Bauxite Part II,

A typical bauxite mining operation starts by locating a deposit and then digging a series of evenly spaced bore holes a few meters deep in a grid with holes spaced approximately one hundred meters or so apart. This is done to effectively map the area where the bauxite is located, so they only mine the bauxite without disturbing any non bauxite containing areas.

Then topsoil is removed (generally 15-30 cm in thickness) and retained. Then below that there is generally a gravel type sub soil 20-80 cm thick. This is removed and retained seperately, exposing the Bauxite bed.

That bed is then surface mined removing the Bauxite rock layer. This layer is crushed to a small size, further milled to an even smaller size and sent to the refinery for processing into ATH, ceramic materials and Alumina which can be used as is or further processed into Aluminum metal at an aluminum smelter.

Upon exhausting the deposit the sides of the pit are smoothed out and any large rock debris is buried. Then they break up the floor of the pit that has been packed by the heavy mining equipment used in the operation. This is done to allow good water drainage and moisture penetration into the remaining sub layers of rock and soil.

The gravelly subsoil is then replaced and smoothed out (landscaped for lack of a better word) followed by the replacement of the topsoil. This is smoothed out (landscaped) to increase water retention and decrease erosion.

The topsoil is then seeded with a mixture of many species (40-60) to insure future plant viability and propagation. Fertilizer is also applied. The site is monitored for a year or so to insure the vegetation has taken root, or remedial seeding is undertaken until they do take root.

Those areas generally will not support forest growth due to the shallow layer of topsoil found over most bauxite deposits. They are generally planted as grassland which can be used for grazing cattle, or later converted to agriculture. Oh I forgot to add, any mining roads to the bauxite mining operation are restored by a process similar to the one above.

Can anyone elucidate me with respect to the remediation and return to nature of a granite quarry? I would be fascinated to hear the process.

#1. I thought ATH and Aluminium were the same. How does the process get further refined into Aluminum?

#2. I can't believe they are that enviro-conscious. That's for realsies?

Posted By Tom M on 01 Nov 2010 03:21 PM
#1. I thought ATH and Aluminium were the same. How does the process get further refined into Aluminum?

#2. I can't believe they are that enviro-conscious. That's for realsies?

Tom,

I hope it's OK to post this here. The bauxite seemed to fit well in this thread but now we are getting into ATH, which is still a processed mineral and Aluminum which last time I checked is still a metal. If anyone thinks we should move this to another thread, please advise. I dont want to take up valuble stone fabrication resources that Andy worked so diligently to provide.

First of all I'm not an Aluminum expert, but years ago I tried to learn everything about the process since ATH is integral to products I have designed and manufactured over the last 22 years. I was fortunate to have great mentors as former directors and people in R&D at resin plants and Alumina refinerys who taught me alot.  I'm going from memory here, so please dont stone the messenger if I leave something out!.

Let me answer question number 2 first. Yes, that’s the real deal! That’s how it’s done in the US, the EU, Australia and other ones I have seen in Asia. It adds some incremental cost, but not much.

For question number 1, I am going to give you the short version. If you need more detail just ask. I have been to refineries in the EU, and also to one of the CHALCO refineries in Henan province China. That refinery was huge. CHALCO (Aluminum Company of China) is the 2nd largest producer of Aluminum in the world.

Oh here is a factoid for you. China is challenged by power outages. The economic growth over here has been staggering. The 3 Rivers Gorge Dam and other hydroelectric projects are complete and generating power. It took several years for the Dam to fill to capacity and they haven’t completed the entire infrastructure to effectively distribute that power. So they ration power to the factories in the form of 1-2 power out days a week and let the general populace enjoy power 24-7 in their homes.

To remedy that situation, the owners of that particular CHALCO branch built their own power plant on their own dime so they can run 24-7. In addition to being the single largest employer in the region they also supply power to the neighboring town. How cool is that? Everyone in that city loves that company! Talk about industry cooperating directly with the local community!

Most of the Bauxite is refined into a refractory grade for aluminum metal production. The remaining (im guessing single digit percentage) is further refined into chemical grade ATH for making ultra pure Aluminum metal or very pure ATH, ceramics etc.

BTW here is another factoid; did you know that one of ATHs uses is as a soft abrasive in some toothpaste? This is due to its hardness which is about 2.5 on the Moh hardness scale. That scale is useful in rating relative mineral and metal hardness. Moh Hardness scale of some common items for reference: (don’t hold me to this, I am going from memory).

Fingernail = 2.5 Moh Hardness
ATH = 2.5 Moh Hardness
Copper Penny (OK it’s mostly zinc) = 3 Moh hardness
Knife steel- = 5.5  Moh hardness
File = 6 Moh hardness
Quartz = about 7 Moh Hardness
Diamond = 10 Moh Hardness

The process overview is that bauxite gets refined into ATH, which is refined into Alumina, which is smelted into Aluminum metal. Im going to leave out the chemical formula for each step so it doesnt get too technical and turn into a snooze fest, but if you want those please don't hesitate to ask.

The crushed bauxite is transported to the refinery. Then it undergoes the Bayer process. There it is crushed to a very fine size in a ball mill, then digested in hot caustic (sodium hydroxide or NaOH and steam) to dissolve the aluminum. The remaining iron oxides which are insoluble are removed via settling and filtration. I used to be a filtration chemist at Brunswick Corp. prior to my time in surfacing manufacturing, so this step brought back old memories .

This insoluble residue is what they call "red mud". The red mud is mostly iron oxides which give the mud its red color, like rust, a common iron oxide which is red in color. They thicken that red mud by evaporation and then wash the mud to recover the caustic (NaOH) which is reused in the process. It also neutralizes the red mud.

For example Bauxite comes in 3 main forms: Gibbsite, Boehmite and Diaspore. The Aluminum companies prefer ore high in Gibbsite (which is basically ATH) because the yields are extremely high and it can be dissolved at a lower temperature reducing energy cost. Also at the lower temperatures silicates are practically insoluble and can also be filtered out. Silica control is crucial and ores with a high silica content cannot be processed so are not mined or they are sorted out.

The diaspore has to go through another process at higher temperatures and pressure, with another chemical reaction to remove the silica which is soluble at those higher temperatures.

They transfer the filtered mixture (which in chemical terms is called a saturated solution) to a precipitator. They add pure ATH crystals to serve as sites for nucleation and cool the solution (making it a super saturated solution) and pure ATH crystals settle out. Then they classify the ATH by size. That ATH can be washed, dried and used in Solid surface or ATH can be further refined in the sinter process which yeilds a very pure and white ATH. Most however goes to the next step called the Calcination Process which produces alumina for making aluminum metal.

Tom, did you ever make hard candy when you were a child? In layman’s terms the Bayer Process is a bit like making hard rock candy from sugar with sand in it. You would boil the sugar in water and keep adding sugar until no more would dissolve (saturated solution) and run it through a coffee filter trapping the insoluble sand. Then you would pour the hot filtered mixture into a jar and let it cool a bit (super saturated solution). Then you would wet a thread and roll it granulated sugar (seed crystals or sites for nucleation) and suspend that thread in the mixture to a depth of about 1 inch from the bottom of the jar. The solution would cool and the sugar would form as crystals on the thread which would grow in size yielding hard rock candy within 24 hours. Yummy!

In the Calcination process, they put the ATH into a kiln  at a temperature of 1100 C to remove the water and make alumina. The alumina then proceeds onto a smelter.

At the smelter they melt the alumina at over 2000 degrees C and then pull the aluminum atom off the oxygen atom via electrolysis. Then they dose in other elements (magnesium, manganese, silicon etc.) to make aluminum alloys with various physical properties for different applications. They cast that into ingots (huge ones) then anneal it (heat treat it) and hot roll it. Then they cold roll it below 550 degrees C or so down to a fraction of a millimeter in gage thickness and roll it onto a huge coil. (maybe 8 feet in diameter). The rolling speeds are extremely fast (more than 20 feet per second)! That coil is then further surface treated for various applications.

For transportation cost reduction the refinery is often onsite with a smelter, or very close to one. And both of those are located near the mining region.

Sorry to be so verbose, but believe it or not that is the short overview. I could write a very long and boring book about this.

Tom, my good friend, if you need any more details on any part of the process please don’t hesitate to ask.

Sensei,
So aluminum and aluminium are the same?

Most granite blocks are split out of the mountain by drilling holes in a line then filling them with hydraulic cement that expands as it hardens which splits the block out. Then its tipped over onto a giant air pillow.

Softer granites and marbles are extracted with diamond wire like explained earlier.