Should I mention industrial experience in an academic CV when applying for a PhD?
As the fellows said, you need to declare something about that rather long gap. Because, the admission committee would be disappointed in a typical applicant with a considerable time interval without any scientific or technical activity.

Furthermore, I would like to pinpoint that the Ph.D. projects would be, roughly, divided into two large categories. One would be pure-research projects, are which supposed to be traced as the cutting-edge study and engineering. The next one would be industrial projects (considerably common in european positions), are which stemming from the desire for new ideas, but are different with the former case in that the mental background from the context of the project would be really functional and precious. One might claim that the industrial experience could be accounted as such background.

Just do it, then!