  We present new low-resolution (R~800) optical spectra of 22 Galactic PNe with
Spitzer spectra. These data are combined with recent optical spectroscopic data
available in the literature to construct representative samples of compact (and
presumably young) Galactic disc and bulge PNe with Spitzer spectra. Attending
to the nature of the dust features seen in their Spitzer spectra, Galactic disc
and bulge PNe are classified according to four major dust types (oxygen
chemistry or OC, carbon chemistry or CC, double chemistry or DC, featureless or
F) and subtypes (amorphous and crystalline, and aliphatic and aromatic).
Nebular gas abundances of He, N, O, Ne, S, Cl, and Ar, as well as plasma
parameters (e.g. Ne, Te) are homogeneously derived and we study the median
chemical abundances and nebular properties in Galactic disc and bulge PNe
depending on their Spitzer dust types and subtypes. A comparison of the derived
median abundance patterns with AGB nucleosynthesis predictions show mainly that
i) DC PNe, both with amorphous and crystalline silicates, display
high-metallicity (solar/supra-solar) and the highest He abundances and N/O
ratios, suggesting relatively massive (~3-5 M_sun) hot bottom burning AGB stars
as progenitors; ii) PNe with O-rich and C-rich unevolved dust (amorphous and
aliphatic) seem to evolve from subsolar metallicity (z~0.008) and lower mass
(<3 M_sun) AGB stars; iii) a few O-rich PNe and a significant fraction of
C-rich PNe with more evolved dust (crystalline and aromatic, respectively)
display chemical abundances similar to DC PNe, suggesting that they are related
objects. A comparison of the derived nebular properties with predictions from
models combining the theoretical central star evolution with a simple nebular
model is also presented. Finally, a possible link between the Spitzer dust
properties, chemical abundances, and evolutionary status is discussed.
