Mineral oil, a colorless and odorless liquid, is a petroleum-derived product. The “making” of mineral oil involves a rigorous, multi-stage industrial process of refining crude oil to achieve the necessary purity and stability for its many uses. This process transforms a raw, dark, and impure substance into the transparent liquid found in everything from pharmaceuticals to heavy machinery.
Origin in Crude Petroleum
Mineral oil is meticulously separated from crude petroleum, a naturally occurring mixture of various hydrocarbons. The initial separation occurs through fractional distillation. Crude oil is heated, causing the hydrocarbons to vaporize, and the hot vapors rise up a distillation column, cooling as they ascend. As the vapors cool, different hydrocarbon chains condense back into liquid at specific temperatures, separating the crude oil based on molecular size and boiling point. Lighter molecules condense higher up, while the heavier, longer-chain molecules (the precursors to mineral oil) condense lower down. This step isolates the correct hydrocarbon fraction, generally consisting of molecules with 26 to 40 carbon atoms.
The Industrial Refining Process
The crude oil fraction must undergo significant further processing to remove impurities and undesirable chemical structures. This refinement determines the final product’s quality and safety. A primary goal is the removal of unstable aromatic hydrocarbons, along with sulfur and nitrogen compounds.
One main technique is solvent extraction, where the base oil is mixed with a chemical solvent (such as furfural) that selectively dissolves the aromatic compounds. This separates the undesirable components from the desired paraffinic and naphthenic hydrocarbons, resulting in a cleaner product called raffinate. The raffinate is often subjected to dewaxing to improve its flow characteristics at lower temperatures.
A more common method is catalytic hydrogenation, also known as hydrotreating. In this process, the oil fraction is subjected to high temperatures and pressure in the presence of a catalyst and hydrogen gas. This treatment removes sulfur, nitrogen, and oxygen compounds while converting unstable aromatic rings into more stable, saturated hydrocarbon structures. This chemical stabilization is essential for creating a highly pure and non-reactive final product.
To achieve the highest purity levels, the oil may undergo a final filtration step, often using materials like activated carbon or clay. This adsorption process removes any remaining trace impurities, color bodies, and odors.
Distinguishing Grades of Purity
The thoroughness of the refinement process dictates the purity and classification of the final mineral oil product. These different grades determine the specific applications for which the oil is considered safe and effective. The least refined version is Technical Grade mineral oil, suitable for industrial uses where human contact is minimal.
For applications involving contact with people or food, purity standards are strict. High-purity mineral oils are classified by designations like United States Pharmacopeia (USP) or National Formulary (NF). To earn a USP or NF designation, the oil must pass rigorous tests, including demonstrating the absence of carcinogenic polycyclic aromatic hydrocarbons (PAHs) and having no detectable aromatics. The USP and NF standards ensure the mineral oil is clear, tasteless, odorless, and chemically inert, making it safe for internal consumption or topical application.
Diverse Applications of Mineral Oil
The various grades of mineral oil lead to a wide spectrum of applications across many different industries. Technical Grade mineral oil is commonly used in industrial settings as a base for lubricants, hydraulic fluids, and metalworking fluids, providing necessary lubrication and cooling properties for machinery.
The highly refined, high-purity grades (NF and USP) have different applications due to their safety profile. In the pharmaceutical industry, USP-grade mineral oil is used as an inert excipient in medications and as an ingredient in laxative preparations. Its non-toxic nature also makes it a popular component in cosmetics, including baby lotions and moisturizers.
Food Grade mineral oil, corresponding to NF and USP grades, is safe for incidental or direct contact with food. It is used as a protective coating for cutting boards, a release agent during processing, and in food preservation applications.