Products for the manufacture of molds and cores used in metal casting and a method for their manufacture and recycle from crushed rock
a technology of metal casting and molds, applied in the field of metal casting, can solve the problems of less common and much more expensive alternatives to quartz sand, inability to meet the thermal or physical characteristics of quartz sand, and increased costs of alternatives
Inactive Publication Date: 2005-02-17
NORAM TECH
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Benefits of technology
[0025] An additional object of the invention is a particle classification system that allows for the simultaneous recovery of two or more distinct grades of foundry quality sand from a single input stream.
Problems solved by technology
In some cases, the thermal or physical characteristics of quartz sand are unacceptable and foundries are obliged to use other sands with better properties.
These non-quartz alternatives are much less common and greatly more expensive than quartz sand and include olivine (ferriferous magnesium silicate), chromite (ferrous chromite, FeCr2O4), and zircon (zirconium orthosilicate, ZrSiO4).
The greater expense of the alternatives to quartz proscribes their general use, and foundries that make particularly demanding precision parts commonly use quartz sand or a recycled sand mixture containing an appreciable fraction of quartz sand for making the external parts of molds, and new non-quartz sand for making the internal parts or cores of the molds.
However, good porosity requires low levels of fine particles, whilst smooth casting surfaces require low levels of large particles; both of these factors limit the breadth of the particle size distribution.
The combination of physical and chemical properties required of a quartz foundry sand limit the number of locations where such products occur naturally.
Sand may therefore need to be shipped over considerable distances, making quartz foundry sand considerably more expensive than local ordinary builder's sand.
Many countries, particularly those located in the drier parts of the world such as northern Africa and the middle East, lack indigenous sources of quartz suitable for use as foundry sand and must import foundry sand at considerable cost from northern and western Europe.
A further factor limiting the number of locations that can supply quartz foundry sand is that much quartz sand, e.g. beach sand, is contaminated with shell or bone fragments or limestone particles that seriously interfere with casting procedures.
Such interference is created by the fact that these contaminants may react with commonly used binders and / or decompose at the temperatures typically used to cast metals.
Not only does quartz present difficulties in availability, the use of quartz has been associated with respiratory ailments.
Hence, quartz sand is the subject of restrictions and precautions in the workplace, and the spent sand, particularly the dust from foundry filters which contains elevated levels of quartz dust, is similarly restricted.
This limits the useful employment of spent quartz sand in concrete and asphalt.
Since different parts of the mold are at different temperatures during casting, they expand unevenly and cracks develop, into which molten metal can penetrate.
After casting, these metal intrusions appear as thin wafers that protrude from the casting and have to be removed in time consuming finishing operations.
At worst, the cast part may need to be scrapped.
Like quartz, the currently available alternatives to quartz are also environmentally suspect.
Olivine is highly alkaline and can contain nickel and in some cases asbestos, all of which can cause irritation to skin and lungs; together with chromite both are considered toxic waste and must be disposed of in special dump sites.
Zircon is weakly radioactive, requiring workplace precautions and dump site limitations.
The sources of currently used alternatives to quartz sand are far fewer in number and most are located outside of the areas where there are large numbers of foundries; this means that they bear considerable freight cost penalties compared to quartz sand.
These factors make these alternative sands as much as ten or twenty times more expensive than quartz sand and they are therefore rarely used as the sole sand in a foundry.
Because spent foundry sand can contain organic materials, acids and heavy metals, environmental authorities usually insist that it must be dumped at an approved site for toxic waste; this adds considerably to the foundry's total sand related costs.
Such thermal processes remove organic binder residues by incineration; they yield sand of fair quality but are energy intensive, costly and not suitable for all sand / binder combinations.
They also lead to emissions of environmentally undesirable gases (oxides of sulphur, nitrogen and carbon).
Such mechanical processes are less costly but the quality of the recovered sand is inferior and its use within the foundry often more restricted than that of new or thermally reclaimed sands.
Recovery of used sand is significantly complicated by the fact that different sand types are sometimes used for the molds and cores.
Once the casting process is complete, it is rarely feasible to separate the used molds and cores from one another, so the different sands used for these two purposes become mixed.
State of the art recycling methods are unable to satisfactorily separate this mixture into its component parts and foundries that use both costly non-quartz sand and cheaper quartz sand must therefore replenish their non-quartz sand with new material after each casting cycle.
In other cases, foundries that would prefer to use and recycle two grades of the same sand, e.g., one for making the mold and another of different particle size distribution for making the core, are unable to do so because limitations in state of the art recycling methods do not allow such closely similar materials to be easily separated.
The proportion of sand that can be recycled can also be limited by the binder system used, since some binders react with quartz at casting temperatures; these include some of the most commonly used binders that contain highly alkaline materials such as sodium silicate or mixtures of phenolic resins with caustic alkalis.
These binder resins are difficult to remove, either by attrition or thermal treatment and, when heated during thermal recycle or subsequent casting, may react with the sand to form silicates of low melting point that seriously compromise the refractory characteristics of the sand.
Foundries are also limited in their choice of classification methods for sand recycling and cannot economically employ methods originally used in large scale manufacture of foundry sand.
Wet classification has inordinately high operating costs and yields effluents that pose environmental hazards.
Sieves are difficult and costly to use with fine materials and, unless the product fractions are carefully remixed, fail to yield products whose particle size distributions give optimal packing characteristics.
Method used
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[0036] In describing a preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
[0037] Foundry sand may be defined in accordance with a number of characteristics which make it suitable for use in casting. These include that such sands are practically free from dust, i.e., particles below 75μ, consist of grains that are rounded rather than angular, have a normal particle size distribution where at least 85% of the particles are between 0.5 and 1.5 of the mean diameter and resist abrasion. Minerals used for foundry sand must have high tensile strength and a sufficiently high sintering temperature, and must not be subject to any chemical change that may cause gas to be evolv...
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Abstract
A system and method for producing foundry quality sand from non-conventional starting materials through the combination of oolitization and classification. Incoming particulate matter is first directed into a controlled energy attrition unit where the particles are made to collide with one another. Such collisions clean and round the particles by chipping away surface projections and coatings without crushing the particles. The particle stream is then directed through a multi-fraction classifier where it is separated into two or more useable grades of foundry sand. An air classifier is preferred for the classification stage.
Description
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention is related to the field of metal casting and, more particularly, to a system and method for producing foundry quality sand from non-conventional starting materials, and for classifying the sand so produced into two or more foundry grade products. [0003] 2. Description of the Related Art [0004] Most foundry sand is made by sieving or wet classifying naturally occurring silica or quartz sand. (As used herein, “quartz sand” is intended to refer to sand containing silica as is found in quartz in crystalline form. As used herein, “non-quartz sand” is intended to refer to sand which does not contain a significant amount of silica.) Quartz sand suitable for casting contains low levels of compounds of alkali and alkaline earth metals, of both organic and inorganically bonded carbon and of halogen and sulphur derivatives. Such sand consists of rounded particles with weight average mean particle sizes of...
Claims
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Patent Timeline
Login to View More IPC IPC(8): B22C5/04B22C5/00B22C5/06
CPCB22C5/18B22C5/06
InventorSPARKS, ROBERT E.HARRIS, KENNETH HILLEL PETER
OwnerNORAM TECH