The Alchemy of Graphite

It's not long before new employees at the company ask me why there are so many different grades of graphite. ‘Alchemy’ and ‘Smoke and Mirrors’, I tell them. And that is the theme of this month’s blog, or at least graphite’s version of it.?

The basic ingredients of synthetic graphite are simple: coke with pitch or tar to bind. Unfortunately, that’s where the simplicity ends.?

There are myriad variants of coke, all with different properties, some of which is totally unsuitable to make graphite with. Each producer has their own coke preferences for their graphite. The same is true for the pitch or tar chosen as the binding agent.?

Then there are different grain sizes of the coke required. This often depends on the application of the graphite being produced but, as a rule of thumb, very small grain coke is used in the production of isostatic graphite (produced under immense pressure) with larger grains of coke used in extruded graphite (where the coke/tar mix is pushed through a mould into bars).?

As well as differences in the raw material composition, there’s also differences between the production processes.?

Graphite production is a two-step process. The first of these requires the coke/tar mix to be heated to 800-1000 degrees centigrade for a few weeks. This process is all about enabling the tar to carbonise leaving carbon atoms in a random orientation, The process could stop here because carbon blocks are good for the smelting of aluminium. Of course, we cannot stop here because there are many more applications for graphite. Each producer closely safeguards their own preferred carbon selection, grain size mix, temperature, and timescale for this first step.?

Step 1A is to densify the material and fill all those cavities created as the volatiles in the tar escaped from the mix, leaving a porous structure ripe for low viscosity pitch impregnation. This is an optional process dependent upon the grade of material being manufactured.?

Step 2 is to graphitise the material. This is achieved by re-heating the material, this time to 2000-2800 degrees centigrade. Electricity is whacked into the long stack of carbon bars, their resistance to the passage of the current heating the bars to this high temperature, the thick layer of insulation around the stack of carbon bars retains the temperature, the insulation is so good the temperature is kept high for 8 – 12 weeks. In days gone by, some unfortunate worker gently raked the insulation away from the surface gradually reducing the insulation thickness allowing the carbon/graphite bars to cool at a predetermined rate. This process is now automatically controlled with massive planers improving the control of the rate of cooling massively. No more are my boots melting, I think I have raked enough today! The thermal process enables the carbon atoms to realign from a random structure and gradually form the layers of graphene (that miracle material you may have heard of), each layer weakly connected to the one above and below making graphite as we know it.?

In summary, while you have very basic simple ingredients to make synthetic graphite, the composition of these varies widely, as does the temperatures and periods used to create the final bars. You’re beginning to see why it’s like alchemy.?

?The final complication is that there will still be variants in the graphite’s technical specifications from a single production run. This is due to the position of a bar being produced in the furnace. Bars in the centre of the graphite stock being produced will have a different temperature profile to those on the edges of the stock. This creates differences in the graphitisation of the coke/tar mix, all dependent on where the bar is situated in the production process. This is the smoke and mirrors – so from one batch there are subtle differences in the material created.?

Producers are very good at identifying these differences and grading the material accordingly. While the producers’ ‘alchemy’ results in a large number of different graphite grades across the market, this is very beneficial to graphite consumers due to the very large range. I can’t think of a single occasion where we haven’t been able to find a suitable grade of graphite for a customer order, no matter how specialised the process it is to be used for.??

While the graphite ’alchemy’ doesn’t make gold, it does ensure that there is always a grade that is fit for your particular purpose, and that’s what is valuable to you.?

That’s it for now, see you next time,?

David???