 7, 312 7 of 11

Figure 7. Dielectric constant (a) and loss factor (b) of Na2CO33 withwith temperature. temperature. Figure 7. Dielectric constant (a) and loss factor (b) of Na2CO3 with temperature. Figure 7. Dielectric constant (a) and loss factor (b) of Na2CO3 with temperature. 3.3. Characterization Characterization of the Roasted Material 3.3. Characterization of the Roasted Material 3.3. Characterization of the Roasted Material 3.3.1. SEM SEM Analysis Analysis 3.3.1. SEM Analysis 3.3.1. SEM Analysis Figure 88 representsrepresents thethe SEMSEM imageimage ofof thethe roastedroasted products.products. ItIt clearlyclearly showsshows thatthat thethe surfacesurface ofof Figure 8 represents the SEM image of the roasted products. It clearly shows that the surface of roastedFigure products 8 represents covered the by SEM a moderate image of quantity the roasted of me melt products.lt after roasted. It clearly It It can shows also that be observed the surface that of roasted products covered by a moderate quantity of melt after roasted. It can also be observed that theroastedthe micro products thermal covered fragmentation fragmentation by a moderate of of the the particles particles quantity in in of the the me mixture mixturelt after roasted. of of sodium sodium It can carbonate carbonate also be andobserved and chromite. chromite. that the micro thermal fragmentation of the particles in the mixture of sodium carbonate and chromite. Thethe microSEM images thermal show fragmentation the aggregation of the of particles the particles in the with mixture some of pores. sodium carbonate and chromite. The SEM images show the aggregation of the particles with some pores. The SEM images show the aggregation of the particles with some pores. a b a b

Figure 8. SEM images (a,b)of microwave roasted reaction products. Figure 8. SEM images (a,b)of microwave roasted reaction products. FigureFigure 8. SEMSEM images images ( (aa,,bb)of)of microwave microwave roasted roasted reaction products. 3.3.2. XRD Analysis 3.3.2. XRD Analysis 3.3.2. XRD Analysis Figure 9 shows the XRD pattern of the product after microwave roasted. The FeO·Cr2O3 and Figure 9 shows the XRD pattern of the product after microwave roasted. The FeO·Cr· 2O3 and Na2COFigure3 reacted 99 showsshows chemically thethe XRDXRD producing pattern pattern of Naof the 2theCrO product product4. Additionally, after after microwave microwave Al2O3 and roasted. roasted. Na2CO The3 Thereacted FeO FeO·Cr toCr generate22OO33 and Na2CO3 reacted chemically producing Na2CrO4. Additionally, Al2O3 and Na2CO3 reacted to generate NaAlONa2COCO323 .reacted reactedAt the samechemically chemically time, Feproducing 2O3 also existed. NaNa22CrO 4 .. Additionally, Additionally, AlAl22O3 andand Na 2COCO33 reactedreacted to to generate generate NaAlO2. At the same time, Fe2O3 also existed. NaAlO2.. At At the the same same time, time, Fe Fe22OO3 3alsoalso existed. existed.

Figure 9. XRD pattern of microwave roasted product. Figure 9. XRD pattern of microwave roasted product. Figure 9. XRDXRD pattern pattern of microwave roasted product.

Crystals 2017, 7, 312 8 of 11 Crystals 2017, 7, 312 8 of 11 Figure 10 shows the XRD pattern of slag after ultrasonic extraction, which shows the existence of Figure 10 shows the XRD pattern of slag after ultrasonic extraction, which shows the existence Fe2O3, Fe3O4 and Al(OH)3. Compared to the XRD patterns of chromite concentrate before microwave of Fe2O3, Fe3O4 and Al(OH)3. Compared to the XRD patterns of chromite concentrate before microwave roasted and ultrasonic extraction, only Fe2O3 and Fe3O4 are existing. Other components removed. roasted and ultrasonic extraction, only Fe2O3 and Fe3O4 are existing. Other components removed.

Figure 10. XRD pattern of slag after ultrasonic extraction. Figure 10. XRD pattern of slag after ultrasonic extraction.

Figure 11 illustrates the XRD pattern of the ﬁnalfinal product.product. The XRD pattern clearly suggests the formation of KK22Cr2OO77. .The The synthesized synthesized potassium potassium dichromate dichromate cr crystalsystals existed existed in monoclinic crystal system. However,However, thethe existence existence of of the the small small amount amount of Kof2CrO K2CrO4 indicates4 indicates that that the pHthe notpH controllednot controlled well; therefore,well; therefore, Na2CrO Na4 2notCrO completely4 not completely transformed transformed into Na 2intoCr2 ONa7.2 FromCr2O7 the. From determination, the determination, the purity the of thepurity resultant of the product,resultant theproduct, potassium the potassi dichromateum dichromate crystals purity crystals is 67.4%. purity is 67.4%.

Figure 11. XRD pattern of the final ﬁnal product.

3.3.3. XRF Analysis Table2 2 represents represents the the brief brief chemical chemical component component of of products products after after microwave microwave roasted roasted and and Table Table3 represents3 represents brief brief chemical chemical components components of slag of slag after after ultrasonic ultrasonic extraction. extraction. After After microwave microwave roasted roasted O, Cr, NaO, Cr, elements Na elements are present are present in large quantities.in large quantities Comparison. Comparison of both tables of both indicates tables thatindicates after ultrasonicthat after extractionultrasonic otherextraction elements other such elements as Fe, such Mg, Al,as Fe, Si and Mg Ca, Al, are Si alland more Ca are than all roasted, more thanwhich roasted, shows whichin the processshows in of the ultrasonic process of extraction ultrasonic is beneﬁcial.extraction is However, beneficial. in However, this process, in this Cr hasprocess, notbeen Cr has completely not been converted.completely converted. By XRF analysis, By XRF a analysis, high conversion a high conversion rate of Cr israte 94.57%, of Cr is which 94.57%, is reasonable which is reasonable and after ultrasonicand after ultrasonic extraction, extraction, the conversion the conversion rate of Cr israte 82.72%, of Cr is this 82.72%, is an acceptable this is an acceptable number to number be optimistic. to be optimistic. Table 2. Brief chemical components of the products after microwave roasted. Table 2. Brief chemical components of the products after microwave roasted. O Cr Na Fe C Mg Al Si Ca O Cr Na Fe C Mg Al Si Ca 65.71 33.27 45.95 6.20 5.11 2.42 1.89 0.83 0.29 65.71 33.27 45.95 6.20 5.11 2.42 1.89 0.83 0.29

Crystals 2017, 7, 312 9 of 11

Table 3. Brief chemical components of the slag after ultrasonic extraction.

O Cr Na Fe C Mg Al Si Ca 46.48 11.18 9.52 17.67 2.82 7.04 4.12 3.11 0.74

3.4. The Purity of the Product

According to the formula of (2)–(4), the purity of final K2Cr2O7 product was calculated. After microwave roasted and ultrasonic extraction, the purity of the product is up to 67.4%, compared with the conversion rate of Cr after microwave roasted that is a little bit down. Since the acidity has not been controlled ideally, the Na2CrO4 having not been completely transformed into Na2Cr2O7. Therefore, the ﬁnal product K2Cr2O7 includes small amount of K2CrO4.

4. Conclusions Using chromite and sodium carbonate for the preparation of potassium dichromate crystals in the microwave field is reasonable because chromite and sodium carbonate having good wave absorbing properties, and it takes only ten minutes in the heating process. The conversion rate of Cr is 94.57% after microwave roasted. The Ultrasonic extraction can strengthen the extraction efficiency and has a good effect on the conversion rate of Cr. Finally, the product obtained shows that the purity of product is up to 67.7% of microwave heating and ultrasonic extraction. The present work suggests that the microwave field is ideal for the preparation of potassium dichromate crystals. However, controlling the reaction parameters such as pH influence the final product purity.

Acknowledgments: The financial support from the National Natural Science Foundation of China (No.51504114) and Kunming University of Science and Technology Foundation for analysis and testing (No.2016M20152128005). This support is gratefully acknowledged. The authors are grateful to the reviewers for the discerning comments on this paper. Author Contributions: Linqing Dai and Shenghui Guo conceived and designed the experiments; Hua Liu performed the experiments and wrote the paper; Yu Duan analyzed the data; Jinhui Peng and Libo Zhang contributed reagents and materials and analysis tools. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Yu, W. Research progress in the field of metallurgy of vanadium titanium magnetite by microwave heating. Iron Steel Vanadium Titan. 2011, 32, 87–96. 2. Tong, Z.; Bi, S.; Yang, Y. Present situation of study on microwave beating application in metallurgy. J. Mater. Metall. 2004, 3, 117–120. 3. Zhang, S.; Peng, J.; Huang, M.; Sun, Y.; Zhang, L.; Fan, X.; Guo, S. Preparation of primary titanium materials from self-reduced pellet of titanium concentrate by microwave reduction. Chin. J. Rare Met. 2006, 30, 78–81. 4. Gu, J.; Ao, N.; Liu, Y. The application and progress of microwave technology in mineral processing process. Min. Metall. 2002, 11, 139–146. 5. Liu, Q.; Xiong, Y. Research on application mechanism of iron ore grinding by microwave. Metallurgy 1997, 3, 25–28. 6. Zhou, X. Structure and microwave dielectric properties of Kunming ilmentite. Chem. Ind. 1992, 3, 7–8. 7. Chen, J.; Li, N.; Wang, S. The heating performance of carbon containing chromite fines in microwave field. J. Beijing Univ. Sci. Technol. 2007, 29, 880–906. 8. Chen, J. Application of Microwave Heating Reduction of Carbon Containing Pellet Melting; Iron & Steel Research Institute: Beijing, China, 2003; Volume 6, pp. 8–13. 9. Jia, Z. Study on ore chromite pellet smelting high carbon ferrochrome. Zhejiang Metall. 1996, 2, 17–22. 10. Bai, X.Y.; Wu, S.; Zhang, S.C. A new microwave heating system for processing ceramic materials. J. Microw. 1999, 2, 114. Crystals 2017, 7, 312 10 of 11

11. Alybakov, A.A.; Gubanova, V.A.; Kudabaev, K.; Tojchiev, N. Paramagnetic centres created by a thermal treatment in potassium dichromate crystals. Cryst. Res. Technol. 2010, 18, 68–70. [CrossRef] 12. Dai, W.; Shu, L. Iron Alloy Metallurgy Engineering; Metallurgical Industry Press: Beijing, China, 1999. 13. Shi, F.; Kongjin, Z.; Yalin, Q. New progress in mineral processing of chromium iron ore. In Proceedings of the China Mineral Processing Technology Exchange Meeting, Haikou, China, 13–17 November 2006. 14. Liu, J. Mineral processing of low grade chrome ore. In Proceedings of the China Iron and Steel Annual Meeting, Beijing, China, 28–30 October 2005. 15. Shi, F. Chromite mine of China and Volgadansk. Geo Exp. 1998, 2, 16–18. 16. Medina-Campos, O.N.; Barrera, D.; Segoviano-Murillo, S.; Rocha, D.; Maldonado, P.D.; Mendoza-Patiño, N.;

Pedraza-Chaverri, J. S-allylcysteine scavenges singlet oxygen and hypochlorous acid and protects LLC-PK1 cells of potassium dichromate-induced toxicity. Food Chem. Toxicol. 2007, 45, 2030–2039. [CrossRef][PubMed] 17. Lushchak, O.V.; Kubrak, O.I.; Nykorak, M.Z.; Storey, K.B.; Lushchak, V.I. The effect of potassium dichromate on free radical processes in goldfish: Possible protective role of glutathione. Aquat. Toxicol. 2008, 87, 108–114. [CrossRef][PubMed] 18. Reedijk, M.F.; Arsic, J.; Kaminski, D.; Van Enckevort, W.J.P.; Vlieg, E. Surface structure of potassium dichromate (KBC) crystals. Surf. Sci. 2003, 526, 133–140. [CrossRef] 19. Thilak, T.; Basheer Ahamed, M.; Vinitha, G. Third order nonlinear optical properties of potassium dichromate single crystals by Z-scan technique. Optik 2013, 124, 4716–4720. [CrossRef] 20. Andre, N.; Paut, O.; Arditti, J.; Fabre, P.; Bremond, V.; Alhmana, T.; Bellus, J.F.; Jouglard, J.; Camboulives, J. Intoxication grave au dichromate de potassium apres introduction nasale accidentelle. Arch. Pédiatrie 1998, 5, 145–148. [CrossRef] 21. Qi, T.G.; Liu, N.; Li, X.B.; Peng, Z.H.; Liu, G.H.; Zhou, Q.S. Thermodynamics of chromite ore oxidative roasting process. J. Cent. Univ. Technol. 2011, 18, 83–88. [CrossRef] 22. Li, J. Study on solid alkali prepared by the oxidation of potassium dichromate experimental. Chifeng Univ. 2005, 5, 25. 23. Li, J. Effect of ultrasonic wave on leaching process. Metallurgy 2001, 30, 28–30. 24. Xu, S.; Zhang, C.; Zhao, T. Application of ultrasonic in extraction process. HN Nonferrous Met. 1994, 11, 350–353. 25. Timothy, M. Sonochemistry: Theory, Applications and Uses of Ultrasound in Chemistry; Ellis Horwood: New York, NY, USA, 1988. 26. Mason, T.J. Advances in Sonochemistry; JAI Press Ltd.: London, UK, 1996; Volume 5. 27. Vinatoru, M. An overview of the ultrasonically assisted extraction of bioactive principles from herbs. Ultrason. Sonochem. 2001, 8, 303–313. [CrossRef] 28. Chen, D.; Lázaro, F.; de Castro, L.D.; Valcárel, M. Direct spectrophotometric determination of total boron in soils with ultrasonic leaching in automatic flow systems. Anal. Chim. Acta 1989, 226, 221–227. [CrossRef] 29. Yanan, L.; Kui, L.; Xiurong, S. Study on Producing Dichromate—Potassium by Chromium Containing Wastewaters. Chem. Ind. Guangzhou 2010, 38, 140–141. 30. Zhang, H.; Xu, H.B.; Zhang, X.; Zhang, Y.; Zhang, Y. Pressure oxidative leaching of Indian chromite ore in concentrated NaOH solution. Hydrometallurgy 2014, 142, 47–55. [CrossRef] 31. Looyenga, H. Dielectric constants of heterogeneous mixtures. Physica 1965, 31, 401–406. [CrossRef] 32. Dick, B.G., Jr.; Overhauser, A.W. Theory of the Dielectric Constants of Alkali Halide Crystals. Phys. Rev. 1958, 112, 90–103. [CrossRef] 33. Akerlof, G. Dielectric Constants of Some Organic Solvent-Water Mixtures at Various Temperatures. J. Am. Chem. Soc. 1932, 11, 4125–4139. [CrossRef] 34. Dwyer, J.J.; Gittis, A.G.; Karp, D.A.; Lattman, E.E.; Spencer, D.S.; Stites, W.E.; Bertrand, G.-M.E. High Apparent Dielectric Constants in the Interior of a Protein Reflect Water Penetration. Biophys. J. 2000, 79, 1610–1620. [CrossRef] 35. Uchino, K.; Nomura, S. Critical exponents of the dielectric constants in diffused-phase-transition crystals. Ferroelectrics 1982, 44, 55–61. [CrossRef] 36. Ghodgaonkar, D.K.; Varadan, V.V. A free-space method for measurement of dielectric constants and loss tangents at microwave frequencies. IEEE Trans. Instrum. Meas. 2002, 38, 789–793. [CrossRef] 37. Liu, C.; Zhang, L.; Peng, J.; Qu, W.; Liu, B.; Xia, H. Dielectric Properties and Microwave Heating Characteristics of Sodium Chloride at 2.45 GHz. High Temp. Mater. Process. 2013, 32, 587–596. [CrossRef] Crystals 2017, 7, 312 11 of 11

38. Huang, H.; Min, L. Study on chromite preparation of chromium oxide instead of potassium dichromate experimental. Shangrao Normal Univ. 2015, 35, 60–62.

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).