Recycling used oil is crucial for conserving resources and protecting the environment. Oil, whether petroleum-based or synthetic, does not wear out; it merely becomes contaminated with use. Proper recycling prevents used motor oil from contaminating fresh water supplies. The process focuses on two main categories: petroleum-based lubricants and plant-based cooking oils, each requiring a distinct path to create new products.
Classifying Oils for Recycling
The recycling path for used oil depends on its original source and the type of contaminants it contains. Used lubricating oils (motor oil, hydraulic fluid, transmission fluid) are contaminated with metals, water, and combustion byproducts, making them primary candidates for re-refining into new base oil. Vegetable and animal-based oils, such as used cooking oil (UCO), follow a separate, specialized chemical conversion process. Mixing these oil types can introduce compounds that complicate the re-refining process. Oils contaminated with hazardous chemicals, such as solvents or high concentrations of halogens, are classified as “waste oil” and require stringent hazardous waste treatment.
Collection and Preliminary Handling
The recycling journey begins with the collection of used oil from generators, ranging from individual vehicle owners to large industrial facilities. Automotive service centers and designated drop-off locations serve as consumer collection points, while commercial services handle bulk pickups. Proper storage is mandatory, requiring containers to be labeled clearly and kept in good condition to prevent leaks and cross-contamination.
At the receiving facility, preliminary handling focuses on removing gross contaminants. Initial testing determines the oil’s quality, checking for excessive water content, solvents, or heavy metals that could interfere with processing. The oil undergoes physical cleaning, where rough filtration removes large solid debris like dirt and metal shavings. Dehydration, achieved by heating the oil in a closed system, removes water and lighter hydrocarbon fuels, preparing the oil for the complex re-refining stage.
The Re-refining Process for Lubricants
The most valuable recycling path for used lubricating oil is re-refining, which converts the material back into high-quality base oil. This complex industrial process begins with pre-treatment to separate water and light fuels. The oil is then heated and subjected to vacuum distillation, a precise thermal separation technique.
Vacuum Distillation
The distillation occurs under a powerful vacuum, which lowers the boiling points of the oil molecules, allowing them to vaporize without thermal breakdown. This process separates the desirable lubricating oil fractions from heavy additives, degraded polymers, and metallic compounds, which remain as a thick residue. The distilled oil vapor is then condensed into different streams based on viscosity, yielding a basic, clean oil known as a distillate.
Hydrotreating and Stabilization
To achieve a quality comparable to virgin crude oil, the distillate requires further purification through hydrotreating. This stabilization process involves reacting the oil with hydrogen gas under high pressure and temperature using a catalyst. The hydrogen chemically reacts with remaining impurities like sulfur and nitrogen, effectively removing them and stabilizing the oil’s molecular structure. The final product is a re-refined base oil that meets the same performance standards as oil derived from newly extracted crude.
Alternative Uses and Final Recycled Products
Not all used oil is suitable or economically viable for re-refining, leading to alternative conversion processes. Used cooking oil (UCO) and yellow grease are processed through transesterification. This chemical reaction involves reacting the oil’s triglycerides with a short-chain alcohol and a catalyst. The reaction converts the oil into two primary products: fatty acid methyl esters (FAME), which is biodiesel, and glycerin as a co-product.
Lower-grade industrial oils, or those too contaminated for re-refining, are often processed for energy recovery. These oils serve as industrial fuel oil for kilns, boilers, or marine applications after filtering and water removal. The recycling industry produces valuable final products that reduce reliance on virgin resources, including re-refined base oil and biodiesel. The heavy residue (“bottoms”) left over from vacuum distillation is incorporated into asphalt and road construction materials. Residual light hydrocarbons removed during processing are often used to power the recycling facility itself.