Phosphate is a key component in agriculture (food production)

• granular mono ammonium and diammonium phosphate pellets for fertilizer
• animal feed supplements
• elemental phosphorous for food-additive applications.

A limited amount of elemental phosphorus is used in other industrial applications.

In 2015, Phosphate rock was mined by 5 companies at 11 mines:

• in 4 states
  • 80% in Florida and North Carolina & 20% in Idaho and Utah
• to produce 27 million tons of marketable product with a value of $2.4 billion fob mine

World Mine Production and Reserves (USGS Minerals Commodity Survey)

There are three sources of Phosphate Rock

• Sedimentary deposits of marine origin
  • Africa, China, Middle East, and United States
• Igneous and metamorphic deposits
  • Brazil, Canada, Finland, Russia, and South Africa
• Biogenic deposits from bird guano

High-quality sedimentary phosphate deposits are mined in the U.S.

The mined phosphate rock is beneficiated by a process in which the phosphate rock is reacted with sulfuric acid to produce an intermediate feedstock of phosphoric acid.

Gangue minerals associated with the deposit impact the cost of processing the ore

The gangue minerals can require larger quantities of reactants (sulfuric acid), which significantly increases production costs, and can increase time require for downstream processing, e.g., filtration; and they can interfere with the ore recovery and concentration in downstream operations, e.g., float processes.

Carbonates are more problematic. Dolomite, for example, shares mineralogical similarities with phosphate, and this makes it more challenging and expensive to remove it.

• Major gangue minerals of primary interest in U.S. operations:
  • dolomite, calcite, silica, and clays
• Certain other gangue components may need to be removed during or after the production of the phosphoric acid:
  • cadmium, radium, uranium
• Clay slimes are removed in log washers or hydrocyclones.
• Silica is removed using the well-established Crago double float process.

I suggest that you go to Mosaic’s website to learn more about the company. Doing so, you will arrive at this "Who We Are" page, and you will note that Mosaic is not only a major producer of phosphate but also of potash!

Figure 9.5.1: Mosaic Company website

Source: The Mosaic Company

Their Florida operations are located in five counties of southwest Florida, as shown on this map. This is a rural area of Florida.

Figure 9.5.2: Mosaic operations in Florida counties

Source: The Mosaic Company

Resource

The phosphate rock deposit consists of a matrix of phosphate pebbles, clays, and sand, along with other gangue such as silica. (Carbonates occur in increasing amounts in the lower quality deposits.)

Key Characteristics

• The matrix is relatively unconsolidated and is typically on the order of 10’ thick.
• It is covered by an average overburden thickness of 30’ ranging from 15’ to 50’.
• The overburden is unconsolidated sand and organic matter.
• The matrix is underlain by a hard limestone.
• The water table is near the surface throughout the region.

Given these key characteristics, you should be able to choose the mining method. Can you do so? Well, at the beginning of the lesson, I told you that they employed the open cast method. Nonetheless, I would hope that you could come to that conclusion on your own. The use of hydraulicking, on the other hand, would not be apparent to you.

Let me tell you the overall sequence of operations to remove any mystery!

• Prepare the site for mining
  • dozers
• Install utilities
  • electric power
  • water drainage
  • “Pitsrdquo; for breaking down the matrix with monitors
  • slurry pumps and pipelines
• Remove overburden
  • dragline
• Mine the ore
  • dragline
• Prepare for the ore for hydraulic transport to the plant
  • hydraulic monitoring to break-up matrix
• Backfill the cut
  • dragline
• Reclaim the site
  • sand tailings pumped to the site
  • level spoil piles and grade
    • dozers
    • loaders
    • hydraulic excavators
  • add top layers
  • revegetate

We’ll take a look at many of these in more detail; but now, let’s start with an aerial view of the mine.

This picture was taken by a Penn State mining engineering intern while riding in the company helicopter!

Figure 9.5.3: Aerial view of Mosaic's phosphate operation

Credit: K. Hutton, © Penn State University, is licensed under CC BY-NC-SA 4.0

The features in this photo will become known to you as we continue.

Best Management Practices (BMP) features and land development

Off to the left side of the photo, you see a waterway that looks like a nicely manicured stream or small river. It is man made, and it is done to control the water table around the mine and ensure that the water table outside of the mine’s perimeter is not lowered or otherwise harmed. At these mines, this “ditch” is known as a best management practice ditch, because it is part of their stormwater pollution prevention plan (SWPPP) required under the Clean Water Act, and is considered a “best practice” to accomplish the intended goal.

Figure 9.5.4: The perimeter ditch
The construction of the BMPs precedes other development activities. Once completed, roadways are constructed into the site, vegetation is cleared, and the topsoil layers are carefully placed away from the active site for use during reclamation. Then, the site will be graded and leveled to provide a working surface for the draglines.

Source: J. Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0

Figure 9.5.5: Land development

Credit: K. Hutton, © Penn State University, is licensed under CC BY-NC-SA 4.0

Once a level site has been prepared, the work of placing utilities will begin. In this photo, they are placing high voltage cable (yellow in color), which will supply power for the dragline as well as the many pumps used on the site.

Figure 9.5.6: Placing high voltage cable

Source: K. Hutton, © Penn State University, is licensed under CC BY-NC-SA 4.0

As work on the electrical power infrastructure is continuing, a series of dewatering holes will be developed. This photo shows the electrical infrastructure, and do note the overhead electrical lines in the background and the electrical installation in the foreground. If you look closely, you can see 6” diameter PVC pipes protruding at the surface of the ground. These are dewatering holes.

Figure 9.5.7: Dewatering holes

Source:J Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0

This next photo gives us a close-up view, and we can see other important features. There is a small pump at the bottom of this hole, and you can see the small discharge line, where the pumped water enters a larger diameter collection pipe that is serving the entire row of dewatering holes. You can also see the electrical cable that goes down the hole to the pump at the bottom. These holes go to the bottom of the matrix, and so, a typical hole would be 50 – 60’ deep. Those pretty pink flags that you see are markers so that equipment, such as tractors and pickup trucks, don’t accidentally run over and damage pipes or electrical cables.

Figure 9.5.8: Close-up view of a dewatering hole

Source:J Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0

This next picture focuses on an electrical installation. As is often the case, this equipment is mounted on a skid so that it can be attached by a wire rope to a dozer and pulled to a new location. This equipment provides the hardware to allow a connection to overhead power lines, if required, a transformer to step the voltage down to a level required by the machines and pumps, and protective devices to allow switching and implementation of other safety features.

Figure 9.5.9: Mobile electrical installation

Source:J Kohler, © Penn State University, is licensed under CC BY-NC-SA 4.0

With the utilities in place, we are ready to install the pits and monitors and to begin overburden removal. Let’s talk first about overburden removal. A typical dragline in use at the Mosaic mines in Florida is shown in the next picture. Remember, we talked about how draglines move – they walk. Look at the two big walking shoes on this dragline, one on each side. The shoe is green and there is a yellow guardrail around the top surface of the shoe. As you might imagine, these things don’t break any land speed records!

Figure 9.5.10: A typical dragline

Source: K. Hutton, © Penn State University, is licensed under CC BY-NC-SA 4.0

Here is another aerial view of the mine, but in this one, you can clearly see the dragline at work.

Figure 9.5.11: Aerial view of the active mining area

Source: K. Hutton, © Penn State University, is licensed under CC BY-NC-SA 4.0

Here is a close-up of the dragline removing overburden. If you look to the left, you can see the last strip or cut that was mined (it is now filled with water).

Figure 9.5.12: Removing the overburden

Source: The Mosaic Company

This short video clip (2:11) will give you a feel for the overburden removal cycle. Note the development activities ongoing to the left of the active cut.

In this video clip, you probably saw the greenish colored material at the bottom. That is the phosphate matrix, and in this next picture, you can see it as well.

Figure 9.5.13: View of the overburden and matrix shelf

Source: The Mosaic Company

Evaluating the Phosphate Matrix

You’ll recall from our discussion of mine planning, that mineral processing plants are designed to perform best when the feed into the plant has characteristics that lie within a fairly narrow range. When the mined material is outside of this range, the practice is to blend loads of material from different mining faces to achieve the desired feed to the plant. This is true in phosphate mining as well. How do you know the qualities of the ore being mined at a given location on a given day? You send in the geologists to take samples and visually inspect the ore. In this next picture, you can see the geologist, accompanied by operations personnel, preparing to sample the phosphate matrix in the drag bucket.

Figure 9.5.14: Geologists sampling matrix from the drag bucket

Source: The Mosaic Company

Now that we have a clearer understanding of this phase of the operation, let’s look at the dimensions of a typical strip or cut.

Figure 9.5.15: Strip or "Cut" dimensions. The mining "cut" here is 320 feet wide. The Overburden is 30 feet from top to bottom, and the Matrix is 10 feet from top to bottom. The lowest layer shown is the Hardrock Bottom, which is comprised of limestone, clay, and dolomite.

Source: The Mosaic Company

Before the dragline can begin to mine the ore, the hydraulic monitors must be set up and ready to go. That work will be occurring while the dragline is removing overburden. Let’s start out by looking at a completed installation which is in operation. In the following video clip (0:35), you will see the dragline dumping the phosphate matrix into a constructed “pit.” There, the monitor blasts the matrix into fine particles. The slurry pours out of the pit, through the iron grate that is designed to keep large chunks from passing, and into a sump where the slurry is drawn into the pump and transported into the slurry pipeline that goes back to the plant. The video clip is 35 seconds in length, but actually represents about 10 minutes of actual work. In other words, you are seeing it in fast motion!

Let’s look at this construction in more detail. Looking at the next picture, we have a hole that has been dug, and they call it a “pit.” The pit is bisected by the steel grate, which they refer to as the “pit trap.” To the left of the grate, we have the hydraulic monitor, and this is where the dragline deposits the phosphate matrix. On the right side of the grate, we have the slurry intake pipe. In this photo, the intake has been lifted above the water line. Normally, the intake is submerged below the water line.

Figure 9.5.16: "Pit" showing slurry intake, "pit gun," and the "trap."

Source: The Mosaic Company

In this next photo, we can see more clearly the construction of the pit and pit trap.

Figure 9.5.17: Pit trap construction

Source: The Mosaic Company

The next step in the construction of the pit is to install the monitor. Small dozers are used to construct the pit and to do the grading around it. In this next photo, look to the right of the worker. Do you see the specially outfitted tractor with the winch/crane assembly? This is what they use to move and position the monitors, pit traps, pumps, and related assemblies.

Figure 9.5.18: Installation of the "pit gun"

Source: The Mosaic Company

The next picture provides a wide-angle view of the pit and related equipment.

Figure 9.5.19: Complete view of the monitor in operation

Source: The Mosaic Company

Now, with this understanding of the individual components, let’s watch a video (8:57) showing the construction and setup of a new pit. Note all the related activities in the background.

Of course, as mining advances, it becomes necessary to move the pit to the next location. This is a process accomplished over a shift. The next video, which is about 6 minutes in length, captures a complete move over a nine-hour period. Fasten your seatbelts!

The design of a slurry transport system is covered in our materials handling course, MNG 404. Here, it is worth noting a few defining characteristics of the system used at the Mosaic mines.

• 2,000 tph capacity
• 1,500 hp or greater pump motors
• up to 62” diameter pump impellers
• pipelines up to 10 mi long
  • 20” diameter pipe
  • pumps every 1 mi
• slurry
  • 35% solids by weight w/ up to 6” – 8“ size fraction

Finally, we can take a look at the placement of overburden into a previously mined cut.

Reclamation is the final part of the sequence of operations, and as you know, it is an ongoing activity. Mosaic does a stellar job of reclaiming the land, and given the growing conditions in Florida, within months after mining has been completed, the area appears undisturbed and back to its original appearance. Actually, that is not always true – often the reclaimed area is suitable for a higher purpose than the overgrown scrub areas that preceded mining. On their website, they discuss the reclamation activities in more detail.

Figure 9.5.20: Reclaimed land in a Mosaic promotional photo

Source: The Mosaic Company

With that, our case study and the module on surface mining comes to an end!