Can KKT be used :
min g(x)
s.t x>=constant
where constant > 0
I have read this
The Kuhn-Tucker method: here says that
This is an alternative, and slightly simpler method for dealing with the common case where there are positivity constraints, that is constraints of the form xi ≥ 0 or − xi ≤ 0.
Minimizing $g(x)$ is equivalent to maximizing $-g(x)$. So, you can restate your initial problem as
$$ \max_{x} -g(x) \quad s.t. \; x \ge a \quad ,\text{ where } a \text{ is some constant}$$
and use the standard KKT method.