PS

*riḥay- - hand-mill, millstone (LS 397; Fronzaroli 1972:633; HALOT 1216)

Akkadian
erû, irû - grinding slab, hand-mill (AHw. 247; CAD E 323)

It's hard to explain the initial e-. One wonders if the word is related to the Sum. e₂-ar₃-ar₃ (Attinger, Glossaire 415). 

Ugaritic
rḥm - (du.) millstone (DUL 724)
Hebrew
rēḥayim - (du.) millstone (BDB 932; HALOT 1216)
Syriac
raḥyā - mill, mill-house, millstone (LSyr.:723; SL 1455)
Arabic
raḥan - mill, millstone (Lane 1057)
Mehri
ərḥā - hand-mill (ML 322)
Jibbali
ráḥaˀ - hand-mill
Harsusi
reḥáˀ - hand-mill (HL 104)
Soqotri
rᵉḥa - rotating hand-mill (two slabs on an axis) (LS 397; CSOL II 564)