Ok, crappy picture, sorry, battery was dying on the camera and the tank lights had already gone off when I remembered to take this photo .. but here is a basic overview. When the time comes that I need to clean out the kalk reactor I'll try to remember to get some closeups of the design.
It's a DIY mixer but I didn't make it, another Canreefer did and I just lazily bought it off him. So it's a DIYBBSE (do-it-yourself-but-by-someone-else).
RO/DI enters from my main 50g RO/DI reservoir into the smaller bucket you see in the back. There's just a humidifier float valve in the bucket so that it doesn't overflow. The dosing pump (piston style) sits on top of the bucket and lifts water from the bucket and pushes it through the reactor. This style of pump will pump against a head pressure of up to 150psi so even a kalk clog in the reactor output shouldn't slow down the process. I bought this pump off another Canreefer, every now and again a dosing pump will come up for sale.
Anyhow RO/DI is pumped at a rate of about 1 drop per second into the kalk reactor. I just trial-and-errored the flowrate to make sure I don't add kalk into the tank at a rate that exceeds evaporation.
The kalk reactor is an 4" acrylic tube with a powerhead attached for mixing. The powerhead is on a timer, runs for about 1/2 hour per day (I just use a mechanical timer, I could run it for less time but I'd have to use an electronic timer instead and I'm too cheap, this works fine). So every week I add a few tablespoons of kalk powder to the reactor, the powerhead keeps it mixed up. There is more powder added than can be dissolved at once, so there is constantly kalk powder at the bottom. This keeps the liquid at a supersaturated state so that it's not really getting diluted as more RO/DI gets added.
I used to just use a powerhead to feed the RO/DI and a needlevalve on the output to throttle back the flow, this worked OK except the needlevalve was prone to clogging and needed to be cleaned every couple of days.
I also used to just have the reactor inline on the float valve on the sump. Again the weakness was that the valve was prone to clogging, also, had to remember to take it offline when doing water changes otherwise it would dump a lot of kalk into the tank when the water level was down in the sump.