A review of the production cycle of titanium dioxide pigment

Materials Sciences and Applications, 2014, 5, 441-458 Published Online May 2014 in SciRes. http://www.scirp.org/journal/msa http://dx.doi.org/10.4236/msa.2014.57048 How to cite this paper: Gázquez, M.J., Bolívar, J.P., Garcia-Tenorio, R. and Vaca, F. (2014) A Review of the Production Cycle of Titanium Dioxide Pigment. Materials Sciences and Applications, 5, 441-458. http://dx.doi.org/10.4236/msa.2014.57048 A Review of the Production Cycle of Titanium Dioxide Pigment Manuel Jesús Gázquez1,2*, Juan Pedro Bolívar1, Rafael Garcia-Tenorio3, Federico Vaca1 1Departamento de Física Aplicada, Universidad de Huelva, Huelva, España 2Departamento de Química, Proyecto Prometeo, Universidad Técnicas Particular de Loja (UTPL), Loja, Ecuador 3Departamento de Física Aplicada II, Universidad de Sevilla, Sevilla, España Email: *manuel.gazque[email protected], *[email protected] Received 19 March 2014; revised 23 April 2014; accepted 2 May 2014 Copyright © 2014 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract Titanium is a very important element for several industrial applications, being one of the ninth most abundant elements in the Earth’s crust (0.63% wt). In this work it will discuss the different mining and industrial activities involved in the production of titanium dioxide. The first step analyzed will treat about the beneficiation mining process of titanium mineral, and secondly, it will discuss the two main processes of the TiO2 manufacturing (sulphate and chloride routes). In addition, we will show different uses of the titanium dioxide pigment as filler in paper, plastics and rubber industries and as flux in glass manufacture, etc. Finally, we will show that the old wastes are currently called co-products since they were valorized, being commercialized by the Spanish industry of TiO2 production in different fields such as agriculture, civil engineering, or cement manufacturing. Keywords Titanium Minerals, Titanium Dioxide Pigment, Properties and Uses of Tio2, Valorization of Co-Products 1. Titanium Minerals: Origin, Deposits and Resources Titanium was discovered in 1791 by the British clergyman and mineralogist William Gregor while analyzing black magnetic sands from Menachan in Cornwall (England). He produced a white metallic oxide from the mineral menachanite, a variety of ilmenite, and named the new element menachite. A few years after Gregor’s discovery, M.H. Klaproth, a German chemist, separated TiO2 from the mineral rutile. Klaproth named the new *Corresponding author. M. J. Gázquez et al. 442 element titanium after the giants of Greek mythology. In 1825, J.J. Berzelius, a Swedish chemist, performed a crude separation of titanium metal. However, it was not until 1910 that M.A. Hunter, an American chemist, produced pure titanium. Ilmenite, the principal titanium ore mineral, was named after the Ilmen Mountains in the south Urals. Titanium, with atomic number 22 and symbol Ti, is one of the transition elements in Group IVB of the periodic table. Its chemistry shows many similarities to those of silicon and zirconium, although its aqueous solution chemistry shows some resemblances to those of vanadium and chromium. Titanium is a silver-white metallic element with a low density, good strength, excellent corrosion resistance, very low electrical and thermal conductivity, and is paramagnetic. Titanium is as strong as steel but 45% lighter. It is 60% heavier than aluminum, but twice as strong. Titanium has a lower coefficient of expansion and lower thermal conductivity than either steel or aluminum alloys. Pure titanium is malleable and ductile but it becomes brittle when contaminated with other elements such as carbon and nitrogen. It can be polished to a high luster. The natural titanium consists of five stable isotopes, 46Ti (8%), 47Ti (7.3%), 48Ti (73.8%), 49Ti (5.5%) and 50Ti (5.4%), but several unstable isotopes (radioactive) are also known, being 51Ti, the radioactive isotope with highest half-live (5.76 min). Titanium is not found as a pure metal in nature due to its strong affinity for oxygen, carbon and nitrogen, making it difficult to obtain in the pure state, but it suffers the phenomenon of “passivation”, which is the process of making a material “passive”, usually by the deposition of a layer of oxide that adheres to the metal surface [1]. In the context of corrosion, passivation is the spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This oxide or nitride film usually has a thickness of few nanometers. 1.1. Titanium Resources Titanium is the ninth most abundant element in the earth’s crust representing approximately 0.6 percent and the seventh metal most abundant in the whole Earth [2] [3] which is found in practically all rocks and sediments, and is present in plants, animals and natural waters. Its elemental abundance is about five times smaller than iron and 100 times greater than copper. Ti metal use is about 200 times less than copper and 2000 times less than iron. The commercial production of titanium dioxide started on the 1920 decade, and the Ti metal started on the 1950 decade due to the increase of demand generated by the aircraft industry, reaching its metal production 166,000 metric tons in 2008 [4]. Titanium ores are basically found in nature as mainly ilmenite (the most important economic mineral), see Figure 1, nominally FeTiO3 but often altered, either as a rock or as sand, rutile (Figure 2(a)), anatase (Figure 2(b)) and brookite, (Figure 2(c)), which although they all have the same formula, differ in their crystalline structure and also as leucoxene (Fe2O3∙nTiO2) [5], an oxidation product of ilmenite which it is composed of finely crystalline rutile. The mineralogical properties of the main titanium minerals are summarized in Table 1. In addition, titanium is common in magnetite, with Ti-rich (2% - 20%) varieties termed titaniferous magnetite or titanomagnetite. Other less common titanium oxide-bearing minerals are pseudobrookite (Fe2TiO5), perovskite (CaTiO3), geikielite ((Mg, Fe)TiO3), and pyrophanite (MnTiO3) [6]. The only silicate mineral with titanium as a major component is titanite, formerly called sphene. Figure 1. Ilmenite ore. Picture obtained from virtual museum of the University of Huelva. M. J. Gázquez et al. 443 (a) (b) (c) Figure 2. a) Rutile form; b) Anatase form; c) Brookite form. Pictures obtained from virtual museum of the University of Huelva. Table 1. Properties of some minerals of titanium [7]. Name, (Formula) %TiO2 Colour Hardness Density Crystal Form Transparency Ilmenite (FeTiO3) 52.6 black 5 - 6 4.5 - 5.0 hexagonal opaque Perovskite (CaTiO3) 58 Black, Brown, reddish-brown or yellow 5.5 4.48 - 4.26 Monoclinic (pseudocubic) Rutile, anatase, brookite (TiO2) 95 reddish-brown, red, yellowish or black 6.0 - 6.5 (rutile) 5.5 - 6.0 (anatase and brookite) 4.23 - 5.5 (rutile) 3.82 - 3.97 (anatase) and 4.08 - 4.18 (brookite) Tetragonal (rutile, anatase) Orthorhombic (brookite) opaque or subtransparent Titanite (sphene) (CaTiSiO5) 35 - 40 Brown, green, grey, yellow or black 5.0 - 5.5 monoclinic transparent to opaque One of the most important sources of titanium is the black mineral sands deposits, which contain mainly heavy minerals with a specific gravity greater than 2.85, and they generally consist of two ore product streams; firstly, titanium dioxide in the rutile form, ilmenite and leucoxene and, secondly, zircon. Ilmenite is used in its natural form and also to manufacture titanium slag and synthetic rutile products as we will see later, Section 2.2. 1.2. Main Deposits The geology of titanium mineral deposits was reviewed by Force [8], and their world production and reserves has been summarized by Gambogui in different years (2009, 2010, and 2011) [4] [9] [10]. More than half of the world’s titanium production is from ilmenite and rutile in shoreline placer deposits in Australia, South Africa, USA, India and Sri Lanka. Most of the remainder is supplied by magmatic ilmenite deposits in Canada, Norway, M. J. Gázquez et al. 444 Finland and USA. The magmatic deposits yield ilmenite with a TiO2 content of 35% - 40%, whereas the shoreline placer deposits provide ilmenite of higher TiO2 content, including altered ilmenite (60% - 75% TiO2), leucoxene (76% - 90% TiO2) and rutile (95% TiO2) [11]. In this sense, the most significant class of magmatic ilmenite deposits is associated with anorthosite or anorthosite-gabbro complexes1, mostly of Precambrian age. These ores typically occur in ilmenite-rich lenses, dikes and sills cutting the anorthosites, and of lesser importance, disseminated in the anorthosites. The ilmenite probably crystallized from an immiscible iron-titanium-rich heavy liquid phase of a ferrodiorite magma [8], which formed syngenetic layers and segregations, as well as auto-intrusions into partly or wholly solidified anorthosite-gabbro. Minerals include ilmenite, titaniferous magnetite (with exsolved titanhematite and ulvospinel), plagioclase, pyroxene, olivine, garnet, biotite, apatite, rutile and pyrrhotite. In a second class of magmatic deposits, perovskite, rutile, brookite and/or ilmenite occur in alkalic ring complexes predominantly composed of nepheline syenites, but are particularly associated with pyroxenite phases. The titanium minerals mostly occur in magnetite-perovskite dikes intruding pyroxenite (Pyroxenite is a plutonic rock that consists of dark minerals in the pyroxene group plus a little olivine or amphibole minerals). The presence of Nb, REE and Th in the oxide minerals of this class of deposit makes refining difficult and there is currently no commercially operating process for recovery of TiO2 from perovskite. On the other hand, some alkalic anorthositic complexes have contact metasomatic deposits of rutile on their margins. Significant tonnages of rutile ore may be developed where swarms of anorthosite dikes and sills permeate country rocks, as in the Roseland District, Virginia [8]. Moreover, we have to take in to account the importance of the alluvial placer deposits. Rutile has been mined from Quaternary alluvial deposits in the Gbangbama district of Sierra Leone, where the deposits are mostly about 10 m and up to 20 m thick, and lie directly on bedrock. In addition, rutile along with zircon and ilmenite is mined from several Quaternary beach and dune heavymineral sand deposits along the east coast of Australia, from Tomago near Newcastle in the south, to North Stradbroke Island, near Brisbane [12]. From the north of Stradbroke Island to Gladstone, the proportion of ilmenite increases at the expense of rutile and zircon. On the west coast of Australia, ilmenite, leucoxene, rutile, zircon and monazite are recovered from Pliocene and Quaternary shoreline deposits, mainly near Bunbury in the south and at Eneabba 225 km north of Perth [13]. In South Africa, heavy-mineral deposits occur in Holocene high dunes at Richards Bay, about 160 km north of Durban. In the USA, Pleistocene beach deposits in Jacksonville District, Florida (Trail Ridge and Green Cove Springs), produce ilmenite, a leucoxene-rutile mixture, and rutile, with zircon, staurolite and monazite as coproducts. Nearby modern beach deposits were mined in the past. Undeveloped resources are known in Georgia, Tennessee and the Carolinas. India produces ilmenite and rutile from beach deposits near Quilon and Manavalakurichi on the south-western coast. Large deposits have also been reported in the Chatrapur area. Malaysia and Thailand produce small quantities of titanium minerals as by-products from shoreline placer tin mining. Sand ilmenite placer deposits have been formed in modern beach environments or are older raised beach deposits formed during the Pleistocene. Economic beach placers are typically 10 m thick, 1 km wide and over 5 km long. A significant part of the iron in sand ilmenite is oxidized to the trivalent state in a natural process affording what is known as weathered ilmenite. Ilmenite sand with a 50% - 60% TiO2, 15% - 25% FeO and 15% - 25% Fe2O3 is a typical feedstock for the TiO2 industries using the sulphate process, but the exhaustion of some large deposits exploited during decades have led to some new deposits being explored for its potential working, some of them in areas of Asia. 2. Titanium Ores Production, Reserves and Purification The term “titanium ore” covers a wide variety of natural or industrial titanium resources that can be used as feed material by TiO2 pigment and titanium metal producer. As we can see in Table 1, the more important commercials titanium ores are basically found in nature as mainly ilmenite (nominally FeO∙TiO2 or TiFeO3) containing 40% - 65% TiO2, with the rest being mainly ferrous and ferric iron oxides with some amounts of other oxide impurities of chromium, manganese, vanadium, magnesium, aluminum, calcium, silicon and others, depending on its geological history, either as a rock or as sand [14]. The titanium and ferrous and ferric iron content in the 1Anorthosite and anorthositic gabbro are rock types consisting of 90% - 100% and 75% - 90% plagioclase feldspar crystals, respectively. M. J. Gázquez et al. 445 ilmenite may vary significantly as it can be present in its primary stoichiometric form (FeTiO3) or as weathered ilmenites and mixtures of both types. The weathered ilmenite is the result from the oxidation and partial dissolution by ground water of the iron, which gradually transforms primary ilmenites to other phases such as pseudorutile (Fe2Ti3O9). The TiO2 content can even exceed the 60% when the ore is altered to leucoxene, which is a mixture of rutile or anatase amorphous TiO2 and iron oxides. Titanium can also be found in nature as rutile, which is a richer form of TiO2 (93% - 96% TiO2) which occurs naturally but is not so often found in deposits valid for commercial use, and is often used to feed the TiO2 and finally can be found as leucoxene (Fe2O3∙nTiO2), a natural alteration product of ilmenite, typically containing more than 65% TiO2 [6] [14]. 2.1. Titanium World Mine Production and Reserves As we can see in Table 2, in 2009 the major producing regions of ilmenite were South Africa (1.05 Mt), Australia (1.02 Mt), Canada (0.65 Mt), China (0.5 Mt) India (0.42 Mt), Vietnam (0.412 Mt), Norway (0.30 Mt), and Ukranie (0.30 Mt), while the total production was 5.3 Mt. On the other hand, the major rutile producers were Australia (0.266 Mt), South Africa (0.127 Mt) and Sierra Leona (0.06 Mt), with a total world production of 0.55 Mt [10]. Nowadays the reserves of ilmenite and rutile are estimated to be about 650 and 42 million metric tons, respectively. For that reason, there is no doubt that ilmenite is the most important titanium resource, and it is sure that this ore will be the main source of titanium in near future. Ilmenite supplies about 91% of the world’s demand for titanium minerals, being its world production of about 5.3 million metric tons in 2009, see Table 2. In the next decade, global demand for TiO2 is expected to continue increasing at an average rate of about 3% annually [4] [9]. Growth higher than the average is expected to occur in the Asia region. China in particular is expected to lead world growth in production and consumption. However, the proliferation of small TiO2 pigment plants using the sulphate-route in China has given a very high TiO2 production capacity in China, reaching 1 Mt/yr by the end of the decade [15]. Growth in aerospace, defense, and industrial uses will strongly influence the demand for titanium metal for the foreseeable future. Based on the announced capacity expansion plans, by 2015, world-wide capacities would be expected to reach 350,000 t/yr [15]. Table 2. Mine production and estimated titanium ore production in thousands of tons [10]. Ilmenite Rutile Mine production Reserves Mine production Reserves 2009 2010 (**) 2009 2010(**) United States 200 200 2000 (*) (*) Australia 1020 1070 100,000 266 280 18,000 Brazil 43 43 43,000 3 3 1200 Canada 650 700 31,000 - - China 500 600 200,000 - - India 420 420 85,000 20 20 7400 Madagascar 47 150 40,000 2 6 NA Mozambique 283 350 16,000 2 2 480 Norway 302 320 37,000 - - South Africa 1050 1120 63,000 127 130 8300 Sri Lanka 30 40 - 11 12 NA Ukranie 300 300 5900 57 57 2500 Vietnam 412 410 1600 - - Sierra Leona - - 61 67 3800 Other countries 34 35 26,000 - - 400 World total 5300 5800 650,000 550 580 42,000 (*) United State rutile production and reserves data are included with ilmenite. (**) estimated. (-) Zero. NA Not Available. M. J. Gázquez et al. 446 2.2. Titanium Ore Purification Alternative titanium feedstock commercially available are obtained from ilmenite by its purification, as it is described below, to obtain “titania slag” and “synthetic rutile”, produced at industrial scale. The limited availability of high quality titanium ores, with high TiO2 and limited iron and heavy metals content (chromium, vanadium, etc.) is threatening the TiO2 business in terms of guaranteed and quality supply and prices, particularly since 2010. 2.2.1. Upgrade of Ilmenite to Slag and UGS To produce the two main titanium products, titanium dioxide (by sulphate and chloride process) [16] and metallic titanium (Kroll process) [17], and taking into account the exhaustion of some deposits, ilmenite ores are often upgraded for commercial purposes into titania “slag” containing typically 70 wt% - 90 wt% TiO2 by electrosmelting processes conducted at very high temperatures (molten state) in electric arc furnaces, where other “main” product is the high purity pig iron (HPPI) of important and specific industrial applications [18]. This process is applied to the rock ilmenite from Eastern Canada and Norway, which is upgraded by a smelting process by important mineral companies, with or without a preliminary pre-reduction stage. The slag obtained from rock ilmenite is commercially supplied to TiO2 manufacturers as a feedstock for the sulphate process, but cannot be used for the chloride TiO2 manufacture due to its relatively high calcium and magnesium content [19]. A further purification of part of the slag produced from rock ilmenite is carried out by thermal treatment [20] [21] and pressure leaching in HCl affording the “upgraded slag” (UGS), with 95% of TiO2 and a lower calcium and magnesium content. UGS is a suitable feedstock for the TiO2 chloride process and titanium metal production. Although the sand ilmenite is often sold to the TiO2 industry as a feedstock for the sulphate process (rarely used for the chloride process which requires a higher TiO2 feedstock such as slag or natural or synthetic rutile) it is also smelted to produce slag by using classical electric furnaces, or a modern DC plasma arc furnace technology. This is the case of the sand ilmenite extracted from several deposits in South Africa and processed to afford a slag with 85% - 87% TiO2 which is suitable and sold for use in the TiO2 chloride process, called for that reason as “chloride slag”, [22]. 2.2.2. Beneficiation of Ilmenite to Synthetic Rutile In the same way, ilmenite ores are also upgraded into “synthetic rutile” products containing 90 wt% - 96 wt% TiO2 by processes consisting in reducing the iron oxide and the “leaching” with mineral acids. Although there are a number of commercialized or proposed processes to produce “synthetic rutile”, the best known are the Becher and the Benilite process [22]. The Becher process starts with the reduction of the iron in ilmenite almost completely to its metallic form in a rotary kiln with coal at ca. 1200˚C. The reduced ilmenite is processed through a screen and magnetic separation, to remove the char, which is recycled to the kiln. It is then subjected to leaching in a NH4Cl solution under aeration to oxidize and precipitate the iron as oxide/hydroxide in fine particles that are separated from the coarser synthetic rutile with hydrociclones to afford the TiO2 beneficiate. In the Benilite process, the iron content of ilmenite is reduced to the ferrous state with heavy oil in a rotary kiln at 850˚C - 1100˚C. The reduced ore is leached in digesters with 18% - 20% HCl at 145˚C. The leached material is then washed and calcined, affording the beneficiate. The leaching acid is regenerated and the iron oxide separated as a by-product. Other processes have either been used or are under development, such as the Rupaque process run at Japan during a number of years, the Austpac process by roasting and leaching with HCl, and the Altair process based on leaching with concentrated chloride solutions. All those process have some similarities with the previously described ones. 3. Manufacture of Titanium Dioxide Pigments In opposition of the popular belief, the most widely used titanium product is not the titanium metal and alloys, but rather is the titanium dioxide (TiO2) pigment that provide whiteness and opacity to a vast range of everyday products from coatings and plastics, to inks and even as flux in glass manufacture, filler in paper, rubber industries [15], cosmetics and food. Annually more than 4.5 million t of TiO2 are produced worldwide [23] and only about 4% - 5% is used to produce metallic titanium [24]. M. J. Gázquez et al. 447 Titanium dioxide was first discovered in the early 1900s and the manufacture of titanium white for use as a pigment (anatase form) was first reported in 1923 in France, where soon replaced the lithopone2 and toxic lead-based pigments in the early 1930s. Titanium dioxide pigments are produced from a variety of ores by two different processes: the sulfate process (about 40% of total TiO2 production) using concentrate sulphuric acid and the chloride process (about 60%) using chlorine gas. Both processes differ in their chemistry and raw material requirements. Because the chloride process has some advantages over the traditional sulfate process in cost and waste management, it has dominated the pigment industry in recent times. 3.1. The Chloride Process The chloride process, commercialized by Du Pont in the early 1950s, offers waste disposal, energy and quality advantages over the sulfate process [25]. This process can use a wide range of feedstock, i.e. rutile, synthetic rutile, high-grade ilmenite or slag, depending of the industry but mainly rutile and synthetic rutile (90% - 95% TiO2) are used, avoiding the iron sulfate waste problem. The chloride process begins with the mixing of raw materials with gaseous chlorine at a temperature of around 900˚C - 1000˚C in a fluidized bed reactor in the presence of coke as a reducing agent (1) [23]. The resulting gas stream contains titanium tetrachloride (TiCl4), oxides of carbon and all the impurity metals from the feedstock in the form of metal chlorides, but impurities such as silica and zirconium may not chlorinated and remain accumulated in the reactor [23] [25]. Significant quantities of gas chlorine are required for low TiO2 content feedstock. The main chemical reactions of this process are the followings: 2TiO2 + 3C + 4Cl2 → 2TiCl4 + 2CO + CO2 (1) TiCl4 (impure gas) → TiCl4 (pure liquit) (2) TiCl4 + O2 → TiO2 + 2Cl2 (3) The gas stream is contacted with recycled liquid TiCl4 which cools it to a level in at which the other metal chlorides separate out as solids by condensation and chemical treatment. The impurities can be reduced to typically about 10 - 20 mg∙kg−1. The purified TiCl4 goes forward with further cooling to be condensed as a liquid (2) and then fed to a high temperature oxidation reactor where it is reacted with oxygen, above 1500˚C, either in a plasma arc furnace or in a toluene-fired furnace to form titanium dioxide and release the chlorine which is recycled back to the beginning of the reaction (3). Residual chlorine associated with the solid TiO2 is removed by aqueous hydrolysis. Finally the pure titanium dioxide is subjected to a range of chemical surface treatments, milling and drying. The wastes generated by the chloride process are mainly coke and ore solids that remain un-reacted during the chlorination process. In addition a waste acid solution, usually called iron chloride waste acid, is also generated when the combined stream of un-reacted coke and ore solids, metal chloride solids, is acidified using water or waste hydrochloric acid (HCl) from the reaction scrubber. The metal chloride impurities are generally environmentally harmful, especially the iron chloride, which are removed and neutralized with lime or limestone, and finally sent for disposal via landfill. Curiously, in one chlorine plant in the United States, the chlorine salts are not neutralized, but they are injected into deep exhausted oils well [22]. In the described process the consumption of chlorine is therefore related to the amount of iron oxide presents in the raw material. About one ton of chlorine is required to produce 5 to 6 tons of titanium dioxide pigment (depending on the iron content in the feedstock used consuming the chlorine as ferric chloride and with about one-third of chlorine ending up as hydrogen chloride). Currently, the chloride process offers tighter product control, is less labor intensive, and is environmentally safer. Currently about 60 percent of the 4.5 million tons of pigment production world-wide is generated by the chlorine process. Although declining in response to concerns about environmentally unacceptable waste, many sulfate plants have introduced innovative techniques deferring their closure. 3.2. Sulphate Process The sulphate process was the first commercialized technology to obtain the titanium dioxide pigment. In this 2Lithopone is a white pigment composed of a mixture of barium sulfate (28% - 30%) and zinc sulfide (68% - 70%) with trace amounts of zinc oxide. M. J. Gázquez et al. 448 process, ilmenite (40% - 60% TiO2) or titanium slag (72% - 85% TiO2) or even a carefully controlled blend, is digested with concentrated sulphuric acid (98%). A highly-exothermic reaction is initiated by the addition of measured quantities of steam, water and diluted sulphuric acid. The general equation for the digestion reaction (dissolution of the raw material) is the following: FeTiO3 + 2H2SO4 → TiOSO4 + FeSO4 + H2O (Dissolution of the raw material) (4) TiOSO4 + H2O → TiO2n.H2O + H2SO4 (TiO2 precipitation) (5) TiO2n∙H2O → TiO 2+ nH2O (TiO2 calcination and conditioning) (6) The resulting liquor contains titanyl sulphate (TiOSO4) and iron sulphate (FeSO4) dissolved in sulphuric acid. If the feedstock used is ilmenite based, a reduction step is required in which iron is added to convert any ferric ions (Fe3+) to the ferrous (Fe2+) form to aid separation later in the process. Therefore to ensure that all the iron is in dissolution, the liquor is passed through scrap metal (iron reduction step). Then, it passes to a clarification tank where the undissolved ore and solids are allowed to settle. The titanium liquor is then concentrated and hydrolyzed to titanium dioxide hydrated (5). The titanium dioxide hydrated precipitates from the ferrous sulphate and sulphuric acid is separated by filtration. The iron sulphate is separated from the titanium dioxide production process liquors by concentration and cooling, as we will see with more detail in Section 5.1. After filtration, the hydrated titanium dioxide slurry is sent to a calciner, where the titanium dioxide crystals grow to their final crystalline size and residual water and H2SO4 are removed (6). The dried titanium dioxide is sent to a finishing phase, which involves any required milling and or chemical treatment, such as surface coating with silica or alumina. Further processing (finishing), is then analogous to the chloride process involving chemical surface treatments (coating), milling and drying operations. About one ton of raw material (ilmenite or ilmenite + slag) is required to produce 0.5 tons of titanium dioxide pigment. Finally, we can say that he sulphate process uses a simpler technology and lower grade and cheaper raw materials to produce a form of pigment called anatase (tetragonal, near octahedral), which is preferred over the pigment from the chloride process for use in papers, ceramics and inks. However, the traditional sulphate process produce lower quality products for most applications and large quantities of waste iron sulphate, due to the high concentration of iron presents in the ilmenite used. The exhaustion of some large deposits exploited during decades have led to some new deposits being explored for its potential working but the trend is to feed the TiO2 sulphate sites containing ilmenite with lower TiO2 than in the past. 4. Properties and Uses of Titanium Dioxide Pigments As mentioned above, about 95% of titanium ore is processed into titanium dioxide which is the most widely used titanium product. TiO2 is a polymorphous and simple inorganic compound, existing in three fundamental crystal forms. All three forms, anatase, rutile and brookite, occur naturally but the latter is rare, and although it has been prepared in the laboratory it is of no commercial interest. Therefore, the main crystal forms commonly available are two, anatase and rutile. Rutile is the most stable form and is thus the most abundant. It has a more compact structure than anatase and this gives rise to important differences in properties between both modifications. Rutile TiO2 has a higher refractive index, higher specific gravity and greater chemical stability than anatase, smelting at 1825˚C. Anatase has no specific melting point as it is irreversibly transformed to rutile before a melting point is reached. 4.1. Properties TiO2 is a simple inorganic compound produced as a pure white powder (known as titanium white). This pigment can be used, in most of its applications, as a pigment to scatter light, because absorbs almost no incident light in the visible region of the spectrum. This pigment scatters light by three mechanisms: reflection from the surface of a crystal, refraction within a crystal, and diffraction, whereby light is bent as it passes near a crystal. Reflection and refraction are maximized by increasing the difference between the refractive index of the pigment and that of the polymer matrix or other material in which it is dispersed. In addition, TiO2 is a good pigment because it has a very high refractive index of 2.70 in comparison with values of only 2.02 and 1.57 for zinc oxide and china clay respectively, as we can see in Table 3. This means that relatively low levels of the pigment are re- M. J. Gázquez et al. 449 quired to achieve a white opaque coating. This high refractive index gives the potential for producing much greater opacity or hiding power, making TiO2 a much better pigment than the other chemicals mentioned. Light scattering by diffraction is most effective when the pigment diameter is slightly less than half the wavelength of the light to be scattered. Hence, the whiteness of titanium dioxide pigments is a function of particle size. Pigments containing smaller-sized particles lead to finished products (e.g., paints, plastics, etc.) that tend to have a bluish tint; pigments with larger-sized particles cause finished products to have a more yellowish tint [26]. On the other hand, we can see in Table 3 that most rutile pigments have a specific gravity between 3.9 and 4.1, while anatase grades have 3.7 or 3.8. Zinc sulphide and Lithopone 30% have slightly higher values of specific gravity, but other white pigments are much denser. An important measure of a pigment’s potential hiding power can be determined by a simple test whereby it is tinted with a standard black pigment, and assessed using an arbitrary scale. The tinting strength values for rutile titanium pigments range between 1550 and 1850 and for anatase between 1150 and 1350. The best of the other white pigments listed in Table 3, zinc sulphide, is only half as powerful as rutile. Other important features of titanium dioxide pigments are excellent resistance to chemical attack, good thermal stability and resistance to ultraviolet (UV) degradation. Rutile pigment is more resistant to UV light than anatase, and is preferred for paints, plastics, especially those exposed to outdoor conditions, and inks. On the other hand, anatase pigment has a bluer tone than the rutile type, is less abrasive and is used mainly in indoor paints and in paper, ceramics, rubber and fibers manufacture. One possible explanation for the high abrasion properties of rutile TiO2 pigments is the multiple facets of the pigment particles [26]. Also, anatase reflects more ultraviolet light than rutile, making more ultraviolet light available for optical brighteners. Both rutile and anatase pigments can be made more resistant to photodegradation by coating the pigment particles, which also improves their dispersibility, dispersion stability, opacity and brightness. Anatase is 10 times more active than rutile and responds to slightly different wavelengths [27]. In paper, the advantage of rutile over anatase is less decisive because the light-scattering advantage is partially off-set by a density disadvantage. Therefore we can say that titanium dioxide is the best white pigment available, but this does not just restrict its use to anything that is white; the opacity is also used in combination with colored pigments to give them the required hiding power they need. Taking into account the properties above mentioned, the major consumer industries for titanium dioxide pigments are mature sectors in high-resource countries where they are used for surface coatings, paper and paperboard and plastics. In addition, consumption of titanium dioxide tends to be parallel to the general economic trends. World consumption of titanium dioxide by end-use in 2001 was: coatings, 55%; plastics and rubber, 24%; paper, 12%; printing inks, 3%; and other, 6%; while in 2005 was: coatings, 58%; plastics and rubber, 23%; paper, 11%; and other, 8% [23] [28]. Some other uses of titanium dioxide are in catalysts, ceramics, coated fabrics and textiles, floor coverings and roofing granules [29]. According to the American Society for Testing and Materials (ASTM, 1988 [30]) D476-84 standard, four types of titanium dioxide pigment exist [31] [32]. Table 3. Refractive index, Refractive tinting strength and specific gravity of some white pigments. Pigments Refractive Index Tinting Strength Specific Gravity White lead 2.00 100 6.7 Zinc oxide 2.02 200 5.6 Zinc sulfide 2.37 900 4.0 Anatase TiO2 2.55 1350 3.7 - 3.85 Rutile TiO2 2.70 1850 3.7 - 4.2 China Clay 1.57 <100 2.6 Lithopone 30% 1.84 300 4.3 Antimony Oxide 2.30 400 5.7 M. J. Gázquez et al. 456 public of Ecuador-(SENESCYT for its acronym in Spanish). The authors would like to acknowledge the financial support received from the company Tioxide-Huelva by the research projects “Valorization of red gypsum from the industrial production of titanium dioxide” (PROFIT, CIT-310200-2007-47) and “Applications of red gypsum and Tionite waste in commercial applications”. The authors also thank to the technical staff for the advisory provided in the explanation of the results. References [1] Budinski, K.G. (1988) Surface Engineering for Wear Resistance. Prentice Hall, Englewood Cliffs, 420. [2] Knittel, D. (1983) Titanium and Titanium Alloys. 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