TITLE; Recovery of metal values from Hot Gas Precipitator dust (HGP dust) of pyrometallijreical Lead-Zinc extraction plant.
INVENTORS;
1. Mr. D. B. MOHANTY (Central Research & Development Lab., Udaipur)
2. Mr. RUSHABH SHAH (Central Research & Development Lab., Udaipur)
3. Mr. ASHISH KUMAR (Central Research & Development Lab., Udaipur)
4. Mr. M L GUPTA (Central Research & Development Lab., Udaipur)
All are Indian citizen.
ASSIGNEE; HINDUSTAN ZINC LIMITED "YASHAD BHAWAN" UDAlPUR-31300li RAJASTHAN, INDIA
Recovery of metal values from Hot Gas Precipitator dust (HGP dust) of pyrometallurgical Lead-Zinc extraction plant
FIELD OF INVENTION; The present invention relates to a recovery of metal values from by-product generated during pyrometallurgical treatment of lead and Zinc concentrate and particularly relates to dissolution of base metal through oxidizing acid leaching followed by precipitation of dissolved metal to produce high grade material.
BACKGROUND OF THE INVENTION;
Hindustan Zinc Ltd (HZL), a part of Vedanta pic, has a Pyro metallurgical lead and Zinc extraction plant (Unit I) at Chanderia Lead Zinc Smelter. The zinc and lead concentrate received from mines is mixed with flux etc and charged in to a sinter machine to produce lumpy masses containing oxides of zinc and lead. This sintered product is charged in to Imperial Smelting Furnace (ISF) for zinc and lead recovery.
Hot gases generated during sintering are loaded with dust. The dust loaded gas is passed through an electrostatic precipitator where the gas is cleaned of dust particles. The dust so collected is called HGP dust. Although, this intermediate product contains approximately 58-62 % of lead, it can not be recycled in the main stream due to its high cadmium content (4-6%).
No significant research has been reported to recover base metal values from a dust characteristically similar to sinter plant of Chanderia Lead Zinc Smelter. The present invention is aimed to propose a simple two stage process scheme developed by extensive research activities to recover base metal values from HGP dust or a dust having similar characteristics.
BRIEF SUMMARY OF THE INVENTION;
The HGP dust treatment process is developed by HZL through its research and development efforts to produce a cadmium sponge and recovering of lead and silver values from left out residue. The process is basically a two stage leaching and precipitation process.
In a single aspect, the process hereby claimed consists of the following steps:
1. Leaching of HGP dust as the first step with oxidizing acid to dissolve cadmium as
cadmium sulphate.
2. Separation of the undissolved residue of lead by filtration with water washing
treatment and precipitation of cadmium rich solution by slowly adding zinc dust
to produce high quality cadmium sponge.
Cadmium sponge thus produced contains 65-90 % Cd, 0.5-15 % Zn, 400 - 700 ppm Pb and 40-80 ppm Fe. This sponge is amenable to different hydrometallurgical or pyrometallurgical processes to produce cadmium metal or can be sold as such.BRIEF DESCRIPTION OF THE DRAWINGS;
Figure 1 is a Process Flow Diagram of Leaching and Precipitation circuit of HOP dust treatment.
DETAILED DESCRIPTION OF THE INVENTION;
As described in Figure 1, HOP dust is leached in the first process stage with the oxidizing acid which is prepared by mixing the sulphuric acid and hydrogen peroxide (both are cooled at <10°C) in a ratio varying from 1:0.4 to 1.2. The leaching is carried out at room temperature. The primary objective of leaching is to dissolve cadmium as cadmium sulphate in the solution. The undissolved solid is separated by filtration and water washed to maximize cadmium recovery in the solution. The undissolved residue is treated in the furnace having total submerge lance technology to recover lead and silver values.
As depicted in Figure 1, zinc dust is slowly added in cadmium sulphate solution to precipitate high quality cadmium sponge. The acidity of the solution is kept between 2 -50 gpl to optimize the zinc percentage in the cadmium sponge. The ratio of cadmium in the solution to zinc dust addition is kept at 1: 0.6 to 1.2 for higher cadmium recovery.
The sponge thus produced primarily consists of mainly cadmium, zinc and is amenable for different hydro / pyrometallurgical processes or can be saleable as such. The present invention can be applied to any dust of similar nature. The present invention is also advantageous in terms of process simplicity as it is two stage process of leaching and precipitation.
The different stages of development work to obtain the process claimed hereby are given below:
SECTION 1 (Raw Material Characteristic)
The HOP dust samples generated from sinter plant of CLZS were thoroughly mixed and representative samples were made which have been the materials subjected to Test work and Engineering, leading to the present invention. The chemical analysis of HGP dust is tabulated in table no 1.
Table No. 1 (Analysis of HGP dust)
(Table Removed)
SECTION 2 (Leaching of HGP dust)
Leaching parameters are as followed:
HGP dust quantity: 0.5 -1 kg Water: 2 - 4 liter H2SO4 (98.5%): 30 - 80 ml H2O2(50%): 12- 112ml Leaching time: 2 - 6 hrs Leaching temperature: 25 - 30 °C Stirrer speed: 100 - 350 rpm
The summary of the Test results are given below: Table No. 2 (Specification of undissolved residue)

(Table Removed)Table No. 3 (Composition of leached liquor)

(Table Removed)Metal recoveries in the solution - Cd: 55-85 %, Zn: 40-70 %, Pb: 0.05 - 1% SECTION 3 (Precipitation of cadmium loaded solution)
Solution used: As given in table No. 3. Parameters are as followed:
The volume of filtrate: 5 liter
Zinc dust added: 24 - 60 gm
Acidity of solution: 2-50 g/1
The summary of the Test results are given below:
Table No. 5 (Composition of cadmium sponge)

(Table Removed)Metal recoveries in the sponge - Cd: 75- 98%, Zn: 5- 15 %.
SECTION 4 (Amenability of cadmium sponge for Metal Extraction)
The cadmium sponge thus produced can be sold as such or it is amenable for cadmium metal extraction by a suitable hydrometallurgical (Acid leaching followed by electrolysis) or pyrometallurgical process.

We Claim:
1. A process for the recovery of valuable metals from HOP dust comprising
Sulfides/Sulphates/Oxides of the metals.
The process consists of:
Leaching of HOP dust with oxidizing acid to dissolve cadmium as cadmium sulphate. Separation of the undissolved lead residue with water washing treatment followed by precipitation of cadmium sulphate solution by slowly adding zinc dust to produce cadmium sponge.
2. A process as claimed in Claim 1, which is believed to be capable of successful
application in treating dust of having similar characteristic. Necessary adjustments
and modifications are to be incorporated depending on the Raw Material
specifications and Project economics.
3. A process as claimed in Claim 1, which can be applied to produce a high grade
cadmium sponge. This sponge can be further treated via a suitable
hydrometallurgical (acid leaching followed by electrolysis) or pyrometallurgical
process to extract high grade cadmium metal (Cd - 99.95%). The achievable
sponge quality and process economics will be the key determining factors for
Technology selection.
4. Process as claimed in Claim 3, this sponge can be directly converted in to
cadmium metal after briquetting and melting at a suitable temperature.
Briquetting pressure 10 - 100 bar and melting temperature 350-600°C.