Regardless of how it is generated — perhaps from heavy equipment washdown or rainfall scouring of residual leakage — oil in collected water must be removed before it can be recycled or discarded. Oil skimmer systems can remove oil from sumps, pits or collection ponds continuously and without interruption from water level fluctuations or floating debris.

The process of washing down heavy-duty equipment is viewed as a critical aspect of routine maintenance in mining. Regular cleaning simplifies maintenance and extends the life of fleet equipment.

For the largest earthmoving equipment, high-powered vehicle cleaning systems efficiently pressure-wash the vehicle from top to bottom, from chassis to tires and wheels. Often applied at the rate of hundreds of gallons per minute, the water can remove thousands of pounds of debris containing diesel, oil, mud and dirt caked with grease.

Given the volume of water involved, the mining industry is increasingly seeking new ways to reduce the consumption of fresh water through more efficient and environmentally friendly water treatment and recycling efforts. Along with a mandate to implement more sustainable water management practices in the face of a global water shortage, operators can also realize significant economic benefits. The potential cost savings include reducing water replacement through commercial suppliers, lowering chemical dosages, and lessening the burden (and requirements) of other aspects of the water treatment process. 

“In mining, water recycling is critical and every gallon that can be reused counts,” said Jim Petrucci, vice president at Oil Skimmers Inc., a Cleveland, Ohio-based manufacturer specializing in oil skimming and separating equipment. “Given that mines are often located in remote areas with limited access to fresh water, the cost to replace lost or evaporated water is particularly prohibitive. So is the cost of treatment and disposal if the water is not recycled.”

To treat water that collects in sumps or pits, the accumulated substances in the water must first be removed to achieve the level of quality required for reuse. As a result, heavy-duty equipment washing systems are typically designed with wash water reclamation systems that continuously treat and recycle hundreds of gallons per minute. To recover as much water as possible, these systems typically settle out the bulk of the mud and solids first in sumps or pits to eliminate any oils before the water is further processed.

“In equipment washing systems, oil that is not removed contaminates wash solutions and serves as a barrier to recycling the water for reuse,” Petrucci said.

Heavy equipment washing is not the only place where oil can accumulate in mining. Due to the extreme demands on the
equipment, small amounts of hydraulic fluid, lubricating oil and fuel can leak from joints, hoses, and other connection points on vehicles over time. When rainwater washes over the equipment, it can generate an oil-water mixture that is typically collected in a large sump or pit nearby. In underground mines, groundwater infiltration and condensation can have a similar
effect as rain.

No matter how it is generated, any oil in collected water must be removed before it can be recycled or discarded. One cost-effective and simple method to remove the oil is using an oil skimmer that continually draws off surface oil from a sump, pit or rainwater collection pond. These systems can be customized to meet the specific requirements based on the location and size of the pit.

“The easiest way to remove oil
from water in a sump or pit is with a skimmer,” Petrucci said. “After skimming the oil, the water can be recycled, returned to the collection pit, or rerouted wherever it needs to go.”

The most efficient type of oil skimmer uses a free-floating collector tube that actively and continuously removes the oil as it rises to the surface of the water. As the tube moves across the surface, oil adheres to the outside. The tube is drawn through a series of ceramic scrapers that constantly remove the oil, which then drains by gravity into a collection vessel. The skimmer is not affected by water level fluctuation or floating debris and solids, removes very little water in the process, and operates continuously with minimal attention or maintenance.

An oil separator may also be required for situations where oil does not separate naturally by gravity or there is a reason to speed the process. Separators utilize a coalescing media that encourages separation and is designed to provide the necessary surface area required for non-emulsified oil droplets to combine or coalesce, forming larger, more buoyant droplets, which rise to the surface more quickly and easily.

Petrucci noted that when mines use an oil skimmer along with a separator, it reduces the costs for downstream wastewater treatment by decreasing the replacement of filters, and addition of chemicals and flocculants.

“Removing any oil in the water prior to additional wastewater treatment allows the entire system to operate more efficiently and reduces the costs,” Petrucci said. “For example, oil that is not removed first can ‘blind’ filters and lead to premature replacement. There are also maintenance costs when changing a filter.”

This article was first published in the September 2021 edition of Engineering & Mining Journal (E&MJ).Just Skimming the Surface