Understanding Melting Rates In Aluminum Furnaces

INTRODUCTION

To understand melting rates in aluminum melting furnaces, and how these rates are calculated, and why they can vary significantly, one needs to understand the “external forces” can influence the final outcome.

ELECTRIC MELTING FURNACES:

With electric melting furnaces, a common ratio of holding capacity to melting rate per hour typically falls within the range of 3:1 to 4:1.

Example:

• If an electric furnace has a holding capacity of 300 kg (660 lb), the melting rate could be 75–100 kg (165-220 lbs) per hour.

Electric furnaces are often optimized for lower melting rates and higher energy efficiency compared to gas-fired furnaces.

GAS MELTING FURNACES:

For crucible-type gas-fired aluminum melting furnaces, the typical ratio of holding capacity to melting rate per hour generally ranges from 2:1 to 3:1. This ratio is often lower compared to electric furnaces because gas furnaces are typically designed to prioritize faster melting rates.

Example:

• If a gas-fired furnace has a holding capacity of 300 kg (660 lb), the melting rate could be 100–150 kg (220-330 lbs) per hour.

Gas furnaces are known for their ability to achieve higher melting rates and faster but often require careful control to maintain efficiency and reduce energy costs.

NOTE: It is possible to “push” furnaces to melt quicker and thus decreasing the holding to melting ratio.  This is done by increasing the melting temperature.  However, one has to be cautious with this since it usually increases the amount of oxidation and dross formation.

MELTING RATES SHOWN ON THE DYNAMO FURNACES WEBSITE:

The figures we provide on our Dynamo Furnaces website are at an “optimum” or “test environment” setting as a benchmark primarily for comparison between the various types of furnaces we have.

THE VARIABLES:

In real world work environment, beyond the furnace design, there are several things that can influence the melting rate per hour.

1) Production Goals:

The specific needs of the operation as to whether it prioritizes holding large amounts of molten aluminum or a faster melt rate, and the desired balance between melting speed and fuel or energy consumption is the first factor that needs to be considered.

2) Metal Charge Composition and Size:

The type, purity, and physical characteristics of the aluminum being melted (e.g., ingots, scrap, or chips) affect the melting rate. Smaller pieces or chips will melt faster than large ingots due to increased surface area.

3) Initial Metal Temperature:

The starting temperature of the aluminum charge impacts the melting time. If the metal starts closer to the melting point, less energy and time are required to melt it.

4) Crucible Material and Condition:

The thermal conductivity and condition of the crucible influence heat transfer efficiency. A well-maintained crucible with high thermal conductivity can melt aluminum faster.

5) Power Supply and Heating Element Efficiency:

The power rating of the furnace and the efficiency of the heating elements directly impact the melting rate. Higher power and more efficient elements lead to faster melting.

6) Ambient Temperature and Environmental Conditions:

Cooler ambient temperatures and drafty or poorly controlled environments can cause heat loss and reduce the melting rate.

7) Metal Oxidation and Dross Formation:

Excessive oxidation or the formation of dross (metallic waste) can reduce the effective melting rate, as energy is wasted in dealing with impurities.

8) Operational Practices:

The way the furnace is loaded, how frequently it is opened, and how it is managed during melting (e.g., stirring practices) can also impact the melting rate. Frequent opening of the furnace lid, for instance, can lead to significant heat loss.

9) Preheating of the Crucible and Furnace:

If the furnace and crucible are preheated or maintained at higher standby temperatures, the melting process can be faster compared to starting from a cold state.

METAL OXIDATION AND DROSS FORMATION

Regarding point # 7 above, excessive oxidation and dross formation in aluminum melting furnaces can be caused by several factors:

1. a) High Melting Temperatures:

Operating the furnace at temperatures significantly higher than necessary for melting aluminum increases the rate of oxidation. Aluminum oxidizes rapidly when exposed to high heat, forming aluminum oxide (Al₂O₃), which becomes part of the dross.

1. b) Prolonged Exposure to Air:

When molten aluminum is exposed to oxygen in the air for extended periods, oxidation occurs more rapidly. Opening the furnace lid too often or leaving the molten aluminum uncovered exacerbates this exposure.

1. c) Moisture Contamination:

The presence of moisture in the furnace, crucible, or aluminum charge can lead to a reaction with molten aluminum, producing hydrogen gas and causing excessive oxidation and dross. This can happen when wet or unclean aluminum scrap is used.

1. d) Contaminated Charge Material:

Using dirty or oxidized aluminum scrap as the charge material introduces impurities that can form dross during melting. Non-metallic contaminants in the scrap can also react with molten aluminum to create more dross.

1. e) Turbulent Stirring or Pouring:

Aggressive stirring or pouring of the molten aluminum can increase the surface area exposed to oxygen, leading to more oxidation. Gentle handling is important to minimize dross formation.

1. f) Poor Fluxing Practices:

Inadequate or incorrect use of fluxes can lead to increased oxidation. Fluxes are used to cover the molten aluminum and protect it from oxidation, as well as to remove impurities. If the flux is not used properly, it can fail to prevent dross formation.

1. g) Alloy Composition:

Some aluminum alloys are more prone to oxidation than others, especially those with high magnesium content. Magnesium oxidizes more readily, contributing to dross formation.

1. h) Improper Furnace Atmosphere Control:

Lack of atmosphere control in the furnace, such as using an inert gas cover (e.g., argon or nitrogen), allows oxygen to come into contact with the molten aluminum and cause oxidation. A controlled or inert atmosphere can reduce the rate of dross formation.

SUMMARY

As we can see there are a considerable number of variables that can influence the meting rate of aluminum furnaces. As a result of all of these variables, we at Dynamo Furnaces have decided to show on our website “optimum” or “test environment” benchmark melting rates. This is a similar concept to automobile manufacturers showing “optimum” milage figures as benchmarks in their figures for “miles per gallon” or “liters per 100 km”.  In a similar way, with melting furnaces, “your milage may vary” based on the factors that have been considered here.

For crucible-type gas-fired aluminum melting furnaces, the typical ratio of holding capacity to melting rate per hour generally ranges from 2:1 to 3:1. This ratio is often lower compared to electric furnaces because gas furnaces are typically designed to prioritize faster melting rates.

AUTHOR / DISCLAIMER

This article was authored by Ed Lange, who is an Aluminum Furnace Consultant, elange2@cogeco.ca. The information presented in this article is based upon the authors 40 plus years of experience in the development, design, and sales of non-ferrous melting, holding, and handling equipment. The author does not in any way, endorse any particular furnace manufacturer, or their respective technologies.