Sulfuric acid, also known as vitriol, is a highly corrosive strong acid that is colorless, odorless and soluble in water. With a molecular formula of H2SO4, it is a lipophobic liquid that is commonly used in chemical industry, petroleum refining and as a battery electrolyte. Sulfuric acid has excellent dissolving properties and is useful for extracting metals from their ores and removing impurities.
Purification Process for Ultra Pure Grade
Commercially available sulfuric acid has purity levels of around 93-98% concentration. However, modern microchip fabrication and other precision applications require an ultra pure grade with tightly controlled levels of contaminants measured in parts per billion or lower. Achieving this stringent level of purification is a complex multi-step process.
The first step for Ultra pure Sulfuric Acid involves purification through fractional distillation to remove water content and other volatile compounds. Liquid sulfuric acid is heated under vacuum conditions, allowing separate collection of higher purity fractions. Simultaneously, ion exchange resins are used to remove dissolved metal ions that could compromise performance.
Following distillation, the concentrated acid undergoes further purification using solvent extraction methodology. This involves repeated contact with immiscible organic solvents like tributyl phosphate, which selectively extract trace contaminants based on differences in solubility. Several extraction-scrubbing cycles are performed to drive the separation towards completion.
Ultrafiltration is another key technique utilized forfinal purification. The concentrated sulfuric acid is passed under pressure through selective polymer membrane filters with pore sizes in the 1-10 nanometer range. This allows separation of negatively charged ionic species, colloidal particles and high molecular weight organic compounds from the purified product.
Stringent Quality Control for Tramp Element Removal
Even with multiple purification steps, achieving part-per-billion level purity specification demands the utmost quality control protocols. Testing using techniques such as ICP-MS (inductively coupled plasma mass spectrometry) and ICP-OES (inductively coupled plasma optical emission spectrometry) identifies and quantifies trace heavy metal ionic impurities present.
Any contaminant elements detected above the allowed threshold, known as "tramp elements", must be removed through further processing. Detailed material tracing finds the source such as pipeline corrosion or filtration membrane degradation. Custom engineered treatment protocols are then developed and validated to eliminate each tramp element. For example, selective ion exchange resins may be tailored to different metals.
Strict Handling Protocols Minimize Recontamination
Maintaining the ultra high purity throughout handling, storage and transport is just as important as the purification process itself. Trace contamination can occur from contact with impure air, dust, moisture or equipment surfaces. Therefore ultra pure sulfuric acid production incorporates highly controlled cleanroom environments and dedicated piping/vessels constructed of corrosion resistant metals like tantalum.
Workers follow detailed SOPs involving protective equipment like full bodysuits, double gloves and HEPA filtered ventilation. Automated processes minimize human intervention. Storage vessels are continuously purged with purified inert gas to prevent reactions with ambient air. Sophisticated leak detection sensors monitor encapsulated transport containers. Together, these stringent precautions minimize any risk of recontamination to parts per trillion levels.
Applications in Microchip Manufacturing
One major end use of ultra high purity sulfuric acid is in the semiconductor industry for microchip wafer cleaning and etching steps in IC fabrication. Even minute contaminants could cause defects rendering entire silicon wafers unusable. The acid's strong oxidizing properties and ability to dissolve organic residues make it effective for removing photoresists and other organic contaminants post-etching or deposition phases.
Its ultra-low trace element content prevents introduction of impurities that could compromise chip performance or yields. With demand for ever smaller circuit features and larger wafer diameters, the industry continues pushing for new generations of purified chemicals. Ultra pure sulfuric acid plays a key role by meeting exacting cleanliness specifications needed for advanced chip manufacturing nodes of 10 nanometers and below.
Quality Assured Supply for Critical Industries
With the growing importance of applications requiring highly purified process chemicals, manufacturers have developed comprehensive quality management systems to assure supply. ISO 9001 certification covering design, manufacturing and distribution ensures production meets stringent international standards.
On-site analytical labs allow real-time testing at PPB levels for over 60 elements. Robust documentation like certificates of analysis provide full batch traceability. Long term supplier partnerships and just-in-time logistics minimize inventory carrying costs. These measures help secure supply of ultrapure sulfuric acid for critical industries and fuel ongoing advances in technologies that have transformed modern society.
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