This will hopefully be a brief clarification regarding the empirical and its relation to the moral law.
Kant writes of the determination of the moral law being due to a pure (non-empirical) principle. However, the empirical is related to the moral law so far as it is necessary to include empirical concepts in ones maxim (e.g., lying is a concept from experience, and with apparent meaning only in experience).
Moral laws can even contain consequences - as long as these consequences are contained in the maxim being judged.
Kant speaks of the principle of the determination of law being pure with regard to how the maxim (which contains empirical concepts, and perhaps consequences) is put in the form of universal law giving (act only according to that maxim which, etc). That the principle used in judging the moral law is pure means that this universal law giving does not rest on experience or derive from it.
The result of the judgment that produces a moral law from a maxim has characteristics that point back to the pure principle. The necessity of the law, and its categorical (non-consequential) character both relate back to the purity of the judgment - despite the judgment containing empirical content left over from the maxim that was judged.