  The recent ALMA observations of the disc surrounding HL Tau reveal a very
complex dust spatial distribution. We present a radiative transfer model
accounting for the observed gaps and bright rings as well as radial changes of
the emissivity index. We find that the dust density is depleted by at least a
factor 10 in the main gaps compared to the surrounding rings. Ring masses range
from 10-100 M$_{\oplus}$ in dust, and, we find that each of the deepest gaps is
consistent with the removal of up to 40 M$_{\oplus}$ of dust. If this material
has accumulated into rocky bodies, these would be close to the point of runaway
gas accretion. Our model indicates that the outermost ring is depleted in
millimetre grains compared to the central rings. This suggests faster grain
growth in the central regions and/or radial migration of the larger grains. The
morphology of the gaps observed by ALMA - well separated and showing a high
degree of contrast with the bright rings over all azimuths - indicates that the
millimetre dust disc is geometrically thin (scale height $\approx$ 1 au at 100
au) and that a large amount of settling of large grains has already occurred.
Assuming a standard dust settling model, we find that the observations are
consistent with a turbulent viscosity coefficient of a few $10^{-4}$. We
estimate the gas/dust ratio in this thin layer to be of the order of 5 if the
initial ratio is 100. The HCO$^+$ and CO emission is consistent with gas in
Keplerian motion around a 1.7 $M_\odot$ star at radii from $\leq 10 - 120\,$au.
