There is quite a bit to unpack here, so I may need to deconstruct your question a bit. Your question is surrounding the fouling rate of a Shell & Tube (S&T) heat exchanger which I will get to, but there are a few things with respect to the RO that must be addressed.
If you have sand in the feedwater to your RO, you have much bigger issues than heat exchanger fouling. Reverse Osmosis is NOT a coarse filtration technology, but is designed as a process to concentration particles in solution. General rules for RO pretreatment; the feed stream needs to be low turbidity and free from larger particles. This pre-treatment is usually accomplished with ultra-filtration (sub 1 μm filtration). While making a hard cut-off for turbidity is a little nebulous, RO feedwater usually needs to be less than 5 on the Silt-Density Index (SDI). If you're designing or working with an RO system, these sound like red flags and should be taken care of. If there is sand in the feed stream, that needs to be filtered out either by settling or by something like large scale sand filters (ironically filter beds made of sand). Now, it sounds like this concentrate is your process stream, so I don't know how much control over that you have, or if that is feasible but it needed to be brought up.
As for your questions about fouling, You're probably right in assuming that there will be a difference between chemical scaling & sediment scaling. All things being considered, it will still likely make much more sense to remove the sand before entering the heat exchanger. If that is not possible, then you'll want to ensure to keep your velocity high (6-10 ft/s or 2-3 m/s) inside the tubes and keep the number of passes low to prevent the sand from settling out. This would effectively eliminate the sand deposition as a fouling issue.
Your assumption about chemical deposition is correct; the most common foulant in RO reject is Calcium/Magnesium, especially since they have retrograde solubility (solubility decreases with increasing temperature). This is commonly known as calcium scale and is most likely what you will deal with, especially if you are increasing the temperature of your process stream in the heat exchanger. As far as fouling rates, there are many models of calcium deposition, and your search should probably include a model that utilizes pH, concentration vs solubility limit & temperature for starters. The caveat here is that you need to have an actual analysis of the feed solutes to determine what may cause you issues, as calcium may not be the only foulant in your feed stream.
How you would tackle calcium deposition in application? Lower the pH, decrease the temperature, adding a solubility modifier / scale prevention chemical, and likely not running the RO at a reduced recovery to prevent reaching the solubility limit.