Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-08-13
2003-11-25
Elve, M. Alexandra (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S595000, C423S598000, C423S599000, C423S600000, C423S594800, C423S594120, C106S401000, C106S450000, C106S451000, C106S453000
Reexamination Certificate
active
06652829
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method of producing crystalline powders of mixed metal oxides by a continuous process. More particularly, the crystalline powders are produced by a continuous rotary calcination technique and utilize fugitive flux mineralizers to enhance mutual migration or diffusion of active species. The powders are useful in a variety of applications but particularly as pigments for use in ceramics, paints and plastics.
BACKGROUND
There are many mixed metal oxide compounds that are important in inorganic technology. They are widely used in diverse ceramic applications such as refractories, ferroelectric devices, as inorganic pigments and the like. Of particular interest is the use of mixed metal oxides as inorganic pigments for applications in ceramics, paint and plastics. Typically, in the inorganic pigment industry, classes of pigments that are well recognized are described in a publication of the Color Pigment Manufacturers Association. For these applications it is desirable to produce pigment particles of fine uniform particle size which are phase pure, defect free and offer optimum color values.
Typically, mixed metal oxide inorganic pigments are commercially, although not exclusively, produced by either: (a) a solid-state reaction process involving the wet or dry blending of various metals, oxides or salts, subsequent calcination at elevated temperatures, to ensure that the desired reaction occurs, followed by comminution (or deagglomeration) to the desired size and washing and drying (if required, to remove unwanted salts); or (b) chemical precipitation which may be followed by calcination and subsequent comminution (or deagglomeration) to the desired size and washing and drying (if required to remove unwanted salts); or (c) combinations of both processes.
Modern practice attempts to maximize dry process options in the interests of economy and energy efficiency by batching and dry blending raw materials prior to calcination. The raw materials used are fine powders typically with particle sizes in the range of 0.2 to 50 &mgr;. It is normally not the purpose of the dry blending process to reduce the particle sizes of the constituent powders, but seeks to distribute them evenly. However, dry blending cannot generally produce raw batches that are homogeneous on a submicron scale. The calcinations are typically 0.1 to 24 hours in length to allow for large scale production; however, this is sometimes insufficient to permit complete diffusion of the active species and reaction of the coarser or more refractory raw materials. Calcination can be achieved in periodic, intermittent kilns, or continuous rotary or tunnel kilns. Final size adjustment is achieved by either wet or dry comminution devices which might include ball milling, attrition milling, micropulverization or jet milling. Wet comminution is followed by a drying operation or a filter, wash and drying operation.
The typical pigment manufacturing process described above causes a number of significant problems for the production of high quality pigments. Some common difficulties are achieving complete reaction at readily achievable low calcination temperatures; production of a single phase product; production of fine sized particles; production of narrow particle size distributions; formation of aggregates and large particles which are difficult or impossible to mill down to the desired size; and elimination of grit and large particles (>2 &mgr; or >10 &mgr;, depending on the pigment application).
It is common practice in the pigments industry, in cases where higher than normal (or preferred) temperatures are required, to ensure that the necessary solid-state reaction takes place by assisting the high temperature reactions by the use of additives (sometimes called fluxes or mineralizers). These additives melt, form eutectics or a reactive vapor phase which is conducive to the formation of the required crystal and/or the mutual migration, or diffusion, of the active species. It is often a further benefit of using such additives that the calcination temperature needed to accomplish the desired reaction is reduced by their use.
The use of fluxes or mineralizers either singly or in combination is largely based on experience because generally there is no reliable manner of predicting which particular mineralizer or combination will enhance the formation of a given color, or amount thereof. Mineralizers are typically employed to enhance liquid phase formation eutectic melt systems and vapor phase reactions. Such mineralizers are typically fluorides, chlorides, sulfates, oxides and other salts which might be used singly or in multiple combinations. Depending upon the application of the pigment, it is frequently necessary to wash the finished pigment to remove residual salts or mineralizers.
One of the limitations of the use of flux mineralizers, particularly those which form fugitive or reactive vapors, is that in order to create the maximum effect, the reactive vapor needs to stay in contact with the pigment batch for a substantial period of the calcination cycle. This requires that the calcination procedure be conducted in a closed container (i.e., a covered or sealed crucible or sagger) to prevent the vapors from escaping at the first opportunity, and to maintain a positive atmosphere of the volatilized flux mineralizer. The calcination itself is, therefore, conducted in a periodic (batch) kiln or continuous tunnel kiln. The use of these materials has, therefore, traditionally precluded the use of continuous (high production rate) rotary kilns when flux mineralizers are required. Also, in conventional processing the nature of the flux mineralizer generally results in it not only reacting with the ingredients of the pigment formulation, but also with the refractory sagger (or crucible). As a result, the full value of the flux mineralizer is often not achieved and the reaction with the sagger refractory material reduces the life (use) cycle of the sagger thereby increasing overall production costs.
In conventional manufacturing in order for the flux mineralizers to be effective in assisting the formation of pigments it is necessary that whether they be gaseous or liquid, they need to be in intimate contact with the powdered ingredients. Good mixing ensures that this occurs for mineralizers that form a melt phase while a sealed sagger system ensures that it occurs when vaporized flux mineralizers are utilized.
SUMMARY OF THE INVENTION
The present invention provides a new and improved method for the manufacture of mixed metals oxide crystalline powders using “fugitive” flux mineralizers in a continuous rotary kiln which avoids the requirement for any granulation or pre-compaction of the raw material feed. The process provides a high quality product at high production rates, and such process minimizes the degree of formation of melts and eutectics, which may in turn plate out onto the furnace wall to form an insulating layer.
In general the present invention provides a method for the synthesis of mixed metal oxide crystalline powders which comprises the steps of preparing a raw material mixture containing at least two different metal cations; adding one or more flux mineralizers and blending it therewith; initiating formation of a mixed metal oxide powder by calcination of the mixture and the flux mineralizer in a continuous indirectly-fired rotary furnace which is capable of entraining the desired atmosphere, whereby the particles of flux mineralizer either melt or vaporize within and around the moving raw material bed charge, displacing the ambient atmosphere and assisting in the formation of the mixed metal oxide particles and thereafter recovering the mixed metal oxide powder.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which t
Barnes Raymond E.
Garcia Sainz Javier
Joyce Ivan H.
Pipoly Richard A.
Elve M. Alexandra
Ferro Corporation
Ildebrando Christina
Rankin, Hill Porter & Clark LLP
LandOfFree
Continuous calcination of mixed metal oxides does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Continuous calcination of mixed metal oxides, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Continuous calcination of mixed metal oxides will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3150110