Mac - I would strongly suggest that you hold on to your money for the time being. You have lenses that cover the range of focal lengths that are most often suggested for portraiture. That range, just to pull everything into focus (pun intended) is 31mm to about 85mm for a camera (like yours) with an APS-C sized sensor. 31mm is considered the "normal" focal length for that size sensor, and 85mm is roughly a 3X telephoto. The most-often recommended focal length for conventional portraits (waist up) would be 50mm.
Use the lenses you have for now. They won't cost you a thing. Your 28-75 would probably do everything you need. If, after doing some work with your current lens(es), you determine that you almost always use a particular focal length then you might consider getting a prime (non-zoom) lens for that type of work. If the 28-75 produces adequate sharpness, color, contrast, etc., then you really wouldn't need another lens. I use my Canon 24-70mm f2.8 for portraits and don't see any reason to change other than the appearance of the lens (BIG) to my subjects.
You could put your money into lighting equipment and be much further ahead than buying lenses that are in the focal length range that you already cover.