Tank leaching

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Tank leaching can be performed as a batch process but is normally operated as a continuous process. Duration of the leaching operation can be from hours up to 5 days, the latter longer retention time is a normal treatment time when refractory gold concentrates are bioleached. Leaching is usually carried out in a series of tanks. In a continuous stirred tank reactor (CSTR) series leaching is co-current, i.e. a pulp of ore/concentrate and leaching reagents are pumped into the first reactor and then allowed to overflow to the following tanks in the reactor series. In the case of sulphide leaching as in bioleaching operations aeration is needed and is accomplished by blowing air under the stirrer. Tank leaching allows for good process control and contact between leaching reagent and material to be leached which therefore results in faster leaching kinetics and better recoveries. Examples of process parameters that can be controlled in tank leaching are:
Tank leaching can be performed as a batch process but is normally operated as a continuous process. Duration of the leaching operation can be from hours up to 5 days, the latter longer retention time is a normal treatment time when refractory gold concentrates are bioleached. Leaching is usually carried out in a series of tanks. In a continuous stirred tank reactor (CSTR) series leaching is co-current, i.e. a pulp of ore/concentrate and leaching reagents are pumped into the first reactor and then allowed to overflow to the following tanks in the reactor series. In the case of sulphide leaching as in bioleaching operations aeration is needed and is accomplished by blowing air under the stirrer. Tank leaching allows for good process control and contact between leaching reagent and material to be leached which therefore results in faster leaching kinetics and better recoveries. Examples of process parameters that can be controlled in tank leaching are:
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reagent concentration
+
*reagent concentration
-
pH
+
*pH
-
dissolved oxygen (DO)
+
*dissolved oxygen (DO)
-
temperature
+
*temperature
To obtain good mixing in the reactors propeller stirrers with downward pumping are normally used and to avoid vortex formation baffles with a width of 1/12 tank diameter are placed symmetrically in the reactor, see figure ……
To obtain good mixing in the reactors propeller stirrers with downward pumping are normally used and to avoid vortex formation baffles with a width of 1/12 tank diameter are placed symmetrically in the reactor, see figure ……
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In oxidative leaching processes where air or oxygen is blown into the reactors the stirrer might be a turbine that has a better oxygen mass transfer capacity than a propeller but at the cost of higher energy consumption. In some cases also air-lift reactors (Pachuca tanks) are used.
In oxidative leaching processes where air or oxygen is blown into the reactors the stirrer might be a turbine that has a better oxygen mass transfer capacity than a propeller but at the cost of higher energy consumption. In some cases also air-lift reactors (Pachuca tanks) are used.
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Tank leaching processes require fine ground ores/concentrates and are more costly to operate than “in-situ, heap and vat leaching operations, therefore tank leaching is performed on materials with higher metal values.
+
Tank leaching processes require fine ground ores/concentrates and are more costly to operate than “in-situ, heap and vat leaching operations, therefore tank leaching is performed on materials with higher metal values.
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== Advantages ==
== Advantages ==

Revision as of 12:27, 19 June 2007

Tank leaching, also known as reactor leaching, is one of several leaching methods. The process of dissolving soluble minerals from a solid ore or concentrate within specially designed vessels.

Contents

Procedure

The ore or concentrate is fed as slurry or pulp into the leaching tank where it is mixed with a lixiviant usually an aqueous solution which acts as a solvent. Process conditions such as type of agitation, temperature and pH are carefully controlled in order to achieve the required leaching efficiency.

Tank leaching can be performed as a batch process but is normally operated as a continuous process. Duration of the leaching operation can be from hours up to 5 days, the latter longer retention time is a normal treatment time when refractory gold concentrates are bioleached. Leaching is usually carried out in a series of tanks. In a continuous stirred tank reactor (CSTR) series leaching is co-current, i.e. a pulp of ore/concentrate and leaching reagents are pumped into the first reactor and then allowed to overflow to the following tanks in the reactor series. In the case of sulphide leaching as in bioleaching operations aeration is needed and is accomplished by blowing air under the stirrer. Tank leaching allows for good process control and contact between leaching reagent and material to be leached which therefore results in faster leaching kinetics and better recoveries. Examples of process parameters that can be controlled in tank leaching are:

  • reagent concentration
  • pH
  • dissolved oxygen (DO)
  • temperature

To obtain good mixing in the reactors propeller stirrers with downward pumping are normally used and to avoid vortex formation baffles with a width of 1/12 tank diameter are placed symmetrically in the reactor, see figure ……

In oxidative leaching processes where air or oxygen is blown into the reactors the stirrer might be a turbine that has a better oxygen mass transfer capacity than a propeller but at the cost of higher energy consumption. In some cases also air-lift reactors (Pachuca tanks) are used.

Tank leaching processes require fine ground ores/concentrates and are more costly to operate than “in-situ, heap and vat leaching operations, therefore tank leaching is performed on materials with higher metal values.

Advantages

  • rapid recovery of the desired metal
  • relatively high extraction.


Disadvantages

  • higher capital and operating costs than heap leaching.

Comments on tank bioleaching

Stirred tank bioleaching is mainly applied on refractory gold concentrates with gold concentrations in the range 10-150 g/tonne. In a refractory gold concentrate very fine gold particles are locked into the sulphide matrix and to liberate the gold for subsequent recovery by cyanide leaching the sulphides have to be leached. Alternative pre-treatment methods to bioleaching are roasting or pressure leaching.

Due to the relatively slow bioleaching kinetics, typically 5 days treatment is necessary, the bioleaching tanks have to be very large to obtain a good production capacity. The bioleaching tanks have volumes up to 1300 m3. Therefore the main operation costs are for mixing and air-supply but also cooling of the reactor and neutralisation of the bioleach liquor are major costs in stirred tank bioleaching. In many of the recently started refractory gold treatment plants bioleaching is the preferred technology.


Examples of tank bioleaching operations

Table …..

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