Reaction of the acidic RO concentrate with phosphate rock at a high solid/liquid ratio effectively neutralized the solution and upgraded the rock by decreasing its Ca/P mass ratio. An extensive series of batch leaching experiments with different solid/liquid ratios and leaching times was undetaken using fine-grained natural phosphate rock obtained from a local mine and a synthesized RO concentrate solution. The solution pH increased soon after mixing with the rock for all five solid fractions tested (Figure ). This increase in pH was more marked with higher solid fractions at all neutralization times.