Barium sulfate, also known as barite, is a naturally occurring mineral with a chemical formula of BaSO4. It forms orthorhombic (monoclinic) crystals. It often appears as thick tabular or columnar shapes, or in dense blocky or granular aggregations. When pure, it is colorless and transparent. But, impurities can give it various colors. Barium sulfate leaves a white streak and has a glassy to semi-transparent luster. It exhibits perfect and distinct cleavage along three directions, has a Mohs hardness of 3 to 3.5, and a density of 4.5.
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Barium sulfate is highly stable. It is almost insoluble in water, and resistant to acids and bases. It is non-toxic and non-magnetic. Yet it has the ability to absorb X-rays and gamma rays, making it useful in various applications.
The chemical formula for barium sulfate is BaSO4, reflecting its composition, consisting of barium (Ba), sulfur (S), and oxygen (O).
Barium sulfate is generally white or colorless. However, it can take on pale colors under certain conditions or due to impurities in some applications.
The molar mass of barium sulfate is approximately 233.39 grams per mole.
Barium sulfate comes in several forms:
5.1. Heavy Barium
Heavy barium is also known as natural barite. It is obtained by processing natural barium sulfate ore (barite) through washing, grinding, and drying. It tends to have more impurities, and its quality primarily depends on the ore itself. It is commonly used as a filler in the production of white pigments or in low-end paint, plastic, and ink industries.
5.2. Precipitated Barium Sulfate
This type is often called industrial barium sulfate or precipitated barite. It is produced through chemical processes. It has high purity, typically exceeding 98%. There are two main methods for producing precipitated barium sulfate. One involves heating barite with coal powder to reduce it, followed by a reaction with sodium sulfate. The other uses natural barium carbonate in a reaction with sulfuric acid. Precipitated barium sulfate has varying properties. It finds applications in industries such as pharmaceuticals, high-quality paints, inks, plastics, rubber, glass, ceramics, and more. Different grades are available. Paint-grade precipitated barium sulfate and plastic-grade precipitated barium sulfate are different.
5.3. Modified Barium Sulfate
Both modified barium sulfate and modified precipitated barium sulfate involve additional treatments of heavy barium or precipitated barium sulfate. It needs to enhance specific properties. Their applications are similar to those of precipitated barium sulfate. It depends on the modified characteristics. When further processed and refined, it is known as modified ultrafine barium sulfate or modified ultrafine precipitated barium sulfate. These two kinds tend to be more expensive.
5.4. Nano-grade Precipitated Barium Sulfate
Nano-grade precipitated barium sulfate is produced by further processing modified precipitated barium sulfate. It requires control of the D50 (median particle size distribution) within the range of 0.2μm-0.4μm. It has a high demand for this processing. There are relatively few manufacturers in China capable of producing it. So price is higher and is primarily used in high-end paints and coatings industries.
Barium sulfate is almost insoluble in water and traditional solvents. But it is soluble in concentrated sulfuric acid.
No, barium sulfate is not radioactive. It is an inert and safe material widely used in medical applications like X-rays and CT scans.
Soluble barium salts have moderate toxicity to humans. Yet, barium sulfate is non-toxic because it is virtually insoluble in water. Accidental barium poisoning usually occurs due to ingesting soluble barium salts mistakenly labeled as BaSO4. In the Cerrovar incident (Brazil, ), nine patients died due to the improper preparation of a radiographic contrast agent. For occupational exposure, regulatory bodies have set an acceptable exposure limit of 15 mg/m3. The National Institute for Occupational Safety and Health recommends a limit of 10 mg/m3. For respiratory exposure, both organizations have established a workplace exposure limit of 5 mg/m3.
Barium sulfate has a wide range of applications:
9.1. Drilling Fluids
Approximately 80% of barium sulfate production is consumed as a refining mineral component in oil well drilling fluids. It increases fluid density and raises hydrostatic pressure within the well. It also reduces the likelihood of a blowout.
9.2. Radiographic Contrast Agent
Barium sulfate is commonly used in medical X-ray imaging. Also used as a radiographic contrast agent in other diagnostic procedures. It is most commonly used to image the gastrointestinal tract during a surgical procedure commonly known as a “barium meal.” It is taken by mouth or through an enema. Suspension is a thick milky solution, usually sweetened or flavored. Although barium is a heavy metal, it often contains toxic soluble compounds. But barium sulfate’s low solubility prevents patients from absorbing harmful amounts of the metal. Moreover, it can be easily removed from the body. It is preferred over alternatives like Thorotrast. With a relatively high atomic number (Z = 56), barium compounds absorb X-rays better than compounds from lighter nuclei.
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9.3. Pigments
In the realm of paint production, most synthetic barium sulfate is used as a component of white pigments. In oil-based paints, barium sulfate is nearly transparent and is used as a filler or for adjusting consistency. Some commercial paint manufacturers sell “permanent white”. It is a mixture of titanium dioxide (TiO2) and barium sulfate. Barium sulfate, combined with zinc sulfide (ZnS), forms an inorganic pigment called lithopone. It is also used as a coating to evenly scatter light on certain types of photographic paper.
9.4. Heat Reflective Coatings
Barium sulfate exhibits high reflectance for visible and ultraviolet light. Researchers have used it as a component in coatings. That can reflect up to 98.1% of all solar radiation, effectively cooling the surfaces to which it is applied. This stands in contrast to commercial white paints, which can reflect only 80-90% of sunlight. Achieving this effect requires a specialized application of barium sulfate. It needs to reduce the coating thickness to as little as 1.5 mm by using hexagonal boron nitride.
9.5. Brightening Paper
In the realm of photography, barium sulfate plays a crucial role as a paper brightener. It is applied as a thin layer, known as baryta. On the base of most photographic papers, it can increase the reflectance of images. The practice was introduced in Germany in . Light-sensitive silver halide emulsions are then coated on top of the baryta layer. This prevents the emulsion from penetrating the paper fibers. It results in more uniform and evenly developed images. Extra layers may be applied for image fixation and protection. Baryta is also used to reduce the weight of inkjet printing papers.
9.6. Enhancing Polymers
Barium sulfate serves as a common filler in the plastics industry. It particularly is used in applications requiring vibration damping. It can make polymers denser. Also, it can improve their resistance to acids and bases, and enhance opacity. Composites containing up to 70% barium sulfate are used in polypropylene and polystyrene plastics. It has superior radiation-blocking properties. this composite material is also used as an X-ray shielding material. In certain specific applications, composites with high barium sulfate content (70-80%) may outperform the more commonly used steel shields.
9.7. Oil Testing
Barium sulfate plays a pivotal role in soil testing. Colored indicators are used to test soil pH and quality. However, the presence of fine soil particles, often clay, can muddy the test mixture and obscure the indicator’s color. We can add barium sulfate to bind with these particles, making them heavier and allowing them to settle at the bottom. The result is a clearer solution for analysis. In colorimetry, barium sulfate is employed as an almost perfect diffuser. Especially when we measure light sources.
9.8. Metal Casting
In the metal casting industry, molds are often coated with barium sulfate to prevent molten metal from sticking to the mold. Since anode plates are cast within copper molds, suspensions of fine barium sulfate in water are applied as coatings on mold surfaces. This prevents direct contact between the liquid copper and the solid mold. Consequently, once the molten copper solidifies into the shape of an anode plate, it can be easily removed from the mold.
9.9. In the Copper Industry
Barium sulfate has a high melting point and is insoluble in water. It is an ideal material for demolding copper anode plates from molds during the casting process. Since anode plates are cast within copper molds. A suspension of fine barium sulfate in water is applied as a coating on the mold surface. This prevents direct contact between the molten copper and the solid mold. So, once the molten copper solidifies into the shape of an anode plate, it can be easily removed from the mold.
9.10. Radiation Measurements
Barium sulfate is sometimes used in combination with PTFE. It is used to coat the inner surface of integrating spheres in radiometric measurements. This material has high reflectivity features. It is close to Lambertian reflectivity. So it is useful for a variety of optical measurements.
The majority of commercial barium comes from baryte (barium sulfate) ores, which are often highly impure. The baryte is processed through thermal chemical sulfate reduction (TSR), also known as carbothermal reduction. It uses coke as a heating agent. This process yields barium sulfide (BaS):
BaSO4 + 4C → BaS + 4CO
In contrast to barium sulfate, barium sulfide is soluble in water and can be easily converted into oxides, carbonates, and halides. To produce high-purity barium sulfate, barium sulfide or chloride is treated with sulfuric acid or sulfate:
BaS + H2SO4 → BaSO4 + H2S
The resulting barium sulfate is often referred to as “blanc fix,” which translates to “eternal white” in French. Blanc fix is a crucial element found in consumer products like paint. In laboratory settings, barium sulfate is produced by combining barium ions with sulfate solutions. Its insolubility makes it one of the least toxic barium compounds. As such, barium sulfate can be used to immobilize (detoxify) barium in waste materials. It is also one of the least soluble sulfate compounds. It is suitable for inorganic qualitative analysis to test for Ba2+ ions and sulfate.
In its natural state, untreated raw materials like naturally occurring baryte. It forms under hydrothermal conditions. It may contain various impurities, quartz, or even amorphous silica.
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