Rio Tinto is refurbishing and reutilizing the Humboldt Mill, a prominent fixture of Michigan’s mining history. In the 1950’s the mill began to service several mining companies, making it an important component of the area’s culture and economy. Since the mid-1990s, the Humboldt Mill has sat idle, falling into disrepair. Even so, this existing Brownfield industrial site is an important community asset.
We received all of the permits necessary to refurbish and operate the mill in 2010. In 2011, we began cleanup, environmental reclamation and equipment upgrades to the facility. Construction on the mill will begin in 2012 to prepare the facility for ore production in 2014.
The mill will be utilized for the concentration of ore and disposal of tailings, just as it has in the past. Ore will be hauled from the Eagle Mine to the mill, where it will be processed into separate nickel and copper concentrates. The concentrates will then be transported via rail to an offsite smelter for further refinement.
By utilizing the mill, we’re able to extract more nickel and copper at lower grades. This leads to a better use of the mineral resource, more jobs, more taxes and a longer mine life.
Ore will be hauled from the Eagle Mine to the Humboldt Mill where it will be processed into separate nickel and copper concentrates. Both concentrates are solid, fine materials that contain a higher proportion of nickel and copper than the ore extracted from the mine.
The milling process involves various stages:
The first stage in the process is crushing. At the Humboldt Mill, we will use three stages of crushing to reduce the size of the ore from run-of-mine (18 inches and smaller) down to 1⁄2 inch or less. At this stage of the process, the ore is dry so we use various dust control measures to provide for the health of our employees and to protect the environment. Of course, all of this activity is being conducted inside the mill.
The next stage in the process is grinding, where we start by adding water to the crushed ore to create a mixture called slurry. Grinding is conducted in two ball mills where we reduce the size of the ore from less than 1⁄2 inch down to approximately 100 microns, creating a texture similar to fine sand. We grind to this size in order to separate nickel and copper from the rest of the minerals.
We then begin the process of concentrating the minerals. Here we use a process called froth flotation, causing some minerals to float while others do not. When we introduce fine bubbles into the slurry, the minerals attach to air bubbles and float to the surface, where they are skimmed off. Initially we float both the nickel and copper together, which is referred to as bulk flotation. We then separate the nickel from copper, in a step appropriately called differential flotation. The results of all this activity are two distinct products: a nickel concentrate and a copper concentrate.
At this stage of the process the concentrates are still in the form of slurry, so we work to remove most of the water prior to shipment. First we thicken the slurry, and then we filter the thickened slurry. The concentrates are then loaded into rail cars and shipped to a facility for further refinement. In the end, they produce nickel metal, copper metal and other necessary materials that are used to manufacture end products like automobiles, stainless steel, cell phones, musical instruments, copper pipe, electrical wire, etc.
The minerals left over once the nickel and copper have been extracted are often referred to as tailings. At the Humboldt Mill, the tailings are sent to the Humboldt Tailings Disposal Facility (HTDF). Here, the solids settle to the bottom and the water is decanted off the top and pumped back to the mill to be reused in the mill process. Excess water will be treated at an on-site treatment plant before being properly discharged.