LABORATORY EXPERIMENT

(KGP002)

LEACHING OF DUST FROM AN ELECTRIC ARC FURNACE

Theory

The yearly consumption of iron and steel within the EU is approximately 150 million tonnes.Roughly 50 million tonnes of thisisproduced by recirculation of iron scrap in electric arc furnaces. The recirculated scrap is often coated with zinc leading to the formation and collection of dust with relatively high zinc content in the gas cleaning system. It is estimated that about 0, 5-1 million tonnes of this type of dust is produced yearly within the EU. Since the dust also contains smaller amounts of Pb, Cd and Cr it is classified as an environmentally hazardous waste. The dust is very fine-grained, which makes it prone to spread by wind and which also increases the leaching rates of heavy metals when exposed to acid rain.The dust is, due to this, regarded as a threat for soil and earthas well as surface- and groundwater.

On account of these possible risksand also because of the possible value in the dust as a raw material, especially regarding zinc, intense research has been conducted during many years with the purpose of making recirculation of the dust possible. Several pyro- as well as hydrometallurgical processes have been developed of which some has been used in industrial applications. Some unsolved problems regardingrecycling of the dust makes the research on the subject still intense. The content of chlorides in the dust leads to problem in pyrometallurgical processes as the chlorides react with the refractories in the furnace.The content of zincferrites constitutes a problem in hydrometallurgical processes where also other heavy metals bring adversities in the following solution purification.

A possible process could be to combine pyro- and hydrometallurgical methods by performing a relatively mild leaching where only the content of free zinc oxide is leached. The leaching residue thatcontains zincferritesand other iron oxides are brought back to the electric arc furnace while the solution, after solution purification, is used for production of zinc oxide.

Aim

The aim with this experiment is to investigate the proportionin percentage of zincoxide and zinc ferrite, respectively,in the dust. Sincethe content of manganese in solution can cause problems in the production of zinc oxide (discoloration of the white zinc oxide) it is of interest to study how much of the manganese that is brought into solution. With the conditions used during the experiment only Mn2+ is soluble which implies that only MnO is being leached from the dust.

Analysisof the dust

Zn / Fe / Pb / Ca / Mn
% / % / % / % / %
Ovako / 21.9 / 29.6 / 0.7 / 9.1 / 2.7

Performance

Place the required experimental equipment in a ventilation hood. The leaching is performed according to:

  • Temperature:80C
  • Leaching solution volume:800 ml water
  • Amount of solids:160 g
  • Amount of NaCl:120 g
  • pH:3.0
  • Leaching time:1 hour

Start by heating up 800 ml watercontaining120 grams NaCl. When the temperature is 75C the heat is turned of and the electric arc furnace dust is added (exothermic reaction). Carefully add the material and wash down eventual dust sticking on to the edges using small amounts of water from a spray can. Add concentrated HCl in small portions with pipetteto keep the pH at a constant value of 3.0during the experiment. In the beginning of the experiment some gas can be developed, due to the presence of small amounts of carbonates in the dust. Regulate the temperature to 80C during the experiment. Remember to keep all of the solid material agitated to avoid cake formation at the bottom of the leaching vessel.

When the leaching is finished the agitator, pH-electrode and temperature sensor are dismantled. Filter the product using anOOR filter. When the filtration is almost finished and the last of the remaining solution is entering into the filter cake, 100 ml wash water is added. When the filtration is ready the total volume (leachate + wash water) is measured. The leaching residue is placed in a weighed bowl and dried at 120C for 2 hours or until it is dry. Weigh the leaching residue when it is dry.

OBS! 1.The total consumption of HCl should be determined (start by measuringout 200 ml of HCl in a measuring cylinder).

OBS! 2.Protect eyes and clothes when handling HCl.

Analysis with AAS

The leachate is analysed for its zinc, iron and manganese content.

Manganeseandiron:1.0 ml leachate + 2 ml HCl is diluted with deionised water to100 ml in a measuring flask.

Zinc:Take 1.0 ml of the solution made for the analysis of iron and manganese and add 2 ml HCl and dilute with deionised water to 100 ml in another measuring flask.

The leaching residueis analysed for its zinc content.

Dissolve a precise amount of leaching residue (~ 1 gram) by adding 25 ml of concentrated HCl in a 100 ml measuring flask. Possiblythe sample needs to be heated carefully to be completely dissolved. When the sample has dissolved, dilute with deionised water up to the marking of the flask

Then take 200 l of the solution + 2 ml HCl and dilute with deionised water to 100 ml in ameasuring flask.

Report

The report shall contain a short introduction, experimental, result and discussion. State the leaching yield for zinc, iron and manganese from the experiment. Calculate the percentage amount of zinc oxide respective zinc ferrite in the dust. Is the material balance okay for the zinc content? Discuss possible deviations. Also,account for how the amount of acid added agrees with the assumption that all metal oxides except zinc ferrite and iron oxides are soluble under the leaching conditions used. Discuss conceivable methods, both pyro- and hydrometallurgical, to avoid manganese in the leachate.

Calculations are accounted for in a separate enclosure.