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Ph Applications in the Inorganic Pigments Industry

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Ph Applications in the Inorganic Pigments Industry Addressing Impossible to Measure pH Applications in the Inorganic Pigments Industry A white paper by Endress+Hauser, Inc. Eric Pfannenstiel, author Eric Pfannenstiel is a Business Development Manager with Endress+Hauser, Inc. in Greenwood, Indiana. Previously he served as a Principle Account Manager with Foxboro. He has more than twelve years in laboratory and process analytical instrumentation. Eric obtained his Bachelor of Science in Chemistry from Adams State College in Alamosa, Colorado. © 2000 Endress+Hauser, Inc. Introduction The pigments industry is characterized by the manufacture of a number of inorganic and organic substances that are produced and marketed as fine powders. These products are used as decorative or protective coatings and colorants for plastics, fibers, paper, rubber, glass, cement, glazes, porcelain enamels, printing inks, and even cosmetics. Inorganic pigments are simple materials which include basic elements oxides, mixed oxides, sulfides, chromates, silicates, phosphates, and carbonates. They are generally classified by color or physical properties. The table below shows common pH pigments. BlackExtenders&Miscellaneous Wshite Pigments ColoredPigment PigmentsOpacifiersPigments TitaniumdioxideIronOxidesCarbonLuminescentPigments ZincoxideComplexInorganicBlacksMetalEffectPigments ZincSulfatePigments NacreousPigments LithoponesMixedmetalOxides TransparentPigments LeadWhites(Spindels,Rutiles, OtherWhitesZirconPigments) UltramarinePigments Cyanide IronBlues CadmiumPigments LeadChromate Pigments iThe commercial manufacturing of these materials is very complex and demands rigorous attention to every aspect of the manufacturing process including tightly controlled pH measurement. Proper pH control throughout the process is one of several variables that contribute to final product quality as determined by both physical and chemical properties. Important physical characteristics include particle size and distribution, particle shape, and agglomeration. Chemical properties include chemical composition, crystalline structure, product purity, and material stability. Control of each property is essential to insure uniform color dispersion and opaqueness throughout the materials to which they are added. The particle size and the difference, between the refractive index of the pigment, and that of the dispersed media determine pigment opacity. A particle having a size of .16 .28 mm provides maximum visible light dispersion. For inorganic pigments to be useful in most applications, average particle size must be between .1 and 10 mm. Any agglomeration of pigment particles can affect its opacity. Thus insuring optimal particle size, distribution, and preventing agglomeration is essential to achieve maximum pigment opacity. One means of accomplishing this is by carefully varying the process pH and temperature to achieve desired particle size. Other key physical properties include lightfastness, ability to resist weathering, heat stability, and chemical resistance. 1 Titanium dioxide and iron oxides are the two most prevalent inorganic pigments manufactured globally. Annual world demand is approximately 3,555 x 106 metric tons and 900,000 metric tons respectively. Carbon blacks are also commonly used. Increasing environmental concerns have drastically reduced the production of chromium, cadmium , and lead based pigments. For the purpose of brevity, this paper describes the manufacturing processes and application of pH measurement in the production of titanium dioxide and iron oxide pigments. Titanium Dioxide Production The base raw material for the production of titanium dioxide is ilmenite, synthetic rutile or titanium slag. Two forms of titanium dioxide are produced: anatase and rutile. Anatase is generally less coarse and is preferred where abrasion is a concern, such as with wear in thread guides or spinning equipment. Rutile has a higher refractive index and corresponding opacity and is more commonly used. Two commercial manufacturing processes, sulfate and chloride, are used to produce these products. The sulfate method uses concentrated sulfuric acid to decompose ilmenite in a digest reactor over approximately a 12-hour period. The exothermic reaction: FeTiO + 2 H SO ®TiSO + FeSO + 2 H O 3 2 4 4 4 2 yields approximately 95-97% solubilized TiO . Scrap iron is then 2 added to reduce residual Fe3+ in solution to Fe2+ to allow iron removal through precipitation. The resulting cake is extracted with water at a temperature of 65°C, the temperature of maximum iron sulfate solubility, and the titanium extract filtered off. By cooling the filtrate solution to 15°C iron (II) sulfate is precipitated in a vacuum crystallizer. Centrifugation or filtration is used to separate the iron sulfate from the TiO filtrate. Temperatures throughout these steps 2 should not exceed 70°C in order to prevent premature hydrolysis of titanium dioxide. Hydrolysis of the titanium dioxide mother liquor is initiated by adding crystallizing seeds to the filtrate at temperatures close to its boiling point (109°C). The resulting reaction: TiOSO + (n+1)H 0 ®TiO nH O + H SO 4 2 2 2 2 4 must be carefully controlled to insure optimal final product physical characteristics. To produce anatase titanium dioxide pigment, anatase microcrystalline seeds are added to the mother liquor in a concentration of .5 1% and the mixture hydrolyzed for 3-6 hours. To produce rutile titanium dioxide pigment, a hydrosol made from a monohydric acid such as hydrochloric acid is added to neutralize the mother liquor. This reaction only takes about one hour. In both instances, free sulfuric acid still entrained in the mother liquor, must be separated from the resulting hydrolysate to prevent possible dissolution. Repeated water washing and filtration of the gel carried out in a vacuum filter removes most of the sulfuric acid. Any residual is removed during the final calcination process. At this time small quantities of various chemicals are doped into the solution to improve the final pigmentary properties. 2 Calcination of the hydrated TiO Gel cake is performed in rotary 2 kilns with an excess of air to prevent possible reduction of the titanium dioxide. Water is removed at temperatures between 200-300°C, sulfur trioxide between 480-800°C, and crystals of TiO 2 grown at higher temperatures. Final temperature for an anatase pigment should reach 800-850°C. Rutile white pigment is produced at temperatures of 900-930°C. This temperature is critical in order to produce pigment particle sizes of 200 400µm. Higher temperatures produce larger particle sizes that do not exhibit good pigmentary properties. The chloride process for manufacturing titanium dioxide accounts for 56% of the world capacity. Finely ground rutile reacts with chlorine and calcined coke in a fluidized bed reactor at temperatures between 800-1200°C: TiO + 2Cl + C®TiCl +CO . Oxygen 2 2 4 2 is added to the reaction in order to maintain the reactor temperature. All base material added to the reactor is dry to prevent the formation of hydrochloric acid. Metallic impurities (magnesium, calcium and zircon) present in the raw titanium feedstock react with chlorine to form metallic chlorides and accumulate in the bottom of the reactor. Volatile chlorides including TiCl are vented as gases through the 4 top of the furnace where they are cooled to less than 300°C. Most impurities, with the exception of vanadium and silicon chlorides, are separated from the titanium tetrachloride in this step. Vanadium and silicon chlorides are then removed by reduction to lower chloride oxidation states and fractional distillation. High purity TiCl is 4 preheated, mixed with hot oxygen, and combusted at 900-1400°C to form titanium dioxide: TiCl + O ®TiO + Cl . Aluminum chloride 4 2 2 2 is added to insure that the final product is rutile. Throughout the process, factors such as reaction temperature, oxygen levels, water, and mixing conditions influence the final quality of the titanium dioxide product. Titanium dioxide derived from the chlorine process is generally preferred because it is lighter in color, has lower capital investment costs, and has less environmental concerns. The disadvantage is the higher quality feedstocks required and the increased abrasiveness of the material. The final step in both processes is pigment finishing. Both rutile and anatase pigments are coated with inorganic oxides to optimize dispersability, dispersion stability, opacity, gloss, and durability. The actual finishing process and coating used is specific to the final product application and the market sector in which it is used. Rutile pigments usually receive a 1-15% inorganic coating; anatase a 1-5% coating. The initial finishing stage disperses the base pigment in water with phosphate, silicate, or organic dispersants. The suspension is milled and classified to remove oversize particles. Selective precipitation with small quantities of colorless hydrous oxides such as P O , SiO , Al O , TiO , and ZrO coats the dispersed 2 5 2 2 3 2 2 particles through specific changes in pH and temperature. Once coated, the pigment is filtered, washed, and dried. During subsequent milling, an organic surface treatment such as polyol or alknolamine is applied for use in paints. Siloxane is added for plastics. The end product is then filtered, dried, and packaged. 3 Iron Oxide Production Iron Oxide represents approximately 40% of the total production of colored, inorganic pigments. Yellow geoethite [a-FeO(OH)], orange lepidocrocite [g-FeO(OH)], red hematite [a-Fe
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