Question

A nitrate ore containing 40 wt% nitrate and 60 wt% inert solids is to be extracted in a multistage counter-current leaching unit. The ore is to be processed at a rate of 1,000 kg h-1 . A 97 per cent recovery of nitrate is required. This is to be in the form of an aqueous extract containing 20 wt% nitrate. The sludge leaving each unit will contain 2 kg water per kg of inert solids. Determine: (a) The feed rate of fresh water to the leaching unit (b) The number of theoretical stages needed to perform the required duty

Answer 1

part a) it is the system of feed of ore + fresh water= Aqueous extract + sludge. Take basis of one hour operation. Then Feed rate (F) =1000 and w kg is fresh water added .It is mentioned in the problem that the sludge leaving each unit will contain 2 kg water per kg of inert solids. The flow is countercurrent. In first unit feed enters and underflow sludge is removed completely ( no solids go into aqoueous phase). so, the weight of sludge will be weight of inert solids(600 kg) + weight of water in it(2*600=1200 kg) + weight of nitrate(0.03*400)= 1812 kg

now, weight of aqueous phase= weight of nitrate(0.97*400=388) +weight of water (0.8* (388+(w-1200))

now applying total balance,

feed =1000

sludge=1812

fresh water=w

aqoueous phase=0.8*w+1037.6

1000+w=1812+0.8*w+1037.6

w=9248 kg per hour

part b will calculated if equilibrium data is given. graphically, no. of stages will be calculated