Aluminum Company of America and Reynolds Metals Co., the nation's two biggest aluminum producers, reported sharp profit declines in the first quarter as demand for the metal continues to sag because of the recession in key industries such as automobiles, construction and appliances.

On Monday Kaiser Aluminum and Chemicals Corp., the other major U.S. aluminum producer, reported a $24.9 million first-quarter loss.

Alcoa reported that earnings declined to $43.8 million (57 cents a share) on sales of $1.2 billion. A year ago the producer had earnings of $101 million ($1.37) and sales of $1.3 billion.

Reynolds reported profits of $13.6 million (68 cents) in the first three months of the year, compared with net income of $33 million ($1.69) a year ago. Its sales fell 13 percent to $732.7 million from $846.2 million. Reynolds' profitable performance, however, reflected an accounting change that added $26.8 million to the bottom line. Without that change, the company would have reported a loss of about $13 million.

Similarly, Alcoa's first quarter statement includes a one-time gain of $21.8 million realized by exchanging common stock for $58.9 million of bonds. Without that gain, Alcoa's first-quarter earnings would have been about $22 million.

"Depressed aluminum prices and the high cost of carrying idle facilities were the two major contributors to lower Alcoa earnings," according to W. H. Krome George, chairman of the nation's biggest aluminum producer. The company said it shipped 397,000 metric tons during the first three months of 1982, compared with 408,000 tons a year ago. Reynolds, based in Richmond, shipped 240,300 tons during the first quarter, 15.4 percent less than the 284,200 tons it shipped during the first three months of 1981.

Kaiser reported Monday that its shipments were off 18.9 percent to 202,000 tons during the first three months.

Although the list price for aluminum ingots is about 76 cents a pound, producers are selling it for about 50 cents, according to George Cleaver, vice president for research at Merrill Lynch, Pierce, Fenner & Smith.

Cleaver said the industry's future will be bleaker. U.S. producers will make about 4 million tons of the metal in 1982, compared with nearly 4.5 million tons last year, he said. Both Kaiser and Reynolds "will lose a lot of money this year and it will not be much better next year," he added.

Although aluminum users have pared their inventories to the bone, producers still have about 1.2 million tons in their stocks, an accumulation of finished metal that would keep prices down even if an unexpected increase in demand for the metal appeared.

Such an increase is demand is unlikely in the short-run and in the long-run aluminum's vitality is questionable, too. Plastics are making inroads into aluminum building products. Aluminum is displacing less steel in the automobile than analysts projected several years ago. For the near-term automobiles, appliances and contruction do not appear poised on the brink of recovery.

The industry is plagued by excess capacity and high energy costs, too. Many Canadian aluminum producers, with plentiful and cheap hydroelectric power, have electricity costs as low as 4 or 5 cents a pound. In the United States where there is less hydroelectric power, costs per pound can vary from 4 cents to 50 cents, according to Cleaver.

Many of the high-cost plants have been shut down or scaled back by domestic aluminum makers.