=mining =heavy equipment
When moving some soft material, such as dirt or coal, it's possible to simply scoop it up with a hydraulic excavator. Doing that on the surface is the cheapest form of mining. But most rocks are too hard to simply scoop, so they must be broken up first. Historically, the main approach has been "drill-and-blast" - drilling holes, filling them with explosive, and blasting. Here are videos of that on the surface and underground. Much of the fracturing happens where shock waves cross, because tensile stress is generated after compressive waves cross and rock has low tensile strength.
With relatively soft rocks, it's
cheaper to use a rotating thing with picks on it to break it up than to
drill and blast.
Here's a video of
longwall mining
with a rotating cutter. Typically, picks use a surface layer of cemented
tungsten carbide, which is hard, but it still gets worn down. A single pick
can produce >10k tons of coal, but might only produce 20 tons of hard rock,
which makes blasting cheaper.
With medium-hardness rock, it's
about equally expensive and fast to use drill-and-blast or a
roadheader. In
populated areas, there are often regulations about explosive use that
mandate roadheader usage even when it's not economically preferable.
When tunneling through very hard rock in places with regulations banning
explosive usage, that's when tunnel boring machines (TBMs) may be used.
They're generally more expensive than other options. Modern TBMs generally
use disc cutters,
which exert enough force on the rock to crush it. Compared to conical picks,
that's less energy-efficient and requires more force, but the disc cutters
last much longer. Elon Musk's
Boring Company
has focused on TBMs because they seemed like the most modern type of system,
but their proposed "improvements" haven't worked out so far.
This is all well-known, and it's
here as background information for what I really want to talk about, which
is hydraulic breakers and vibro rippers.
A
hydraulic breaker
is basically a hammer and chisel, with a very heavy hammer pushed forward by
hydraulics. There's something that appeals to me on an aesthetic level about
that: millenia ago, humans mined with hammer and chisel, and now we can
instead use a big robot with a big hammer and big chisel.
In the US
and Europe, they're used in some small quarries, which exist because
trucking is often more expensive than mining, but larger quarries generally
use explosives. For underground mining, hydraulic breakers have been used
extensively in China, but in the US, roadheaders are
strongly preferred. Why is that?
If you just look at the machines and
consumables, hydraulic breakers are generally cheaper than roadheaders, but
they're also slower. The reason they're used in China more than the US is
labor costs, which are more than half the net cost of using them
for tunneling in the US.
So, the problems of hydraulic breakers are, in order:
- speed
-
labor cost
- fuel cost
- hydraulic oil cost
The obvious solutions are:
- use more
- automation
- use electricity
- electromechanical systems
Diesel fuel is more expensive
than electricity, and the pollution is a problem, especially underground.
I'm a big fan of electrification of heavy machinery. I also think that
central hydraulic systems are obsolete - that
electrohydrostatic
or electromechanical actuators are now always a better option - and
companies like Caterpillar just don't know it yet.
Regarding speed,
vibro
rippers can be a few times as fast as hydraulic breakers for softer
rocks, and I think there's still some room for improvement in operating
principles of such machines, since this topic hasn't had as much thought put
into it as, say, nuclear weapons.
The biggest difficulty here is
automation, which is basically a software problem.
Another example of returning to system types used before mechanization of mining is rail conveyor systems, which are starting to replace trucks and conveyor belts. Conveyor belts are cheaper to operate than trucks over any distance, and rail is more efficient and cheaper to install than conveyor belts. Centuries ago, humans pushed mine carts around, and now trucks are getting replaced by automated mine carts. This reversion makes some sense because human labor is energy-limited, energy efficiency is now becoming more important than capital costs of machinery, and microcontrollers make more complex control schemes possible.