The Process of Re-refining Used Lubricating Oil

Lubricating oil is essential for vehicle engines, but what happens to the oil once it has served its purpose? Every year, millions of litres of used oil are generated worldwide. According to the American Petroleum Institute (API), approximately 50% of all lubricating oil becomes used oil after its lifecycle. However, improper disposal poses severe environmental risks—one litre of used oil can contaminate up to one million litres of freshwater, leading to water pollution, soil degradation, and toxic emissions.

Fortunately, used oil can be recovered through a sustainable process called oil re-refining. This method removes contaminants, restores its properties, and produces high-quality base oil that can be used again—reducing dependence on the extraction of virgin crude oil. Re-refining preserves natural resources that are otherwise exploited for oil extraction and refining. Moreover, millions of litres of oil get saved from being discarded, significantly reducing air pollution and greenhouse gas emissions compared to traditional refining.

This article explores the science behind oil re-refining, major oil re-refining techniques, and the role of oil re-refining in promoting circular economy principles.

What is Oil Re-Refining?

It is a process that effectively removes contaminants, metal particles, and degraded additives, restoring the oil’s properties to near-original levels, thus leading to resource conservation. Unlike simple recycling or incineration, re-refining reduces the need for new oil production since the refining process involves the usage of Used Oil as raw material, conserves natural resources, and contributes to environmental protection.

The re-refining journey begins with the collection of used oil from various sources like automotive service centers and industries. Once gathered, the oil is subjected to rigorous testing to ensure it's fit for processing. The first major step in any oil re-refing process is dehydration, where water and light fuels are removed through evaporation. Following this, the oil undergoes vacuum distillation, separating it into different fractions. The crucial lubricating fraction is then treated to eliminate impurities, often using methods like hydrotreating or solvent extraction. The final result is high-quality base oil, ready to be blended with additives to produce fresh lubricants.

By embracing the re-refining process, we not only conserve resources but also significantly reduce environmental pollution, contributing to a more sustainable future.

The Used Oil Re-Refining Process: The Science behind It

Re-refining is the process of taking used lubricating oil, removing contaminants, and restoring it to a high-quality, reusable state, having multiple environmental, economic, technical, and social benefits.
Instead of being discarded or incinerated, used oil goes through processes based on available technology. Below are a few common methods:

Used Lubricating Oil Re-Refining Technologies in India

The re-refining of used lubricating oils is an essential process to reclaim valuable base oils and reduce environmental impact. Several advanced technologies are used to ensure that re-refined oils meet the highest quality standards as per the Bureau of Indian Standards (BIS) and are suitable for reuse in various applications. Here's an overview of the key technologies involved:

• Solvent Extraction:

  Solvent extraction, also known as liquid-liquid extraction, is an advanced refining technique used to remove impurities from used oil based on their varying solubility in organic solvents. This process efficiently separates unwanted contaminants, such as oxidation byproducts, sludge, and heavy metals, while preserving the high-quality base oil fraction.
  • Pre-Treatment & Filtration: The used oil undergoes filtration and dehydration to remove water, dirt, and large particulates before solvent treatment.
  • Solvent Addition & Mixing: A suitable organic solvent (e.g., furfural, N-methyl-2-pyrrolidone (NMP), or phenol) is introduced to the pre-treated oil. The mixture is agitated to allow selective dissolution of contaminants into the solvent phase while leaving the base oil relatively unaffected.
  • Phase Separation: The process results in two distinct liquid phases:
    • Extract Phase: Contains impurities, degraded materials, and unwanted compounds dissolved in the solvent.
    • Raffinate Phase: Enriched with purified base oil, separated from the contaminants.
  • Solvent Recovery & Distillation: The raffinate phase undergoes vacuum distillation to remove residual solvent, leaving behind high-purity base oil. The solvent from both phases is recovered and recycled for repeated use, reducing waste and operational costs.

• Hydrotreating:

  Hydrotreating is a highly efficient chemical refining process used to upgrade the quality of recovered base oils. It removes contaminants such as sulfur, nitrogen, oxygen, and unsaturated hydrocarbons, resulting in a cleaner, more stable, and higher-performing base oil.
  • Pre-Treatment & Filtration: The used oil undergoes pre-treatment, which includes filtration and dehydration to remove water, sediments, and large particulate matter.
  • Heating & Pressurization: The pre-treated oil is heated to temperatures ranging from 300–420°C (572–788°F). Simultaneously, hydrogen is introduced under high pressures of 30–150 bar (435–2,175 psi) to facilitate the chemical reactions.
  • Catalytic Reaction: The heated oil and hydrogen mixture is passed over a hydrotreating catalyst, typically made of metals like nickel-molybdenum (Ni-Mo) or cobalt-molybdenum (Co-Mo) supported on alumina.
  • Separation & Purification: The hydrotreated oil is sent to a separator to remove gaseous byproducts like hydrogen sulfide and ammonia.
  • Final Product: The resulting base oil is clear, odorless, and free of impurities, making it suitable for use in high-performance lubricants.

• Membrane Technology:

  Membrane technology uses selective adsorption and desorption to recycle used oils into high-quality base oils.
  • Pre-Treatment: Removal of contaminants like water, glycol, alcohol, and emulsions.
  • Nanofiltration: Uses a precision membrane to separate the oil from undesirable non-hydrocarbon substances at the molecular level.
  • Polishing: Removes oxidized substances that could affect the oil's color and odor or leave residual particles.

• Other Treatment Processes: Acid Clay Treatment Process

  The conventional acid/clay treatment process for re-refining used oils involves several steps:
  • Settling and Decantation: Allowing the oil to settle, separating the contaminants.
  • Dehydration: Removing water from the oil.
  • Acid Treatment and Sludge Separation: Treating the oil with acid to neutralize impurities, followed by separating the sludge.
  • Clay Treatment and Diluent Removal: Using clay to remove additional impurities and diluents.
  • Upgrading or Additive Blending: Enhancing the oil with additives to improve performance.

  Merits of Acid-Clay Treatment

  • Simple Technology: The process is easy to implement, making it accessible for small-scale operations.
  • Low Capital Investment: The required equipment is relatively inexpensive, reducing initial setup costs.
  • Widely Available Equipment: Most of the equipment is made from carbon steel, which is commonly available.
  • Easily Sourced Chemicals: The process relies on readily available chemicals, ensuring a consistent supply.

  Demerits of Acid-Clay Treatment

  • Low Oil Yield: The refining efficiency is relatively low, leading to a lower output of reusable oil.
  • Waste Disposal Challenges: Acid sludge and oil-contaminated clay pose significant disposal and environmental challenges.
  • Corrosion Issues: The process can lead to corrosion of equipment, increasing maintenance costs.
  • Environmental Hazards: Improper waste management can result in pollution and ecological harm.

The Path to a Circular Economy

Used oil is a valuable resource that does not degrade but simply becomes contaminated over time. While improper disposal poses serious environmental risks, including groundwater contamination and air pollution from combustion, re-refining offers a sustainable alternative. By restoring used oil to high-quality base oil, re-refining reduces waste, conserves natural resources, and minimizes environmental impact. As studies suggest, adopting re-refining as a standard practice is the most effective way to manage used oil responsibly while supporting a circular economy.

With the introduction of Extended Producer Responsibility for Used Oil Management in India, the accountability of producers, including manufacturers and importers, to ensure responsible recycling and disposal practices has increased tremendously. This involves meeting established recycling targets, adhering to environmental standards, and promoting sustainable waste management initiatives. The recent Hazardous and Other Wastes (Management and Transboundary Movement) Second Amendment Rules, 2023, marks a pivotal step toward eco-friendly practices in India.