Patrick, using the simple principle of solubility and the common ion effect, I have explained to you how you can either concentrate an impurity in the mother liquor or in the solid. It is that simple. To do what you are describing, you must know what you are doing. If I cannot explaine some simple facts of chemistry to you, then I'm sorry.
You have to know what you are about, before you undertake it.
And, I said that what I had given was hypothetical, but nonetheless, your procedure as outlined will concentrate all halide and phosphate impurities along with other soluable sodium salts in the concentrated Borax solution.
I'm sorry, but that is a chemical fact.