It's supposed to be counter-productive, just like all Combiner recipes are. However, it allows you to turn a bit of flint and cobble into gravel if you have none around.
I was referring to the Purification Chamber recipe. I thought your intent was to have a means of making gravel to turn into flint to make flint a renewable resource 'in house'.
I really like your idea of a by-product. However, the reason for which differs slightly from yours. For T4 ore processing, you're still going to need to create the Chemical Injection Chamber and all the other machines, just as T3 will still require the Purification Chamber, and as T2 currently still requires the Enrichment Chamber. I do, however, think there should definitely be some form of byproduct for T4 that would allow you to make some form of other very high end machine.
Perhaps it can be used as a fuel or amplifier for the process of the higher-end machines? For example, the IC2 UU Matter creation process takes far less energy to run if it has scrap. That kind of thing.
After all, if this is a byproduct from your primary ore refining process, you're going to end up with a LOT of it very rapidly, so it behooves you to give the player something to constantly spend it on. That implies an ongoing, rather than static, cost. Some random ideas:
* This byproduct is used as a fuel for a higher-tier generator.
* This byproduct is used to create a consumable 'amplifier' which are put in upgrade slots. Inserting multiple amplifiers extends duration, but does not stack effect (or perhaps will sit in the upgrade slot until the time for the last one runs out, then a new one is consumed, much like a fuel slot). There are several types of amplifiers:
- Speed Amplifier. For (balance check) ticks, the machine runs faster, as though it had one additional speed upgrade
- Efficiency Amplifier. For (balance check) ticks, the machine runs using x% (5? 10?) less energy consumed
- Productivity Amplifier. For (balance check) ticks, the machine has an x% chance of producing a bonus resource of the same type it would normally produce. Possibly hard/impossible/OP to implement?
* This byproduct is used to create a material which is desirable for higher-tier machines. Possible requirement for upgrading to Elite Factories? Used for the Digital Miner?
* This byproduct is consumed in the machine to produce 400% output, much like Flint is used in the Purification Chamber.
Again, just throwing out some ideas