1. Introduction
Ilmenite concentrate is used in the production of titanium metal as a feedstock. Electric melting of ilmenite concentrates to produce titanium slag and pig iron is accompanied by high dust emissions since the charge is fed in a loose state. Silicon contained in the charge is sublimed in the process of melting and falls into the thin sleeve filters together with gases entrained into the gas duct system, condensing in the form of amorphous silica SiO2. High silica content in the dust makes it impossible to return it back to the process, so it is stored in designated storage fields for production wastes.
Considerable amounts of titanium are lost together with the dust generated in the electric smelting process of ilmenite concentrate. Its content in the dusts reaches 50%. Additional extraction of titanium from the dusty waste will not only reduce losses but also allow the acquisition of additional commercial products.
Some of the most demanded products in the market of titanium raw materials are titanium dioxide and amorphous silica. Titanium dioxide is used as a white pigment in the paint, optical coatings, pharmaceuticals, ceramics, food, and paper industries. Titanium dioxide has been used in the photoelectrochemical decomposition of water to produce hydrogen [
1,
2], to purify water from organics [
3,
4,
5,
6,
7,
8,
9], to clean the environment from toxic substances [
10,
11], in high-performance solar cells [
12], as a material with antibacterial and antiviral effect for medical applications [
13,
14], and to create self-cleaning surfaces [
15].
Due to the lack of effective technology, most of the waste from titanium production is not currently recycled. Processing is focused on traditional raw materials, the main of which are ilmenite ores and concentrates.
There are two industrial methods of titanium raw material processing for the production of titanium dioxide: sulfate or sulfuric acid and chlorine. In the sulfate method, the titanium-containing raw material (usually ilmenite concentrate) is treated with concentrated sulfuric acid, and the sulfate solution containing sulfuric salt is decomposed to produce titanium dioxide [
16,
17,
18,
19,
20]. Sulfate technology allows the use of poorer and cheaper raw materials; however, it has several drawbacks, the main of which is the formation of large amounts of waste solutions. According to the chlorine technology [
21], rutile or ilmenite is firstly subjected to the action of chlorine gas in the presence of carbon (coke, etc.) at high temperature, and titanium tetrachloride is formed, which is then oxidized by oxygen to its dioxide at 1300–1800 °C. Compared with the sulfate method, the chlorine method is more environmentally friendly. However, it is selective towards raw materials and requires the processing of high-quality rutile.
Fluoroammonium refining is becoming one of the promising methods of rare metal extraction. Ammonium fluoride, ammonium bifluoride, or their mixtures are used as fluorinating agents.
Ammonium hydrofluoride, unlike fluorine, hydrogen fluoride, and hydrofluoric acid, under normal conditions does not represent a significant environmental hazard and becomes a strong fluorinating agent when heated. The physicochemical basis for the process of fluorination with ammonium bifluoride is that oxygen-containing compounds of transition and many non-transition elements in interaction with NH
4HF
2 form very convenient for processing fluoro- or oxofluorometallates of ammonium, whose physicochemical properties ensure product solubility and the possibility to separate mixtures by sublimation [
22]. A great advantage of these complex salts is their selective tendency to sublimation or thermal dissociation to non-volatile fluorides which guarantees a deep separation of the components, and the stepwise separation of NH
4F vapors allows forthe collection of the desublimate of the latter and use in a closed cycle.
In [
23], during the fluorination of titanium slag with ammonium hydrofluoride, at 380 °С, the degree of sublimation of ammonium hexafluorosilicate was 99%, after sublimation of silicon hexafluoride, titanium dioxide with impurities of other oxides remained in the solid product. The separation of titanium dioxide from other components was carried out using a solution of ammonium hydrofluoride. The precipitation of the titanium compound from the ammonium fluoride solution was carried out by adding a 25% solution of ammonia water. However, titanium compounds (NH
4)
2TiOF
4 or (NH
4)
3TiOF
5 precipitated from the solution. A further shift of the equilibrium towards the formation of Ti(OH)
4 required multiple washing of the precipitate with ammonia water. The content of TiO
2 in the resulting product was more than 90%. A method using another fluorinating agent, ammonium fluoride NH
4F, is known [
24]. The method consists in treating the initial flotation quartz-leucoxene concentrate with ammonium fluoride at a mass ratio of 0.6–1.25:1 and 195–205 °C. The compounds of silicon and titanium were separated by heat treatment of the resulting product at 295–305 °C and the sublimation of ammonium fumoride and obtaining the residue of artificial rutile containing 90–95% titanium dioxide. The method for processing titanium-containing ilmenite concentrate raw material [
25] includes fluorination of raw materials, thermal treatment of the pro-fluorinated mass, separation of fluorination products through sublimation, and pyrohydrolysis of the residue after sublimation to produce iron oxide. Ammonium fluoride, ammonium bifluoride, or their mixtures was used in the fluorination process as a fluoride reagent in a stream of inert gas. Subliminal products were collected with water to produce a solution of ammonium fluorotitanate, and hydrated titanium dioxide was precipitated by water ammonia solution, followed by heat treatment of the precipitate to obtain anhydrous titanium dioxide. In the method [
26], the fluorination of an ilmenite or quartz-leucoxene concentrate was performed at a temperature of 110–195 °C or without heating, and the subsequent separation of silicon from titanium was carried out by sublimation of ammonium silicofluoride at a temperature of 305–450 °C or by aqueous leaching. The content of silicon dioxide in the titanium-containing residue was 0.3 wt%. The titanium content in silicon sublimes was less than 1 wt%. In another method [
27], the ilmenite concentrate was treated with a solution of ammonium fluoride or hydrodifluoride with the separation of titanium from insoluble fluoroammonium salts of iron. Titanium fluoroammonium salts precipitated from the solution were mixed with finely dispersed silicon dioxide, then the mixture was pyrohydrolyzed at a stepwise increase in temperature to 850–900 °C with exposure at each stage for 20–60 min. Titanium dioxide with an anatase structure containing 99.5% TiO
2 and 0.5% SiO
2 was obtained.
As the review of fluoride methods for processing titanium-containing raw materials shows, the separation of silicon from titanium after treatment with a fluorine-containing agent was carried out both by sublimation of silicon fluoride and by leaching with the transfer of silicon into solution in the form of a silicofluoride compound.
The ammonium fluoride processing method makes it possible to regenerate the used fluoride reagents rather well. This has significant advantages over the sulfate method, which produces a large amount of dilute waste sulfuric acid contaminated with various impurities. This makes it difficult to return sulfuric acid back to the process. Also, the method requires a high content of titanium (not less than 46 wt% TiO2) in the ore material. Moreover, the decomposition of titanium-containing raw materials is carried out with concentrated sulfuric acid, which poses a certain danger, since gas and reaction mass are released in this case. In the chlorine method during the processing of ilmenite, difficulties arise at the stage of separation of titanium, silicon, aluminum, and iron chlorides due to the proximity of their physical and chemical properties, and it is also necessary to strictly observe the technological regulations and safety measures due to the existing danger of phosgene formation during chlorination in the presence of carbon-containing reducing agents. Despite the fact that the use of the ammonium fluoride method requires the use of corrosion-resistant equipment and high sealing of technological stages, this method reduces the number of technological operations. The number of reagents, with the possibility of their regeneration, improves the quality of the products obtained and creates the possibility of using a safer and more environmentally friendly method.
Production waste is a complex multi-component raw material that is formed in technological processes and accumulates in its composition components similar in properties. The processing of such raw materials is already a problem. The ammonium fluoride method makes it possible to separate the target components with high selectivity and obtain end products of the appropriate quality from them.
Information about the use of fluoroammonium treatment in the available patent and scientific literature refers to natural titanium-containing raw materials. There are sporadic studies on the application of the fluoroammonium-processing method to titanium slurries with the production of calcium nitrate and titanium dioxide [
28,
29]. Silicon with alkali forms a water-soluble sodium silicate; therefore, in our previous studies [
30], to separate silicon from titanium, electric smelting dust of ilmenite concentrate was leached with a sodium hydroxide solution. The influence of sodium hydroxide solution concentration, duration, leaching temperature, and S:L ratio on the leaching process was studied. The optimum parameters of sodium hydroxide leaching of electric smelting dust of ilmenite concentrate were determined: NaOH concentration of 110–115 g/dm
3; S:L ratio of 1:5; a temperature of 80–90 °С; and a duration of 90–120 min. The silicon extraction in the alkaline solution was 77.7%. Physicochemical studies of electrical melting dust of ilmenite concentrate showed that the silicon is in the form of a magnesium silicate phase, which, as a result of alkaline leaching, is not completely decomposed, partially remaining in the cake.
Studies were performed using high-temperature fluoroammonium processing to ensure the most complete decomposition of silicon-containing phases and to separate the silicon impurity from titanium. Taking into account the differences in the physicochemical properties of the dust constituents, it was of interest to determine the optimal conditions for ammonium fluoride processing with the separation of silicon and titanium and the production of products in the form of their oxides with a high content of the main component.
2. Materials and Methods
Materials: all of the reagents used were ammonium bifluoride, aqueous ammonia, and hydrochloric acid were of a grade not lower than “chemically pure”.
The fine dust of electric smelting of ilmenite concentrates were provided by the “Ust-Kamenogorsk Titanium-Magnesium Plant” JSC, the content of the main components of the dust is shown in
Table 1.
X-ray diffraction analysis (XRD) of the dust (
Figure 1) showed that the substance of the dust sample is in an X-ray amorphous state and the diffractogram background is high, iron in the dust is mainly in the trivalent state, and the harmful impurity silicon is connected with titanium, magnesium, and iron.
Analysis methods: X-ray diffraction analysis was performed on a D8 ADVANCE “BRUKER AXS GmbH” diffractometer (Karsruhe, Germany), Cu-Kα emission. The database PDF-2 International Center for Diffraction Data ICDD (Swarthmore, PA, USA) was used.
X-ray fluorescence analysis was performed using an Axios PANalytical spectrometer with wave dispersion (Almelo, The Netherlands).
The chemical analysis of the samples was performed using an Optima 8300 DV inductively coupled plasma optical emission spectrometer (Perkin Elmer Inc., Waltham, MA, USA).
Experimental procedure: to carry out the processes of sublimation of silicon or titanium fluorides, the dust or residue from the sublimation of silicon, respectively, was thoroughly mixed with ammonium bifluoride in the required ratio. The charge sample was placed in an alundum boat and installed in a LOP LT-50/500–1200 tubular electric furnace. Argon was supplied through a horizontal pipe, and the furnace was heated to a predetermined temperature and maintained at this temperature for a certain period of time. At the end of the experiment, sublimates of ammonium hexafluorosilicate or titanium fluorides were captured at the end of the tube, and the gas-air mixture was captured in a flask with ammonia water. Preliminary experiments on the fluorination of dust from electric smelting of ilmenite concentrate determined the rate of argon supply, which makes it possible to remove fluoride fumes from the reaction zone. The argon feed rate for the used installation was 1.0–1.5 dm
3/h. The degree of fluorination and sublimation of silicon was estimated from the change in the content of the controlled component in the solid residue according to the formula:
where
С0 is the amount of silicon in the initial dust, g;
Ci is the amount of silicon in the residue after fluorination and sublimation, g.
The fluorination and sublimation plants are shown in
Figure 2.
Silicon fluoride sublimates were dissolved in water and treated with water in a ratio of solid to liquid equal to 1:10. After dissolution in water, silicon sublimes were subjected to ammonia hydrolysis. The hydrolysis of a solution of ammonium and oxonium hexafluorosilicates was conducted as follows: in a solution heated to 40 °C containing hexafluorosilicate ion, 10% or 25% ammonia solution was added in portions with active stirring to pH 7.5–8, which upon reaching it was necessary to keep the suspension for 80–90 min by stirring to form and precipitate silica flakes.
During the process of pyrohydrolysis of sublimes of titanium fluorides, a weighed sample of titanium fluorides was placed in an alundum boat and loaded into an electric furnace. After heating to 100 °C, steam was supplied to the furnace, where a boat with a sample of sublimes of titanium fluorides was previously installed. The steam rate was 1.5–2.0 dm3/h.
Titanium dioxide was purified from impurities by hydrochloric acid solution in thermostatic reactors with a volume of 0.5 dm3. Purification was made under established conditions and constant stirring of the pulp. Pulp stirring was performed with a glass stirrer. The stirring speed was 450 rpm.