Dust rings: Not possible during mining.
There are 2 theories of how the dust rings around Saturn and Uranus were formed:
Millions of years ago the planet had a large moon orbiting around it. It got destroyed and the remains are now the dust and larger particles in the rings
They date back to the formation of the planet, when a heap of dust particles started to coalesce in one location and started rotating. Stuff rotated fast enough that not everything got pulled into the emerging planet and instead stayed as a disk around it.
The only reason there are rings instead of a dust shell is that either there was something orbiting the planet, or that the planet's rotation during creation is responsible for it. Since you've got a lone asteroid, neither of these cases can be fulfilled. After all, I very much doubt that all your mining stations keep to the same equatorial orbit. Especially if they are trailing debris behind them in their orbit - it'd get much too dangerous much too quickly.
(why have the processing station in orbit? What do they do with the unwanted rocks after they got all the ores? Dump down to Ceres? That would be very energy efficient - it's much cheaper to have the processing station on planet due to the fuel requirements to carry all the raw materials up to orbit)
(where is the equatorial orbit on an object with no rotation? Or does Ceres rotate?)
Yes, the Kessler Syndrome as mentioned in a comment, will eventually force debris to condense around an equatorial ring, but for that you would need a lot of debris at the same height. On Earth, that would be geosynchronous or low earth orbit, because we deliberately shoot stuff up there. On your asteroid, mining would not be as precise - as a result your debris will be distributed much more evenly and thus take a lot longer for the Kessler Syndrome to take effect.
(Not all rock and dust thrown up by your mining operation gets exactly the velocity and -vector needed to settle in an orbit. Only a tiny fraction of it does that, the rest either escapes to space or falls down to the asteroid.).
Dust shell: Maybe.
A dust shell is more in the realms of the possible. Compare it to the space debris around earth - it is orbiting in more or less stable circles, but those circles are angled in every direction at any height.
In case your asteroid is part of an asteroid field, however, it is very likely that your dust shell will be depleted by other asteroids passing closeby and catching some of the debris in their own gravitational field. Depending on how dense your field is, it might prevent a dust shell from forming entirely.
Size of the dust shell
With the Orbital Velocity Calculator, it means that if your ships can manage 500m/s you'd have your Low Ceres Orbit at around 270km. Slower orbital speeds means a higher orbit, larger orbital speeds means a lower orbit. That gives you a shell around Ceres starting at around 270km radius (most particles) and stretching out to several 10000km radius (those that barely manage escape velocity).
Visibility of dust shell
A shell starting at 270km height should be visible (Ceres' diameter is only about 900km, and it has no atmosphere to block visibility). However, you will probably not see a lot.
If you take the entire mass that you need for visible dust rings and spread it out to a sphere, not much will be left.
Maybe, if you view Ceres against a bright object, you might detect some distortion around Ceres. Or, if the debris is highly reflective, you might notice it when sunlight reflects of stuff where Ceres' dark side is supposed to be.
Easy Visibility? no.
Visibility of dust rings
If you just can't live without dust rings, you could do comparisons to Saturn.
The mass of Saturn's rings is estimated at 1.5*10^19kg. Saturn's mass at 5.7*10^26kg is about 7 orders of magnitude bigger. For Ceres (9 * 10^20kg) the mass of similarly visible rings would be 2.5*10^13kg (ignoring potential square-cube laws between ring mass and planet mass).
Assume that only 1 in 1000 debris particles from your mining operation receives a stable orbit vector (it's probably a lot lower still), and you will need to produce 2.5*10^16kg debris to get your Saturn-like rings. That is a measurable fraction of the planet's entire weight! (1/40000).
Not to mention that it's turned absolutely lethal for all orbiting processing stations a long time before that. Earth has 'only' a couple million objects larger than 10cm in orbit, and it's already considered crowded in LEO and GEO.