  We investigate the circumstellar dust properties of the oxygen-rich bipolar
proto-planetary nebula IRAS 18276-1431 by means of two-dimensional radiative
transfer simulations of the circumstellar dust shell. The model geometry is
assumed to have a torus and an envelope. The parameters of the dust and the
dust shell are constrained by comparing the SED and NIR intensity and
polarisation data with the models. The polarisation in the envelope reaches 50
-- 60 % and is nearly constant in the H and K_S bands in the observations. This
weak wavelength dependence of the polarisation can be reproduced with a grain
size distribution function for the torus: 0.05 micron <= a with
n(a)=a^{-(p=5.5)}exp(-a/{a_c=0.3 micron}). The power index p is significantly
steeper than that for interstellar dust. Similar results have also been found
in some other PPNs and suggest that mechanisms that grind down large particles
may also have acted when the dust particles formed. The spectral opacity index
beta is found to be 0.6+/-0.5 from the millimeter fluxes. This low value
indicates the presence of large dust grains in the torus. We discuss two
possible dust models for the torus. One has a size distribution function of 1.0
micron <= a <= a_max=5,000.0 micron with n(a)=a^{-(p=2.5)} and the other is 1.0
micron <= a <= a_max=10,000.0 micron with n(a)=a^{-(p=3.5)}. The former has
beta of 0.633, but we are not able to find reasonable geometry parameters to
fit the SED in the infrared. The latter has beta of 1.12, but reproduces the
SED better over a wide wavelength range. With this dust model, the geometric
parameters are estimated as follows: the inner and outer radii are 30 AU and
1000 AU and the torus mass is 3.0 M_sun. Assuming an expansion velocity of 15
kms^{-1}, the torus formation time and mass-loss rate are found to be \sim300
yrs and \sim10^{-2}M_sun yr^{-1} respectively.
