The bottom stream from the FCC has a number of different names. For your HCO to have an ash content of 0.16% by weight makes me believe that this is the stream out of the bottom of the FCC fractionator. This is sometimes called slurry or main column bottoms (MCB).
Typically, the FCC slurry is processed through a Delayed Coker at up to 10% of the total feed. Normally, the ash content of the vacuum tower bottoms controls the ash content in the petroleum coke. Blending in FCC slurry into the feed may not impact the ash content of the coke, depending on the ash content of the vacuum resid. If you are attempting to make petroleum coke with an ash content of less than 0.25% by weight, the combined feed (vacuum resid and FCC slurry) would need to have a combined ash content of approximately less than 0.08% by weight. If you assume a 30% by weight coke yield and all the ash ends up in the coke a 0.08% ash in the feed results in a 0.24% ash in the coke. If the desalters in the crude distillation unit are working well, a large portion of the solids that become ash in the petroleum coke are removed in the desalter brine.
A concern that is sometimes raised with the solids in FCC slurry that is processed through a Delayed Coker is erosion of the heater return bends or in the piping between the heater and the coke drums. I have not seen erosion caused by the solids in FCC slurry in these areas. If the solids loading in the slurry is higher than normal and the particle sizes are larger than normal, erosion could occur at the very high velocity portions of the fired heater in the return bends. Those same areas are exposed to high erosion rates during on-line spalling or steam-air decoking. I think the probability of erosion from the solids in the FCC slurry causing any significant metal loss is low.
Fuel grade petroleum coke can be sold with an ash content of 1% by weight and higher. There are many factors that contribute to the limits of ash in the feed to the Delayed Coker. Many Delayed Cokers process FCC slurry as part of the feed at similar ash content values.