  We investigate mass ejection from accretion disks formed in mergers of black
holes (BHs) and neutron stars (NSs). The third observing run of the LIGO/Virgo
interferometers provided BH-NS candidate events that yielded no electromagnetic
(EM) counterparts. The broad range of disk configurations expected from BH-NS
mergers motivates a thorough exploration of parameter space to improve EM
signal predictions. Here we conduct 27 high-resolution, axisymmetric, long-term
hydrodynamic simulations of the viscous evolution of BH accretion disks that
include neutrino emission/absorption effects and post-processing with a nuclear
reaction network. In the absence of magnetic fields, these simulations provide
a lower-limit to the fraction of the initial disk mass ejected. We find a
nearly linear inverse dependence of this fraction on disk compactness (BH mass
over initial disk radius). The dependence is related to the fraction of the
disk mass accreted before the outflow is launched, which depends on the disk
position relative to the innermost stable circular orbit. We also characterize
a trend of decreasing ejected fraction and decreasing lanthanide/actinide
content with increasing disk mass at fixed BH mass. This trend results from a
longer time to reach weak freezout and an increasingly dominant role of
neutrino absorption at higher disk masses. We estimate the radioactive
luminosity from the disk outflow alone available to power kilonovae over the
range of configurations studied, finding a spread of two orders of magnitude.
For most of the BH-NS parameter space, the disk outflow contribution is well
below the kilonova mass upper limits for GW190814.
