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.

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 O ], and brown 2 3 maghemite [g-Fe O ] comprise the primary pigment colors. Iron 2 3 pigments are characterized by low chroma and excellent lightfastness. They are nontoxic, non-bleeding, and inexpensive. They do not react with weak acids and alkalis, they are not contaminated with manganese, and do not react with organic solvents. Although naturally occurring deposits are common, most are not rich enough to allow use in pigmentary applications. Synthetic iron oxide pigments are preferred for coloring plastics, paper, rubber, and magnetic recording tapes. Their advantages include chemical purity, uniform particle size and distribution, and decreased environmental concerns as compared to their heavy metal counterparts. Iron oxide reds are available in colors ranging from orange through pure red and violet. Varying shades are controlled by particle size, shape, and surface properties. Four primary methods are used in the production of iron oxide reds (red hematite [a-Fe O ]): 2 3 1. Two-stage calcination of FeSO 7H 0 4 2 2. Precipitation from an aqueous solution 3. Thermal dehydration of yellow geoethite 4. Oxidation of black oxide (Fe O ). 3 4

The final product of each process is Fe O , however specific physical 2 3 properties are determined by the manner of preparation. Thermal dehydration of yellow goethite yields a product with the lowest density; two stage calcination the highest. This paper describes the two-stage calcination process, which is most commonly used. In this process, iron (II) sulfate heptahydrate is initially dehydrated to a monohydrate: FeSO 7H O®FeSO H O + 6H O. 4 2 4 2 2 Subsequently the monohydrate is thermally decomposed at temperatures exceeding 650°C to yield Fe O . Final color depends 2 3 on the particle size formed and is controlled via regulation of the time and temperature of the calcination process. Iron oxide yellow, [a-FeO(OH)], is manufactured in colors ranging from light yellows to dark bluffs. Final color is determined by particle size that is generally between .1-.8mm. Due to their resistance to alkalis, oxide yellow is often used to color cement. Three commercial processes are used; the Penniman-Zoph process, the precipitation process, and the Laux process. The Penniman-Zoph process requires the preparation of ferrous sulfate nucleating seeds or particles and subsequent oxidation to ferric oxide. Initially ferrous sulfate is reacted at low temperatures to yield the hydroxide: FeSO + 2NaOH®Fe(OH) + Na SO 4 2 2 4 The hydroxide is then oxidized to yield the hydrated ferric oxide: 2Fe(OH) + .5 O ®Fe O H O + H O. Ferric Oxide seeds are 2 2 2 3 2 2 transferred to reaction tanks containing ferrous sulfate and scrap

4 iron in solution. The mixture is heated to between 70-90°C, air added, and the solution circulated to initiate seed growth. The following reactions occur during this process: 4FeSO + 6H O + O2®4FeO(OH) + 4H SO 4 2 2 4 4H SO + 8 FeSO + 2O2®4Fe (SO ) + 4H O 2 4 4 2 4 3 2 4Fe (SO ) + 4Fe®12FeSO 2 4 3 4 The net reaction being 4Fe + 3O + 2H O®4FeO(OH). Process 2 2 variables which directly effect product quality and determine the shade of yellow include the temperature of the reaction, the pH of the solution, the circulation rate of oxygen, the circulation rate of the ferrous sulfate solution, and the size and shape of the seed particles. The longer the reaction is allowed to proceed, the larger the particle sizes developed and the deeper the color. The reaction is terminated once the desirable hue is achieved. The precipitate is washed of any residual soluble salts, dried, ground, and packaged. The precipitation process involves the hydrolysis of ferric solutions with alkalines such as sodium or calcium hydroxide. Ferrous salts, which are commonly waste by-products from other metallurgical reactions, are often used. Ferrous chloride or sulfate is oxidized and the precipitate washed, separated by sedimentation, and dried. Yellow oxide produced by the Laux process is actually a by-product of the manufacture of aniline. By reacting nitrobenzene with iron and water in the presence of aluminum or ferrous chlorides, high quality iron pigments are produced. Aniline is filtered off and the pigments separated from unreacted iron. The Importance of pH Measurement In titanium dioxide and iron oxide production, pH is a crucial measurement. Control of process pH is among many process variables which ultimately determines end-product quality. Particle size, distribution, opacity, color, product purity, and production yields can all be affected by pH. Maintaining proper pH is essential to optimize oxidation-reduction states, achieve desired particle size and color, and to selectively precipitate desired end products. Production yields, efficiencies, and product purity are also directly affected. Insuring accurate pH measurement is essential to plant operation. pH measurement is used throughout the sulfate process for titanium dioxide production. However it is considered a crucial control point in the product finishing area for both the sulfate and chlorine processes. Inorganic coatings are selectively precipitated onto the TiO particle to increase product durability and to achieve desired 2 pigmentary properties. Precipitation is controlled by carefully varying the pH and temperature during the finishing process. Ideally pH must be controlled within several tenths in order to precipitate the hydrous oxide onto the TiO substrate with the desired thickness 2 and uniformity without precipitating titanium dioxide. The finished product is subsequently filtered, washed, and dried. pH measurement is also essential in the manufacturing of both red and yellow oxides. Red oxide is primarily derived from the calcination

5 of iron (II) sulfate hydrate. A key control point is measurement of the pH during the dissolution of iron in sulfuric acid. Accurate measurement in yellow oxide production is also extremely important. Similar to TiO finishing, pH and temperature are varied to achieve 2 the desired end product pigmentary properties. Color and particle size is directly attributable to the process pH, temperature, recirculation flow, and reaction time. Application Challenges

Accurate, reliable measurement of pH in pigment manufacturing applications brings unique challenges and frustrations. The process conditions themselves are very harsh and not conducive to in-line pH measurement. Processes are generally quite acidic, often running at values below 0 pH. Due to their batch nature, temperatures are often cycled or run at elevated levels. Entrained solids, fine particulate, and high salt content characterize the suspensions. Each of these factors leads to high maintenance labor and material replacement costs. Common problems associated with these measurements include reference junction poisoning, abrasion of the pH measuring glass, sensor drift, probe breakage, probe fouling, constant cleaning and calibration, and ultimately frequent probe replacement. In such applications it is not unusual to replace sensors within days of initial installation. It is not uncommon in pigment manufacturing facilities to clean and/ or re-calibrate pH electrodes multiple times throughout the day. This is further complicated by the means in which the sensors are installed. Often pH electrodes are installed in situ via extension pipe into a tank. This makes it awkward to retrieve the sensor for routine maintenance. Because sensor replacement is so common, many companies have adopted plug-in sensor connections to facilitate quick sensor replacement. Unfortunately temperature compensation is generally not built into this connector, and the resulting measurement is not temperature compensated. Attempts have been made to install the pH measurement sensor in slipstreams or bypass lines to facilitate isolation of the sensor from the process during replacement. However, these lines often become plugged resulting in static flow. Yet another problem is the batch nature of the process. Often the temperature and pH of the process is cycled to achieve optimal product quality and physical properties. Temperatures can exceed the upper limitations of the sensor. The pH can reach values which are so acidic that destruction of the pH sensor is inevitable. Once the batch is completed and the reaction tank emptied, the sensor is often left dry. Thus, in each of these scenarios, the sensors must be manually removed from the process and reinserted as process conditions allow. This further adds to the associated costs of the measurement point.

6 Following is an approximation of expected annual costs associated with a single measurement point:

Typical Sensor Cost Replacement Sensor Frequency Yearly Sensor Cost

$ 225.00 each 2 weeks - 26 sensors/year $5,850.00

Cleaning Frequency Calibration Frequency Associated Labor Costs

Once per shift (assumes burden labor rate of 3 daily Daily, 15 minutes each - $50.00/hour, actual rates vary) 15 minutes each - 91.25 hours/year $18,250.00 273.75 hours/year

AAtA0nticipatedTotalYearlyCost$$024,100.00

Application Solution The ideal solution to the challenges presented with these applications is to provide a sensor that has infinite life and requires no maintenance. Unfortunately this can not be realized with existing technology. However, a solution can be provided which drastically increases the useful sensor life, while reducing the overall maintenance requirements. Automated pH holders are not new to the market. The concept is quite simple. Provide a means of automatically controlling the introduction and removal of a pH sensor into and out of the process. Add to this the capability to automatically clean and assess the sensor health both in situ and while removed. While simple in concept, in actuality the same rigorous process conditions that affect existing sensor measurement performance impact the holder design criteria. In addition, a suitable pH electrode must be supplied which provides accurate measurement under existing chemical, thermal, and mechanical conditions. Several basic system design conditions must be met. The holder must permit the electrode to be withdrawn and reinserted into the process automatically without interruption of the process. For safety reasons, only holders equipped with limit switches for end position monitoring should be used. If power supply, air supply, or mechanical fault were to occur within the holder, it could remain undetected and consequentially result in costly damage. For this reason, the right electrode holder materials and seals for each application must be carefully selected. Most problems encountered with automatic pH measuring points are related to electrode holder wear. Additionally, the holder must not execute any undesirable movements in the event of a power or compressed air supply failure. Following a power failure, the system must automatically initiate a start-up in a predefined manner. When a pH electrode does fail, it must be easily removable from the holder without allowing leakage of process solution to the outside.

7 The primary task of the pH measuring analyzer is to analyze and display the pH value based on the signal from the pH electrode. However, a number of additional functions are being increasingly integrated into the measuring analyzer. In addition to monitoring the electrode glass membrane breakage and fouling, they must include alarm and control functions. Given this growing complexity, measuring instruments with a plain-text user interface should be chosen to simplify operation and minimize the risk of operator error. In order to increase accuracy, the analyzer should have both Nernst temperature compensation and solution temperature compensation. The latter should be used whenever calibration is performed, since buffer solutions are temperature dependent. In most cases, automated pH systems require an electro-pneumatic control cabinet that executes the control commands from the analyzer. To assure reliable operation of the entire system, all components of the system must be integrated. For the pigment applications noted, a pneumatic holder Retractable pH Holder constructed of Kynar (PVDF), PEEK, Titanium, or Hastelloy is used. A microprocessor based controller/transmitter and pneumatic interface provide integrated system control and measurement output to the distributed control system. An integral ball valve built into the holder provides isolation of the sensor from the process during cleaning, calibration, and routine sensor replacement. Cleaning cycles are programmed into the controller for local control and binary coded inputs are used to additionally allow the distributed control system to remotely retract, insert, or initiate a sensor cleaning as process conditions require. Pressure sensors incorporated into the design of the holder provide feedback regarding holder position (inserted/retracted). Interlocks prevent reintroduction of the holder insertion shaft with the sensor removed. A retraction security lock insures that process pressure does not force the sensor from the process, should loss of system air occur. An external injector integrated into the system allows introduction of rinse water and cleaning agents into the wash/rinse chamber of the holder. A typical cleaning cycle is described in the diagram. Additional cleaning programs available include interval cleaning, interval measurement, and a weekly program. The rinse,

8 cleaning, and dwell period set points are adjusted as needed depending on the batch operation requirements. Utilities required include 120 VAC, rinse water (29-145 psi), and dry,oil-free compressed air (60-90 psi). The pH sensor used is a gel filled, 12 mm, pH sensor with 100 ohm RTD and single Teflon reference junction rated to a working pressure of 90 psig and maximum temperature of 130°C. An optional liquid filled sensor with flowing zirconium reference junction and external electrolyte reservoir has also been used due to the high salt concentration and fine particulate materials found in the process. Sensor diagnostics and holder problems are detected and indicated on the analyzer display. When servicing is required, individual components can be easily replaced because of the systems modular design. A representative system is shown below. The holder itself is installed on a reaction or finishing tank Typical Automatic Cleaning pH Measurement System recirculation line. Operating temperatures and pH values vary throughout the production cycle depending upon the desired end product. In yellow iron oxide production, temperatures range from 40-60°C and pH is maintained at specific set points between 1-4 pH depending upon the time in the reaction cycle. In TiO 2 finishing, the temperature is also cycled and pH variations are controlled between 5-7. Sensors are manually calibrated prior to the initiation of each batch. In the yellow oxide [a-FeO(OH)] process, sensors are initially retracted from the process as the pH is less than -2 pH. Rather than continuously monitoring pH, the sensor is introduced at predetermined intervals into the process as measurement and control data is required. Sensors are also retracted as reactor temperatures exceeding the rating of the sensor are reached and upon completion of the batch. This technique increases average sensor replacement intervals from a matter of days or weeks to months. Automated cleaning of the sensor reduces manual maintenance intervention and calibration requirements. This is especially important in TiO finishing where solids content can 2 approach 60%. Multiple daily manual cleanings are instead replaced by semiannual, scheduled recommended maintenance of the holder. Calibration frequency is also decreased as sensor life and performance is increased. The net benefit to plant operations is realized in instrumentation reliability, enhanced confidence in the accuracy of the measurement point, and decreased operating and maintenance costs. Although significant savings are achieved in material replacement costs, the greatest savings come from reduced maintenance requirements.

9 Following is an example of expected initial equipment costs and approximated annual costs associated with a single automated measurement point:

Required Hardware Approximate Costs

Retractable Kynar pH Electrode Holder $ 5,530.00 System Controller and Analyzer 5,345.00 Sensor 194.00 Injector 1,215.00 Cables 180.00 Installation 400.00

AAtpproximateInstalledHardwareCost$$012,864.00

ExpectedAnnualMainteneance EstimatedCosts

ReplacementSensorFrequency 4peryear YearlySensorCosts eachsensor$194.00 MaintenanceParts (annualo-ringreplacement)1,100.00

AAsA0ssociatedYearlyCosts $$01,876.00

MaintenanceFrequency Quarterly(8hours/yearforsensor MaintenanceLabor replacementandholdermaintenance) CalibrationFrequency Weekly(15minuteseach) CalibrationLabor 13hours/year

AAsssociatedYearlyCosts(021hoursat$50.00/hour)$1,050.0

TTsT0OTALYearlyCosts $$02,926.00

The calculated return on investment of the automated, retractable system versus the fixed measuring point installation is as follows:

Firstyearsavings(LaborandMaterials) $21,174.00($24,100-2,926)

I0nitialSystemHardwareCosts $12,864.0

RRteturnOnInvestment ~~s7.3months

The required hardware and maintenance part prices are actual manufacturer list prices. Estimated labor requirements are indicative of typical automated pH measuring systems in these applications. Actual associated labor costs will vary depending on the actual plant and are provided merely as an estimate. Regardless, the estimated initial costs show a return on investment of approximately 7.3 months. Future savings are estimated at nearly $21,000.00 per year per measurement point. When multiplied by the number of potentially applicable measurement locations within a typical pigment plant, annual savings can be substantial. Additional savings and benefits to the plant are realized in improving product quality through tighter process control and measurement accuracy.

10 In conclusion, automated, retractable pH systems provide a solution in the process measurement of pH in difficult pigment applications. Flexible system design allows the operator to tailor the operation to increase sensor life, reduce maintenance requirements, and improve measurement accuracy. Net benefits to the plant are realized in reduced maintenance and operating costs as well as increased product quality and tighter process control.

Actual holder installation in TIO 2 finishing tank application